stomach
1119
46
RetpAhc
A
Lesser curvature nodes
Right paracardial nodes Left paracardial nodes
Left gastric nodes
Short gastric arteries
Left gastric artery
Coeliac trunk
Coeliac nodes
Common hepatic nodes
Common hepatic artery
Short gastric
Right gastric artery nodes
Hepatic artery
Suprapyloric
nodes
Gastroduodenal
artery
Left gastroepiploic artery
Splenic artery
Inferior Left gastroepiploic nodes
pancreaticoduodenal
artery
Infrapyloric nodes
Right gastroepiploic nodes
Right gastroepiploic artery
Superior mesenteric artery
Right gastroepiploic nodes
B
Coeliac trunk Left gastric artery
Common hepatic artery Splenic artery
Common hepatic nodes Splenic nodes
Hepatic artery
Hepatoduodenal nodes
Short gastric
Gastroduodenal artery arteries
Splenic hilar nodes
Left gastroepiploic
Retropancreatic artery
nodes
Left gastroepiploic
Anterior pancreatic nodes
nodes Inferior pancreatic nodes
Common hepatic
Superior mesenteric artery Common hepatic
nodes nodes
Pancreas
Superior mesenteric Retropancreatic Portal vein
vein nodes
Superior mesenteric Right gastroepiploic vein
artery
Infrapyloric nodes
Superior mesenteric nodes
Superior mesenteric vein
Fig. 64.14 The main lymph node stations of the stomach (A) and upper abdominal viscera (B). The sagittal relationships of the node groups around the
neck of the pancreas are shown bottom right. The para-aortic nodes are among the highest nodes for these viscera but have been removed for clarity.


AbdominAl oesophAgus And stomAch
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8
noitces
Posterior vagus Anterior vagus cuboidal in shape, and their nuclei are rounded and euchromatic. They
contain secretory zymogen granules and their abundant cytoplasmic
A RNA renders them strongly basophilic. Parietal (oxyntic) cells are the
source of gastric acid and of intrinsic factor, a glycoprotein necessary
Hepatic nerves
for the absorption of vitamin B . They are large, oval and strongly
12
eosinophilic, and have centrally placed nuclei.
Parietal cells occur intermittently along the walls of the more apical
Anterior half of the gland but can reach as far as the isthmus; they bulge laterally
gastric nerves
into the surrounding connective tissue. They have a unique ultrastruc-
ture related to their ability to secrete hydrochloric acid. The luminal
Coeliac nerves Greater posterior side of the cell is deeply invaginated to form a series of blind-ended
gastric nerve channels (canaliculi) that bear numerous irregular microvilli covered
B by a plasma membrane rich in H+/K+ ATPase antiporter channels. The
Greater anterior latter actively secrete hydrogen ions into the lumen; chloride ions
gastric nerve follow along the electrochemical gradient. The mitochondria-rich cyto-
plasm facing these channels contains a tubulo-vesicular system of abun-
dant fine membranous tubules directed towards the canalicular surface.
The precise structure of the cell varies with its secretory phase: when
stimulated, the number and surface area of the microvilli increases up
to five-fold, probably as a result of the rapid fusion of the tubulo-
Pyloric nerves vesicular system with the plasma membrane. This process is reversed at
the end of stimulated secretion, when the excess membrane retreats
Pyloric branches back into the tubulo-alveolar system and microvilli are lost.
Fig. 64.15 The distribution of the vagal nerves to the stomach. The two Mucous neck cells are numerous at the necks of the glands and are
most common variations in the anterior vagus are shown in pink. Key: scattered along the walls of the more basal regions. They are typical


A, multiple main trunks; B, low origin of the hepatic/pyloric branch lying mucus-secreting cells, displaying apical secretory vesicles, containing
close to the lesser curvature. mucins, and basally displaced nuclei; their products are distinct histo-
chemically from those of the superficial mucous cells.
Stem cells are relatively undifferentiated mitotic cells from which the
other types of gland cell are derived. They are relatively few in number,
gastric vessels and nerves (Figs 64.16–64.17). The microstructure
and are situated in the isthmus of the gland and the bases of the gastric
reflects the functions of the stomach as an expandable muscular sac
pits. Stem cells are columnar and possess a few short apical microvilli.
lined by secretory epithelium, although there are local structural and
They periodically undergo mitosis; their progeny migrate either apically,
functional variations in this pattern.
to differentiate into new surface mucous cells, or basally, to form
mucous neck, parietal, chief or neuroendocrine cells. All of these cells
Mucosa
have a limited lifespan, especially the mucus-secreting types, and so
they are constantly replaced. The typical replacement period for surface
The mucosa is a thick layer with a soft, smooth surface that is mostly mucous cells is 3 days, and that for mucous neck cells is 1 week. Other
reddish brown in life but pink in the pyloric region. In the contracted cell types appear to live much longer.
stomach, the mucosa is folded into numerous folds, or rugae, most Neuroendocrine (enteroendocrine) cells occur in all types of gastric
of which are longitudinal. They are most marked towards the pyloric gland but more frequently in the body and fundus of the stomach. They
end and along the greater curvature. The rugae represent large folds in are situated mainly in the deeper parts of the glands, among the chief
the submucosal connective tissue (see below) rather than variations in cells. They are basally situated, pleomorphic cells and display irregular
the thickness of the mucosa covering them, and they are obliterated nuclei surrounded by granular cytoplasm that contains clusters of large


when the stomach is distended. As elsewhere in the gut, the mucosa is (0.3 µm) secretory granules. Neuroendocrine cells synthesize a number
composed of a surface epithelium, lamina propria and muscularis of biogenic amines and polypeptides that are important in the control
mucosae. of gut motility and glandular secretion. They are part of the system of
dispersed neuroendocrine cells. In the stomach, they include gastrin-
Epithelium producing G cells; somatostatin-producing D cells; histamine-producing
ECL (enterochromaffin-like) cells; serotonin-producing enterochromaf-
When viewed microscopically at low magnification, the internal surface fin cells; and ghrelin-producing P/D(1)-type endocrine cells (Sakata
of the stomach wall appears honeycombed by small, irregular gastric and Sakai 2010).
pits: there are approximately 60 to 100 gastric pits per square millimeter
of gastric mucosa, each pit having a diameter of approximately 70 µm Cardiac glands
and a depth of about 0.2 mm (Lillibridge 1964) (see Figs 64.16–64.17). Cardiac glands are confined to a small area near the cardiac orifice;
The base of each gastric pit receives several long, tubular gastric glands some are simple tubular glands, others are compound branched tubular
that extend deep into the lamina propria as far as the muscularis mu- glands. Mucus-secreting cells predominate; parietal and chief cells are
cosae. Simple columnar mucus-secreting epithelium covers the entire present but sparse.
luminal surface, including the gastric pits, and is composed of a con-
tinuous layer of surface mucous cells that release gastric mucus from Pyloric glands
their apical surfaces to form a thick, protective, lubricant layer over the Pyloric glands empty via groups of two or three short, convoluted tubes
gastric lining. This epithelium commences abruptly at the Z line at the into the bases of the deep gastric pits of the pyloric antrum; the pits
cardiac orifice, where there is a sudden transition from oesophageal occupy about two-thirds of the mucosal depth (Fig. 64.18). The glands


stratified squamous epithelium. are populated mainly by mucus-secreting cells but they also contain
neuroendocrine cells, especially G cells, which secrete gastrin when
gastric glands
activated by appropriate mechanical stimulation (causing increased
Although all gastric glands are tubular, they vary in form and cellular gastric motility and secretion of gastric juices). Although parietal and
composition in different parts of the stomach. They can be divided into chief cells are scarce, parietal cells are always present in both fetal and
three groups: cardiac, principal and pyloric. The most highly specialized postnatal pyloric glands, and may also appear in the duodenal mucosa,
are the principal glands. proximally near the pylorus, in adult tissue.
Principal gastric glands Lamina propria
The principal glands are found in the body and fundus, three to seven The lamina propria forms a connective tissue framework between the
opening into each gastric pit. Their junction with the base of the pit is glands. It contains small masses of lymphoid tissue, gastric lymphatic
the isthmus; immediately basal to this is the neck; and the remainder follicles, which resemble solitary intestinal follicles (especially in early
is the base. The walls of the gland contain at least five distinct cell types: life). It also contains a complex periglandular vascular plexus involved
chief, parietal, mucous neck, stem and neuroendocrine. in the maintenance of the mucosal environment, e.g. the removal of
Chief (peptic) cells (see Fig. 64.16) are the source of the digestive bicarbonate produced in the tissues as a counterpart to acid secretion,
enzymes pepsin and lipase. They are usually basal in position and and neural plexuses rich in both sensory and motor terminals.


stomach
1121
46
RetpAhc
Fundus
Oesophagus
Cardiac orifice
Body Surface mucous cell
Pylorus
Duodenum
Mucus
Pyloric canal
Pyloric antrum
Mucous neck cell
Mucus
Parietal cell
H+
Gastric pit
Intrinsic
factor
Cl−
Chief cell
Isthmus
(neck)
Mucosa
Gastric gland Pepsin
(body)
Neuroendocrine cell
Base
G cells – gastrin
D cells – somatostatin
ECL cells– histamine
Fig. 64.16 Principal regions of the interior of the stomach and the microstructure of tissues and cells within its wall. Undifferentiated, dividing cells are
shown in white. Abbreviations: ECL, enterochromaffin-like.
Muscularis mucosae variably developed in different regions of the stomach and not easily
separated. Clasp-like semicircular smooth muscle fibres are present on
The muscularis mucosae is a thin layer of smooth muscle fibres lying
the right side of the oesophagus and sling-like oblique gastric muscle
external to the layer of glands, arranged as continuous inner circular
fibres on the left side; the latter radiate horizontally towards the greater
and outer longitudinal layers, and a discontinuous external circular
curvature and maintain the angle between the oesophagus and stomach.
layer. The inner layer sends strands of smooth muscle cells between the
The circular muscle layer is thicker in the distal pyloric antrum, where
glands; their contraction probably assists in emptying into the gastric
it forms the anular pyloric sphincter. The outer longitudinal layer is
pits.
most pronounced in the upper two-thirds of the stomach.
Submucosa The muscularis externa enables the stomach to produce churning
movements that mix food with the gastric secretions. When the muscles
contract, they reduce the volume of the stomach and throw the mucosa
The submucosa is a variable layer of loose connective tissue. It contains
into longitudinal folds or rugae (see above). Muscle activity is control-
thick bundles of collagen, numerous elastin fibres, blood vessels and
led by a network of unmyelinated autonomic nerve fibres and their


nervous plexuses, including the ganglionated submucosal (Meissner’s)
ganglia, which lie between the muscle layers in the myenteric (Auer-
plexus.
bach’s) plexus.
Muscularis externa
Interstitial cells of Cajal
The muscularis externa is a thick muscle coat immediately under the
serosa, with which it is closely connected by subserous loose connective Like smooth muscle elsewhere in the gut, interstitial cells of Cajal are
tissue. From innermost outwards, it contains oblique, circular and lon- present within the submucosa and muscular layers of the stomach.
gitudinal layers of smooth muscle fibres (see Fig. 64.6). The layers are These spindle-shaped cells contact nerve fibres and form gap junctions


AbdominAl oesophAgus And stomAch
1122
8
noitces
E
P
P
Fig. 64.17 A low-power micrograph showing the stomach wall, thrown
into longitudinal folds or rugae that are visible macroscopically. The E
surface epithelium is infolded microscopically to form gastric pits. Gastric
glands extend through the thickness of the mucosal lamina propria and
Fig. 64.18 A micrograph showing the pyloric region of the stomach.
open into the bases of the gastric pits. A muscularis mucosae layer and
Pyloric glands are stained with the periodic acid–Schiff (PAS) technique to
submucosa follow the contours of the rugae. Part of the external
show mucin (magenta) in the gastric pits and glands. Pale-staining cells
muscularis layers is seen below left. (Courtesy of Mr Peter Helliwell and
are the larger parietal cells (P) and smaller enteroendocrine cells (E).
the late Dr Joseph Mathew, Department of Histopathology, Royal
(Courtesy of Dr JB Kerr, Monash University with permission from Kerr JB
Cornwall Hospitals Trust, UK.)
1999 Atlas of Functional Histology. London: Mosby.)
with smooth muscle cells, consistent with their role in modulating
GASTRIC MOTILITY
motor neurotransmission. Interstitial cells of Cajal are involved in the
generation of rhythmic gastric slow-wave contractions and play an
important role in gastric motility. They have been implicated in the In simple terms, the proximal region of the stomach (fundus and upper
pathogenesis of gastric motility disorders such as diabetic gastric paresis body) functions mostly as a temporary storage compartment, regulating
(Negreanu et al 2008) and in the development of gastric stromal intragastric pressure and the onward passage of chyme into the distal
tumours (Roggin and Posner 2012). stomach. Proximal gastric tone decreases with swallowing (receptive
relaxation) and in response to gastric distension (gastric accommoda-
Serosa or visceral peritoneum tion). Its activity is modulated by enterogastric reflexes (e.g. acid,
protein or fat in the duodenum inhibits proximal gastric muscle con-


traction) and gut hormones (e.g. cholecystokinin, gastrin, glucagon and
The serosa is an extension of the visceral peritoneum. It covers the entire
vasoactive intestinal polypeptide). The vagus nerve plays a key role in
surface of the stomach other than along the attachments of the greater
these reflexes. In contrast to this activity, the muscle of the distal region
and lesser omenta to the greater and lesser curvatures, respectively,
of the stomach exhibits strong phasic contractions that increase in
where the peritoneal layers are separated by vessels and nerves, and over
amplitude towards the pylorus; these occur about three times per
a small posterosuperior area near the cardiac orifice, where the stomach
minute and act to grind the food mechanically and propel it towards
contacts the diaphragm at the reflections of the gastrophrenic and left
the pylorus. Solid material lags behind liquids, and when the contents
gastropancreatic folds.
of the antrum reach the contracted pylorus, only the liquid and small
particulate solids can be expelled into the duodenum; larger solid
Endoscopic mucosal resection
matter is retropulsed back into the stomach for further breakdown
(Patrick and Epstein 2008).
Endoscopic mucosal resection (EMR) is a technique that can be applied
in the oesophagus and stomach in order to excise dysplastic lesions or
early cancers that remain confined to the mucosa (Pech et al 2008).
Bonus e-book images
EMR is a misnomer since the technique involves resection of both the
mucosa and submucosa. The technique is inappropriate for tumours
that have invaded the submucosa since these are associated with a
significant risk of lymph node metastases and therefore require formal Fig. 64.13 The endoscopic appearance of oesophageal and gastric
surgical resection (oesophagectomy or gastrectomy) and lymphadenec- varices in portal hypertension.
tomy (Ancona 2008).
KEY REFERENCES
Ancona E, Rampado S, Cassaro M et al 2008 Prediction of lymph node status A study that demonstrated that reflux of gastric acid and bile have a


in superficial esophageal carcinoma. Ann Surg Oncol 15:3278–88. synergistic effect in the pathogenesis of Barrett’s oesophagus.
A study that reported that intramucosal cancers and those invading the
García RM, Tomás NP, Pozo CD et al 2013 Laparoscopic treatment of acute
upper third of the submucosa are associated with significantly fewer nodal
gastric volvulus. Cir Esp 91:189–93.
metastases than tumours invading deeper into the submucosa. This
A case series that included 10 patients with acute gastric volvulus treated by
demonstrated the potential for endoscopic treatment of superficial lesions
laparoscopic gastropexy. The technique was shown to be safe and effective in
and the need for oesophagectomy and lymphadenectomy where there is
the emergency setting.
invasion beyond the upper third of the submucosa.
Japanese Research Society for Gastric Cancer 1998 Japanese Classification
Dvorak K, Payne CM, Chavarria M et al 2007 Bile acids in combination with
of Gastric Carcinoma. Tokyo: Kanehara; Gastric Cancer 1:10–24,
low pH induce oxidative stress and oxidative DNA damage: relevance
25–30.
to the pathogenesis of Barrett’s oesophagus. Gut 56:763–71.


1123
46
RetpAhc
Key references
A detailed, widely accepted description of the lymph node fields of the upper Pech O, Behrens A, May A et al 2008 Long-term results and risk factor analy-
abdominal viscera, particularly in relation to malignancy. sis for recurrence after curative endoscopic therapy in 349 patients with
high-grade intraepithelial neoplasia and mucosal adenocarcinoma in
Mittal RK, Goyal RK 2006 Sphincter mechanisms at the lower end of the
Barrett’s oesophagus. Gut 57:1200–6.
esophagus. PART 1 Oral cavity, pharynx and esophagus. GI Motility
A paper that showed that endoscopic therapy (endoscopic resection and
online. doi:10.1038/gimo14.
photodynamic therapy) was safe and highly effective at treating patients
A review that discusses the principle anatomical mechanisms that prevent
with high-grade dysplasia and intramucosal (T1a) cancer associated with
reflux at the gastro-oesophageal junction and explains how failure of one or
Barrett’s oesophagus.
more of these may lead to progressive gastro-oesophageal reflux disease.
Silverstein FE, Tytgat GNJ 1991 Atlas of Gastrointestinal Endoscopy, 2nd ed.
Paquet KJ 2000 Causes and pathomechanisms of oesophageal varices devel- New York: Gower Medical Publishing.
opment. Med Sci Monit 6:915–28. A pictorial atlas that includes over 900 endoscopic images along the entire
An explanation of how chronic liver diseases cause a pathological increase length of the gastrointestinal tract. The accompanying text discusses normal
in portal pressure, leading to the development of portosystemic collaterals, and abnormal features and therapeutic techniques.
including oesophageal varies. This article reviews the pathogenesis, diagnosis
Yousef F, Cardwell C, Cantwell MM et al 2008 The incidence of esophageal
and treatment of oesophageal varices.
cancer and high-grade dysplasia in Barrett’s oesophagus: A systematic
Paterson WG 2001 The normal anti-reflux mechanism. Chest Surg Clin N review and meta-analysis. Am J Epidemiol 168:237–49.
Am 11:473–83. A systematic review and meta-analysis that evaluated the incidence of


An article that describes the normal mechanisms that prevent reflux of oesophageal cancer and high-grade dysplasia in Barrett’s oesophagus,
gastric contents into the oesophagus, including the resting tone of the lower concluded that the risk of neoplastic progression was lower than previously
oesophageal sphincter, the mechanical effect of the crural sling, and the described, and questioned the need for routine surveillance of all patients
specialized anatomy of the proximal stomach. with non-dysplastic Barrett’s oesophagus.


Abdominal oesophagus and stomach
1123.e1
46
RetpAhc
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upper third of the submucosa are associated with significantly fewer nodal
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metastases than tumours invading deeper into the submucosa. This
opment. Med Sci Monit 6:915–28.
demonstrated the potential for endoscopic treatment of superficial lesions
An explanation of how chronic liver diseases cause a pathological increase
and the need for oesophagectomy and lymphadenectomy where there is
in portal pressure, leading to the development of portosystemic collaterals,
invasion beyond the upper third of the submucosa.
including oesophageal varies. This article reviews the pathogenesis, diagnosis
Arul GS, Parikh D 2008 Oesophageal replacement in children. Ann R Coll and treatment of oesophageal varices.
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low pH induce oxidative stress and oxidative DNA damage: relevance sis for recurrence after curative endoscopic therapy in 349 patients with
to the pathogenesis of Barrett’s oesophagus. Gut 56:763–71. high-grade intraepithelial neoplasia and mucosal adenocarcinoma in
A study that demonstrated that reflux of gastric acid and bile have a Barrett’s oesophagus. Gut 57:1200–6.
synergistic effect in the pathogenesis of Barrett’s oesophagus. A paper that showed that endoscopic therapy (endoscopic resection and
photodynamic therapy) was safe and highly effective at treating patients
García RM, Tomás NP, Pozo CD et al 2013 Laparoscopic treatment of acute
with high-grade dysplasia and intramucosal (T1a) cancer associated with
gastric volvulus. Cir Esp 91:189–93.
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laparoscopic gastropexy. The technique was shown to be safe and effective in Piasecki C 1974 Blood supply to the human gastroduodenal mucosa with
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prevent reflux at the gastro-oesophageal junction and explains how failure of right colon: precise structure of the major veins and gastrocolic trunk
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gut – a gastroenterologist’s point of view. World J Gastroenterol 14:
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6285–8.
with non-dysplastic Barrett’s oesophagus.


CHAPTER
65
Small intestine
OVERVIEW
A
The small intestine consists of the duodenum, jejunum and ileum. It 1st
extends from the distal end of the pyloric canal to the ileocaecal junc-
tion and has a mean length of 5 metres (3–8.5 metres) when measured
intraoperatively in the living adult (Teitelbaum et al 2013). The duode-
num extends from the stomach to the duodenojejunal junction. The 2nd
remaining small intestine is often referred to as the ‘small bowel’, the
proximal two-fifths of which is referred to as the jejunum and the distal
three-fifths as the ileum. There is no clear boundary between the two
parts, but there is a gradual transition in morphology from the proximal 4th
to distal ends of the small bowel. The distal 30 cm or so of the ileum
is often referred to as the terminal ileum, which has some specialized
3rd
physiological functions.
The duodenum is in the upper abdomen and is mostly retroperito-
neal. The jejunum and ileum occupy the central and lower parts of the
abdominal cavity and usually lie within the boundary formed by the
colon. The small bowel is attached to the posterior abdominal wall by
B
a mesentery that allows the intestinal loops to be mobile. In the supine Neck of
position, loops of jejunum may be found anterior to the transverse gallbladder
colon, stomach and even lesser omentum, whereas in the erect position,
Quadrate lobe
loops of ileum may descend into the pelvis anterior to the rectum. The
greater omentum covers the upper jejunum and ileum to a variable Right end of the
extent. The jejunum and ileum are covered by visceral peritoneum on Hepatic gastrocolic omentum
flexure
all but their mesenteric borders, where the peritoneum is reflected to
enclose the adipose tissue of the mesentery. The small bowel mesentery Attachment of the
transverse mesocolon
abuts about 20% of the circumference of the muscular wall of the ileum
and somewhat less of the jejunum.
DUODENUM
Superior
mesenteric artery
The adult duodenum is approximately 25 cm long and is the shortest,


widest and most predictably placed part of the small intestine. The Superior
proximal 2.5 cm is intraperitoneal and the remainder is retroperitoneal. mesenteric vein
The duodenum forms an elongated ‘C’ that lies between the level of the
C
first and third lumbar vertebrae in the supine position. The lower ‘limb’
Common hepatic
of the C extends further to the left of the midline than the upper limb. duct
The head and uncinate process of the pancreas lie within the concavity Portal vein Gastroduodenal
of the duodenum, which is ‘draped’ over the prominence formed artery
by the lumbar spine; the duodenum therefore curves in an anteropos- Right kidney Left kidney
terior direction as well as forming a ‘C’. The duodenum lies entirely
above the level of the umbilicus. It is described as having four parts
(Fig. 65.1).
FIRST (SUPERIOR) PART
The first, and most mobile, part of the duodenum is about 5 cm long.
It starts at the duodenal end of the pylorus and ends at the superior
duodenal flexure. The proximal 2.5 cm is intraperitoneal while the
distal 2.5 cm is covered by peritoneum on its anterior and superior Left renal
surfaces and forms the inferior boundary of the epiploic foramen. vessels
The lesser omentum is attached to its upper border and the greater
omentum to its lower border. The first 2–3 cm of the duodenum is lined Right renal Left ureter
vessels
by relatively smooth mucosa and readily distends on insufflation during
Aorta
endoscopy. This part is frequently referred to as the duodenal ‘cap’. Right ureter
During contrast radiology, it shows a few longitudinal folds continuous Left gonadal
with the pylorus (Mather Cordiner and Calthrop 1936) and has a tri- Right gonadal vein artery
angular appearance; it is often visible on plain radiographs of the
Right gonadal artery
abdomen as an isolated triangular gas shadow to the right of the first
or second lumbar vertebra. The first part of the duodenum passes supe- Inferior vena cava


riorly, posteriorly and laterally for 5 cm before curving sharply inferi- Fig. 65.1 A, The four parts of the duodenum. B, Anterior relations of the
1124 orly at the superior duodenal flexure. It becomes more retroperitoneal duodenum. C, Posterior relations of the duodenum.


Duodenum
1125
56
RETPAHC
during this part of its course, until peritoneum only covers its anterior because the anterior surface of the first part is covered only by
aspect. The section from the duodenal cap to the superior duodenal peritoneum.
flexure lies posterior and inferior to the quadrate lobe of the liver. The common hepatic and hepatoduodenal lymph nodes lie close to
Beyond the duodenal cap, the internal appearance is characterized the first part of the duodenum (see Fig. 64.14B) and can be visualized
by circumferential mucosal folds that remain pronounced, even during using endoscopic ultrasound; this may be important in the staging of
endoscopic insufflation (Fig. 65.2). gastric, pancreatic or bile duct tumours. The proximity of the common
The first part of the duodenum lies anterior to the gastroduodenal bile duct to the first part of the duodenum allows endoscopic ultra-
artery, common bile duct and portal vein, and anterosuperior to the sound examination of the distal common bile duct and the formation
head and neck of the pancreas. The gastroduodenal artery lies immedi- of a surgical anastomosis between bile duct and duodenum (choledo-
ately behind the posterior wall of the duodenum; a penetrating peptic choduodenostomy) when required.
ulcer on the posterior wall may erode into the gastroduodenal artery The junction of the first and second parts of the duodenum lies
or one its branches and cause dramatic haemorrhage. A penetrating posterior to the neck of the gallbladder.
peptic ulcer on the anterior wall may perforate into the peritoneal cavity
SECOND (DESCENDING) PART
The second part of the duodenum is approximately 8 cm long. It starts
at the superior duodenal flexure and runs inferiorly in a gentle curve,
convex to the right side of the vertebral column and extending to the
lower border of the third lumbar vertebral body. It then turns sharply
medially at the inferior duodenal flexure, which marks its junction with
the third part of the duodenum. It is covered by peritoneum only on
its upper anterior surface, lies posterior to the gallbladder and the right


lobe of the liver at its start, and is crossed anteriorly by the transverse
colon. The right end of the gastrocolic omentum and the origin of the
transverse mesocolon are attached to the anterior surface of the duode-
num by loose connective tissue. Below the attachment of the transverse
mesocolon, the connective tissue and vessels forming the mesentery of
the upper ascending colon and hepatic flexure are loosely attached to
its anterior surface. This part of duodenum is at risk of injury during
surgical mobilization of the ascending colon and hepatic flexure.
The second part lies anterior to the hilum of the right kidney, the
right renal vessels, the lateral edge of the inferior vena cava and the right
psoas major (see Fig. 65.1C). The head of the pancreas and the common
bile duct are medial and the hepatic flexure is above and lateral. Part of
the pancreatic head is sometimes embedded in the medial duodenal
wall, and pancreatic ‘rests’ in the duodenal wall may produce small
filling defects on contrast radiology. The internal appearance is similar
to that of the distal portion of the first part of the duodenum, with
pronounced mucosal folds (Fig. 65.3). The common bile duct and
pancreatic duct enter the medial wall, where they usually unite to form
a common channel, which frequently contains a dilated segment known
as the hepatopancreatic ampulla (of Vater) (p. 1175). The narrow distal
end of this channel opens on the summit of the major duodenal papilla,
Duodenal cap Duodenojejunal flexure a mucosal elevation situated on the posteromedial wall of the second
Fig. 65.2 The contrast radiographic appearance of the duodenum part, 8–10 cm distal to the pylorus. A duodenal mucosal fold often
showing a distended duodenal cap and the remainder of the duodenum partially encircles the major papilla, forming a hood (Horiguchi and
up to the duodenojejunal flexure. Kamisawa 2010) (see Fig. 65.3). A second, accessory pancreatic duct is
Fig. 65.3 The endoscopic appearance of the
duodenum. A, Duodenal cap (first part). B, Second


part, showing the pronounced, ‘branched’
mucosal folds. C, Major duodenal papilla. D, Third
part.
A B
D
C


SmAll inTESTinE
1126
8
nOiTCES
sometimes present and opens about 2 cm proximal to the major papilla
on a minor duodenal papilla (Suda 2010, Kamisawa et al 2010). Gastroduodenal artery
Duodenal diverticula Supraduodenal artery
Posterior superior
pancreaticoduodenal
The duodenum is the most common site for a diverticulum in the small
artery
intestine. Diverticula are usually solitary and may be congenital (con-
taining all layers of the duodenal wall) or acquired (protrusion of the Anterior superior Right gastroepiploic artery
pancreaticoduodenal
mucosa and submucosa through a defect in the muscular coat of the
artery
bowel wall). They are typically located on the medial wall of the second
Communicating artery
part of the duodenum, intimately related to the head of the pancreas,
and the major duodenal papilla is frequently found either on the
Posterior inferior
mucosal fold at the mouth of a diverticulum or arising from the mucosa pancreaticoduodenal
within it. Diverticula may complicate interpretation of contrast radio- artery
graphs of the duodenum or biliary system, and may cause difficulties
Anterior inferior
during attempted endoscopic cannulation of the major duodenal
pancreaticoduodenal
papilla. Most are asymptomatic but they may be complicated by bleed-
artery
ing, inflammation, perforation and, occasionally, pancreatitis or biliary
Inferior pancreaticoduodenal artery
complications (Fotiades et al 2005). Duodenal branches
First jejunal artery
THIRD (HORIZONTAL) PART
Anterior vessels
Posterior vessels
The third part of the duodenum starts at the inferior duodenal flexure (behind pancreas or duodenum)
and is approximately 10 cm long. It runs from the right side of the lower
Fig. 65.4 The arterial supply of the duodenum. Only representative
border of the third lumbar vertebra, and passes to the left and slightly branches of the small vessels, which may be multiple, are shown.
superiorly, anterior to the inferior vena cava and abdominal aorta,


becoming continuous with the ascending fourth part (see Fig. 65.1C).
It lies posterior to the transverse mesocolon, and is crossed anteriorly
by the origin of the small bowel mesentery and the superior mesenteric
vessels. The lower portion of its anterior aspect is covered by perito- VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
neum, which is reflected anteriorly to form the posterior layer of the
root of the small bowel mesentery. The anterior surface of the left Arteries
end, close to the junction with the fourth part, is also covered by
The main vessels supplying the duodenum are the superior and inferior
peritoneum.
pancreaticoduodenal arteries. The first and second parts also receive
The third part lies anterior to the right ureter, right psoas major, right
contributions from other sources, including the right gastric, supraduo-
gonadal vessels, inferior vena cava and abdominal aorta (at the origin
denal, right gastroepiploic, hepatic and gastroduodenal arteries (Fig.
of the inferior mesenteric artery), and inferior to the head of the pan-
65.4). Branches of the superior pancreaticoduodenal artery may con-
creas. Anteroinferiorly, loops of jejunum lie in the right and left infra-
tribute to the supply of the pyloric canal, anastomosing to a minor
colic compartments. The mid portion of the third part lies in the angle
extent with the gastric arteries within the muscular layer of the
between the superior mesenteric artery anteriorly and the abdominal
pyloroduodenal junction.
aorta posteriorly; narrowing of this angle may occur from loss of
perivascular adipose tissue or spinal straightening and is a rare cause of Gastroduodenal artery
duodenal obstruction (Merrett et al 2009). The gastroduodenal artery usually arises from the common hepatic
artery behind or above the first part of the duodenum. It descends
behind the retroperitoneal portion of the first part of the duodenum to
FOURTH (ASCENDING) PART the left of the common bile duct. At the lower border of the first part
of the duodenum, it is commonly described as dividing into the right


The fourth part of the duodenum is 2.5 cm long. It starts just to the left gastroepiploic and superior pancreaticoduodenal arteries but this ana-
of the aorta, runs superiorly and laterally to the level of the upper tomical arrangement is rare (Bradley 1973, Bertelli et al 1995, Bertelli
border of the second lumbar vertebra, then turns sharply anteroinferi- et al 1996) and its usual branching pattern is as follows. As it descends
orly at the duodenojejunal flexure to become continuous with the behind the first part of the duodenum, it usually gives off the posterior
jejunum. The inferior mesenteric vein lies either posterior to the duo- superior pancreaticoduodenal artery, several retroduodenal branches
denojejunal flexure or at its lateral margin beneath a peritoneal fold. that supply the first part and proximal portion of the second part of the
The duodenojejunal flexure is a useful landmark to locate the vein duodenum, and a supraduodenal artery that supplies the anterosupe-
radiologically or surgically. The aorta, left sympathetic trunk, left psoas rior part of the proximal duodenum (Bianchi and Albanèse 1989). As
major, left renal and left gonadal vessels are all posterior relations, the gastroduodenal artery emerges below the first part of the duode-
the left kidney and left ureter are posterolateral, and the transverse num, it usually gives off the right gastroepiploic artery and several
colon and mesocolon are anterior, separating it from the stomach (see pyloric branches. It then descends on the anterior surface of the pan-
Fig. 62.8). The inferior border of the body of the pancreas is superior. creas, where it divides into the anterior superior pancreaticoduodenal
The peritoneum of the root of the small bowel mesentery descends over artery and pancreatic branches. Although the gastroduodenal artery
its anterior surface. usually branches from the common hepatic artery, it may occasionally
At its left lateral limit, the fourth part becomes progressively invested originate from other sources, including: as a trifurcation with the right


in peritoneum, such that the duodenojejunal flexure is suspended from and left hepatic arteries; the coeliac trunk; the superior mesenteric
the retroperitoneum by a double fold of peritoneum, the suspensory artery; or from the right or left branch of the hepatic artery.
ligament of the duodenum (or ligament of Treitz). The ligament of The gastroduodenal artery or one of its branches may be a source of
Treitz is in two parts; the first part may contain skeletal muscle fibres haemorrhage from a penetrating posterior duodenal ulcer (see above)
and runs from the right crus of the diaphragm to connective tissue or it may be the site of aneurysm or pseudoaneurysm formation; for
around the coeliac trunk, and the second part contains smooth muscle these reasons, it is an important vessel for interventional radiologists.
and descends from connective tissue around the coeliac trunk to the
duodenum, passing behind the pancreas anterior to the left renal vein. Superior pancreaticoduodenal arteries
The ligament is often absent or rudimentary in adults and its function There are usually two superior pancreaticoduodenal arteries: a posterior
is unknown (Kim et al 2008). The ligament of Treitz is avascular; the and anterior. The posterior superior pancreaticoduodenal artery is
vascular supply to the fourth part of the duodenum enters its wall from usually a separate branch of the gastroduodenal artery and is given off
the posteromedial aspect. behind the upper border of the first part of the duodenum. It descends
The duodenojejunal flexure is an important landmark in the radio- to the right, anterior to the portal vein and common bile duct, where
logical diagnosis of incomplete rotation and malrotation of the small the latter lies behind the first part of the duodenum. It then spirals
intestine (p. 1054). around the right side of the bile duct to run behind the head of the


Jejunum
1127
56
RETPAHC
pancreas, crosses posterior to the retropancreatic segment of the Lymphatic drainage
common bile duct (which is embedded, to a variable degree, in the
Duodenal lymphatics run to superior and inferior pancreaticoduodenal
head of the pancreas), and anastomoses with the posterior division of
lymph nodes, and from there to supra- and infrapyloric, hepatoduo-
the inferior pancreaticoduodenal artery (Bertelli et al 1996). The pos-
denal, common hepatic, coeliac and superior mesenteric nodes.
terior artery supplies branches to the head of the pancreas, the first and
Lymphatic drainage to para-aortic nodes has also been described (Hirai
second parts of the duodenum, and several branches to the lowest part
et al 2001).
of the common bile duct.
The anterior superior pancreaticoduodenal artery is usually a termi- Duodenal feeding
nal branch of the gastroduodenal artery and descends in the anterior
groove between the second part of the duodenum and the head of the
Available with the Gray’s Anatomy e-book
pancreas or on the anterior surface of the gland parallel to the groove
(Bertelli et al 1995). It supplies branches to the first and second parts
of the duodenum and to the head of the pancreas, and then passes
posteriorly to anastomose with the anterior division of the inferior INNERVATION
pancreaticoduodenal artery.
The duodenum is innervated by both parasympathetic and sympathetic
inferior pancreaticoduodenal artery neurones.
The inferior pancreaticoduodenal artery usually arises from the superior Preganglionic sympathetic neurones have their cell bodies in the
mesenteric artery or its first jejunal branch, near the superior border of intermediolateral columns of the grey matter in the fifth to the twelfth
the third part of the duodenum (Bertelli et al 1996). It crosses behind thoracic spinal segments. Their fibres travel via the greater and lesser
the superior mesenteric vein and passes behind the uncinate process splanchnic nerves to the coeliac plexus and synapse in the coeliac and


of the pancreas, where it divides into anterior and posterior branches. superior mesenteric ganglia; postganglionic axons are distributed to the
The anterior branch passes to the right, immediately inferior and then duodenal wall via peri-arterial plexuses on the branches of the coeliac
anterior to the lower border of the head of the pancreas, and runs trunk and superior mesenteric artery. The sympathetic nerves are vaso-
superiorly to anastomose with the anterior superior pancreaticoduode- constrictor to the duodenal vasculature and inhibitory to duodenal
nal artery. The posterior branch runs posteriorly to the right behind the musculature.
head of the pancreas, and anastomoses with the posterior superior The preganglionic parasympathetic supply is carried by vagal fibres
pancreaticoduodenal artery (Bertelli et al 1997). Both branches supply that travel from the coeliac plexus and synapse on neurones in the
the pancreatic head, its uncinate process, and the second and third parts duodenal wall. The parasympathetic supply is secretomotor to the duo-
of the duodenum. Occasionally, the anterior and posterior branches denal mucosa and motor to the duodenal musculature.
arise separately from the superior mesenteric or first jejunal artery.
Referred pain
Jejunal artery branches In common with other structures derived from the foregut, pain arising
Branches from the first jejunal branch of the superior mesenteric artery from the proximal duodenum is poorly localized and referred to the
supply the fourth part of the duodenum and frequently anastomose epigastrium. It is mediated by afferent fibres that accompany the sym-
with a terminal branch of the anterior superior pancreaticoduodenal pathetic neurones.
artery. The fourth part of the duodenum therefore receives a potential
collateral supply from the coeliac trunk and superior mesenteric artery,
which means that it is not commonly affected by ischaemia.
JEJUNUM
Veins
Submucosal and intramural veins give rise to small veins that accom- The jejunum has an external diameter of about 4 cm and an internal
pany corresponding named arteries. The venous anatomy of this region diameter of about 3 cm. It has a thicker wall than the ileum and a rich


is variable and not well characterized. The superior pancreaticoduode- arterial blood supply. The plicae circulares (see below) are most pro-
nal vein runs superiorly on the posterior surface of the head of the nounced in the proximal jejunum, where they are more numerous
pancreas, posterior to the distal common bile duct, and usually drains and deeper than elsewhere in the small bowel (Figs 65.5–65.6). They
into the portal vein. The inferior pancreaticoduodenal vein runs inferi- frequently ‘branch’ around the lumen and may appear to be stacked
orly and usually drains into the superior mesenteric vein or its first on top of each other, giving the jejunum a characteristic appearance
jejunal tributary. Small veins from the first and upper second parts of during single contrast radiography (see Fig. 65.6), computed tomo-
the duodenum drain directly into the portal vein, and veins from the graphic (CT) enterography (Fig. 65.7A) or magnetic resonance (MR)
third and fourth parts may drain directly into the superior mesenteric enterography (Fig. 65.7B). The plicae circulares are also clearly visible
vein. Numerous small anastomoses are present between veins of the by capsule endoscopy, in which images of the small bowel are transmit-
second and third parts of the duodenum and retroperitoneal veins ted wirelessly from a small swallowed camera that is the size of a pill
(Murakami et al 1999). (Fig. 65.8).
Fig. 65.5 Typical cross-sections through the
Mesentery proximal jejunum (A) and terminal ileum (B). The
Vasa recta mesenteric attachment is wider in the jejunum,
and two leaves of vessels enter the bowel wall.
The latter is also thicker in the jejunum.
Mucosa
Peyer’s patch
Plicae circulares
Submucosal
arterial plexus
Circular muscle
Longitudinal muscle
Serosa
A B


Small intestine
1127.e1
56
RETPAHC
When gastric emptying is functionally impaired – as, for example, in Superior mesenteric Superior mesenteric Stomach
gastroparesis, nutrition may be delivered directly into the duodenum vein artery
or jejunum via a nasal transpyloric feeding tube.
A
Ileum Jejunum
Stomach Transverse colon
B
Ileum Jejunum
Fig. 65.7 A, Computed tomographic (CT) enterography. Coronal slice
showing small intestinal loops and superior mesenteric vessels.
B, Magnetic resonance (MR) enterography. Coronal slice showing the
small intestine and transverse colon.


SmAll inTESTinE
1128
8
nOiTCES
In the supine position, the jejunum usually occupies the upper left Jejunal feeding In patients with a functioning intestine who cannot
infracolic compartment, extending down to the umbilical region. The tolerate adequate oral or intragastric feeding, jejunal feeding is often
first one or two loops often occupy a recess between the left part of the the preferred option. Compared to parenteral (intravenous) nutrition,
transverse mesocolon and the left kidney. On supine radiological exam- enteral feeding is associated with fewer complications. Furthermore, it
ination, the jejunal loops are characteristically situated in the upper maintains the integrity of the gut mucosa (thereby reducing bacterial
abdomen, to the left of the midline, whereas the ileal loops tend to lie translocation from the gut lumen), decreases the likelihood of aspira-
in the lower right part of the abdomen and pelvis. This distribution can tion from gastro-oesophageal reflux, and is less of a stimulus for pan-
be reversed in small bowel obstruction due to rotation of the dilated creaticobiliary secretion. Jejunal feeds can be delivered via a nasojejunal
bowel around its mesenteric attachment. or gastrojejunal tube, or directly into the jejunum; gastrojejunal and
jejunal tubes may be inserted endoscopically, radiologically or surgi-
cally. The end of the feeding tube must lie beyond the duodenojejunal
flexure to prevent reflux of feed into the duodenum and stomach.
ILEUM
The ileum has a median external diameter of about 3 cm, an internal
diameter of about 2.5 cm and tends to have a thinner wall than the
jejunum (see Fig. 65.5B). The plicae circulares become progressively less
obvious in the distal ileum; they tend to be single and flatter (see Figs
65.6–65.7; Fig. 65.9). The mucosa of the terminal ileum immediately
proximal to the ileocaecal junction may appear almost flat at endos-
CC JJ copy, although the villi can be seen when viewed close up (see Fig. 65.9)
JJ and at capsule endoscopy (see Fig. 65.8).


In the supine position, the ileum lies mainly in the hypogastric
II
region and right iliac fossa. The terminal ileum frequently lies in the
JJ
pelvis, from where it ascends over the right psoas major and right iliac
II vessels, to end by opening at the ileocaecal junction in the right iliac
II fossa.
ANATOMICAL DIFFERENCES BETWEEN
THE JEJUNUM AND ILEUM
A
While there is no clear boundary between the jejunum (the proximal
two-fifths of the small intestine beyond the duodenum) and the ileum
(the distal three-fifths), there are general anatomical differences between
these regions. The wall of the jejunum is thicker and more vascular, has
a greater number of more prominent plicae circulares, and contains less
CC
TTII
PPCC
PPCC
II
II
B
Fig. 65.6 Barium studies of the jejunum and ileum. A, Barium follow-
through. The feathery appearance of the small intestine is due to the
plicae circulares and is most prominent in the jejunum. The constrictions
(arrows) are the result of peristalsis. B, Small bowel enema (enteroclysis).
The plicae circulares are clearly demonstrated by this technique.
Abbreviations: C, caecum; I, ileum; J, jejunum; PC, plicae circulares; TI,
terminal part of ileum. Fig. 65.9 The endoscopic appearance of the terminal ileum.
A B C D
Fig. 65.8 The small intestine visualized by capsule endoscopy (PillCam®). A, The proximal jejunum at 1 h 34 mins. B, The distal jejunum at 1 h 51 min.
C, The proximal ileum at 2 h 25 min. D, The distal ileum at 3 h 34 min. Note the presence of plicae circulares and visible intestinal villi. (Courtesy of
Simon Gabe.)


Small intestine
1128.e1
56
RETPAHC
Fig. 65.12 A temporary loop ileostomy in an infant. The skin marking
around the stoma is from the stoma bag adhesive. (Courtesy of Professor
Mark Stringer, Christchurch Hospital, New Zealand.)
Fig. 65.11 A Meckel’s diverticulum containing ectopic mucosa at its tip.
(Courtesy of Professor Mark Stringer, Christchurch Hospital, New
Zealand.)
The mean length of the small bowel, measured from the duodenojeju- Small bowel transplantation has become a standard clinical procedure
nal flexure to the ileocaecal junction in vivo, is about 5 m but can range for selected patients with intestinal failure. The small intestine may be
from 3 to 8.5 m (Teitelbaum et al 2013). Males have a longer small transplanted in isolation or along with the liver and/or other abdomi-
bowel than females, and height is positively associated with small nal organs such as the pancreas (multivisceral transplant). Postopera-
bowel length (Teitelbaum et al 2013). Short bowel syndrome results tively, patients are recommenced on parenteral nutrition, and enteral
from surgical resection, congenital deficiency or disease-associated loss feeding is started via a feeding jejunostomy once intestinal motility has
of absorption, and is characterized by the inability to maintain protein- returned. Protein is delivered as peptides and fats as medium-chain
energy, fluid, electrolyte or micronutrient balances when on a conven- triglycerides, which are absorbed directly into the mesenteric veins and
tionally accepted, normal diet (O’Keefe et al 2006). Left untreated, not via the lymphatics (which are divided when the graft is taken from
short bowel syndrome leads to dehydration, malnutrition and weight the donor). Parenteral nutrition can usually be discontinued 3–6 weeks
loss. Adults with the greatest risk of developing intestinal failure fall after transplantation but may continue to be used to supplement enteral
into three groups: those with an end-jejunostomy and <100 cm of feeding when stoma losses are large.
residual small bowel, those with a jejunocolic anastomosis and <50 cm Intestinal dysmotility is common after intestinal transplantation.


residual small bowel, and those with a jejunoileal anastomosis and There is no extrinsic nerve supply to the graft but intrinsic small bowel
<35 cm of residual small bowel (despite the presence of an ileocaecal motility usually recovers within 48–72 hours after transplantation
junction and colon) (Nightingale and Woodward 2006, Jeppesen (Gunning and Friend 1998). Adequate gastric emptying is often delayed
2013). for several weeks, despite pyloroplasty and the administration of drugs
After surgical resection, the remaining small bowel undergoes an to enhance gastric motility. Reinnervation of the allograft via its vascular
adaptive process that involves morphological and functional changes. pedicle is a slow process (Walther et al 2013). Intestinal motility is also
The small bowel dilates and villus height and crypt depth increase, affected by any inflammatory response caused by rejection. Denervation
expanding the absorptive surface area. Adaptation begins soon after and division of the lymphatic drainage of the intestinal allograft do not
intestinal resection and may continue for up to 2 years (Jeppesen and appear to seriously impair longer-term function. In animal models,
Mortensen 2002). However, there are differences between the ability lymphatic drainage is re-established within 21–28 days, which is con-
of the proximal and distal small bowel to adapt; the likelihood of sistent with observations in patients (Kocandrle et al 1966).
regaining intestinal autonomy is greatest in patients with a retained
segment of ileum and colon in continuity, as compared to patients
with a residual duodenojejunal segment and an end-jejunostomy
(Jeppesen 2013).


ileum
1129
56
RETPAHC
A B Fig. 65.10 Specimens of the jejunum (A) and
ileum (B) from a cadaver where the superior
mesenteric artery was injected with red-coloured
gelatin before fixation. Subsequently, the
specimens were dehydrated and then cleared in
benzene, followed by methyl salicylate. The largest
vessels present are the jejunal and ileal branches
of the superior mesenteric artery and these are
succeeded by anastomotic arterial arcades, which
are relatively few in number (1–3) in the jejunum,
becoming more numerous (2–6) in the ileum. From
the arcades, straight arteries (arteriae recta) pass
towards the gut wall; frequently, successive
straight arteries are distributed to opposite sides
of the gut. Note the denser vascularity of the
jejunal wall. (Specimens prepared by MCE
Hutchinson; photographs by Kevin Fitzpatrick on
behalf of GKT School of Medicine, London.)
lymphoid tissue than the ileum. There are also differences between the Short bowel syndrome
mesenteric vessels in the jejunum and ileum (Conley et al 2010). The
jejunal mesentery, measured from the superior mesenteric artery to Available with the Gray’s Anatomy e-book
the mesenteric border of the bowel, is shorter than the ileal mesentery,
and the jejunal arteries are slightly larger than their ileal counterparts.
Small intestinal transplantation
The jejunum typically contains 1–3 tiers of vascular arcades, whereas
there are often 2–6 tiers in the ileum (Fig. 65.10). The arteriae recta in
Available with the Gray’s Anatomy e-book
the ileum are more numerous, shorter and narrower than in the
jejunum. The jejunal and ileal arteries, arcades and arteriae recta
are muscular arteries capable of influencing splanchnic blood flow,
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
which can vary between 10% and 35% of cardiac output (Rosenblum
et al 1997).
Solitary lymphoid follicles are scattered throughout the small intes- Arteries


tinal mucosa but are most numerous in the distal ileum. Aggregated Branches from the superior mesenteric artery supply the jejunum and
lymphoid follicles, Peyer’s patches, are circular or oval masses contain- ileum. The arteries divide as they approach the mesenteric border of
ing 5–260 follicles. They vary in size, shape and distribution, most the intestine (see Fig. 65.5), giving off numerous branches that extend
measuring 2–8 cm and visible macroscopically as dome-like elevations, between the muscular layers before forming a submucosal arterial
usually along the antimesenteric border of the intestine. They are rarely plexus that supplies the mucosa. Although there is a rich anastomotic
present in the duodenum; small, circular, few in number and impalpa- network of arteries within the intestinal mesentery, anastomoses
ble in the distal jejunum; and larger, more numerous and often palpa- between the terminal branches close to the intestinal wall are few. The
ble in the ileum (particularly in the terminal 25 cm; Van Kruiningen intramural and submucosal arterial networks consist of small-calibre
et al 2002). Villi are small or absent over the larger follicular masses. vessels only. Consequently, division or occlusion of several consecutive
Lymphoid aggregates are most prominent in early childhood and, when vasa recta may produce segmental ischaemia of the bowel, while divi-
enlarged in viral infections, may form the apex of an intussusception. sion of more proximal arterial branches in the small bowel mesentery
They become less prominent around puberty, and decrease further in may not cause ischaemia because of collateral flow through vascular
number during adult life (Cornes 1965). arcades.
Meckel’s diverticulum A congenital ileal diverticulum (of Meckel) Superior mesenteric artery
is found in 2–3% of individuals and represents the remnant of the The superior mesenteric artery originates from the abdominal aorta
proximal part of the vitellointestinal duct. It projects from the antime- 1 cm below the coeliac trunk, at the level of the lower border of the


senteric border of the terminal ileum and is commonly located between first lumbar vertebra in the transpyloric plane (Figs 65.13–65.14). The
50 and 100 cm from the ileocaecal junction. It is variable in length angle of its origin from the aorta is acute (mean value 45°, range
(usually 2–5 cm in adults) and often possesses a short ‘mesentery’ of 38–60° and greater in individuals with a greater body mass index;
adipose tissue containing a vitellointestinal artery (from the termina- Ozkurt et al 2007); this can make cannulation via the transfemoral
tion of the superior mesenteric artery) that extends from the ileal route somewhat difficult. The artery is usually surrounded by fat, lym-
mesentery to its base (Fig. 65.11). The lumen of the diverticulum phatics and neural tissue at its origin, which helps to increase the angle
usually has a calibre similar to that of the ileum. The tip is normally and distance between it and the aorta, thereby preventing compression
free but occasionally it may be connected to the anterior abdominal of the duodenum where it is crossed by the artery. The artery descends
wall near the umbilicus by a fibrous band. The mucosa is typically ileal, anterior to the uncinate process of the pancreas and the third part of
but small heterotopic areas of gastric body type epithelium, pancreatic, the duodenum, and posterior to the splenic vein and the body of the
colonic or other tissues may also occur in the wall of a diverticulum. pancreas. The left renal vein lies behind it and separates it from the
Unopposed acid secretion by heterotopic gastric body type epithelium aorta (Fig. 65.15B). Within the small bowel mesentery, the superior
may give rise to ulceration and bleeding in the adjacent normal ileal mesenteric artery crosses anterior to the inferior vena cava, right ureter
mucosa. Diverticular inflammation may mimic acute appendicitis; and right psoas major. Its calibre progressively decreases as successive
since Meckel’s diverticulum and the appendix are both derived from branches are given off to the jejunum and ileum, and its terminal


midgut structures, pain from either structure is referred to the perium- branch anastomoses with the termination of the ileocolic artery.
bilical region. Rarer complications of a Meckel’s diverticulum include The superior mesenteric artery usually gives off the inferior pancrea-
intestinal obstruction, intussusception, perforation, calculi and tumours ticoduodenal, middle colic, right colic and ileocolic branches from its
(Sagar et al 2006, Uppal et al 2011). right side, and jejunal and ileal branches from its left side. Its jejunal
and ileal branches form vascular arcades within the small bowel mesen-
Jejunostomy and ileostomy In clinical practice, a stoma is a tery. The last of these arcades forms an irregular and incomplete ‘mar-
surgically created opening from a hollow viscus to the skin, classified ginal artery’ of the small intestine. Straight arteries, the arteriae recta,
according to its location. An intestinal stoma may be either an end are given off from the most distal arcades and pass directly to the small
stoma, in which only the proximal end of the divided bowel is anasto- intestine.
mosed to the skin, or a loop stoma, in which both the proximal and Anatomical variations in the origin and branching pattern of the
distal ends of the bowel are exteriorized on the surface (Fig. 65.12). superior mesenteric artery are well described (Winston et al 2007,
The output of a jejunostomy is greater than that of an ileostomy and Horton and Fishman 2010) (see also p. 1144). It may be the source of
more likely to result in excessive fluid, electrolyte and nutrient losses. the common hepatic, gastroduodenal, accessory or replaced right


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nOiTCES
Middle colic artery Fig. 65.13 The superior mesenteric artery and its
branches. The outlines of representative ileal and
Right branch Middle branch Left branch Inferior pancreaticoduodenal artery
jejunal loops, appendix, caecum, ascending and
transverse colon are shown for reference. Only the
origin of jejunal and ileal branches is shown. For
details of arcades, see Figure 65.10.
Right colic artery
Jejunal branches
Ileocolic artery
Superior division
Inferior division
Anterior Ileal branches
caecal branch
Posterior
caecal branch
Appendicular artery
Terminal ileal branch
hepatic, accessory pancreatic, splenic or rarely the inferior mesenteric ileum, caecum and vermiform appendix. It ascends in the mesentery to
artery. Also rare is a superior mesenteric artery arising from a common the right of the superior mesenteric artery, passing anterior to the right
coeliacomesenteric trunk (Rountas et al 2013). ureter, inferior vena cava, third part of the duodenum and uncinate
The remnant of the vitellointestinal artery (the embryonic artery that process of the pancreas. It joins the splenic vein behind the neck of the
originally connected the intestinal circulation to the yolk sac) is usually pancreas in the transpyloric plane (lower border of L1 vertebra) to form
obliterated; when present, it forms the artery supplying a Meckel’s the portal vein.
diverticulum. It is occasionally represented in the mesentery by a fibrous The superior mesenteric vein receives jejunal, ileal, ileocolic, right
strand from the termination of the superior mesenteric artery to the colic, middle colic, right gastroepiploic and inferior pancreaticoduode-
ileum. nal veins. A major proximal jejunal branch usually runs transversely
behind the superior mesenteric artery to enter the right posterolateral
Jejunal branches There are usually 4–6 jejunal branches, which aspect of the superior mesenteric vein (Kim et al 2007). The right colic


arise from the left side of the upper portion of the superior mesenteric vein is highly variable. When present, it may drain into the superior
artery (see Figs 65.13–65.14). They are distributed to the jejunum via mesenteric vein directly, or may join the right gastroepiploic or inferior
1–3 tiers of arterial arcades, the most distal of which gives rise to straight pancreaticoduodenal vein to form a ‘gastrocolic trunk’, which then
arteries. The latter run almost parallel in the mesentery before being drains into the superior mesenteric vein (Yamaguchi et al 2002).
distributed alternately to either side of the small bowel, forming two Although the inferior mesenteric vein usually drains into the splenic
distinct ‘leaves’ of vessels within the mesentery separated by a relatively vein, it may join the superior mesenteric vein directly or its confluence
avascular plane (see Fig. 65.5A). with the splenic vein (Graf et al 1997).
This vascular arrangement allows a dilated segment of small bowel
Lymphatic drainage
to be bisected longitudinally and tubularized to double its length, a
potentially useful technique to achieve small bowel lengthening in The lymphatic system of the small intestine regulates tissue fluid homeo-
short bowel syndrome (Bianchi 1984). stasis, participates in immune surveillance, and transports dietary fat
Small twigs from the jejunal arteries supply regional mesenteric and fat-soluble vitamins from the gut lumen. It is organized into two
lymph nodes. networks. Firstly, lacteals from the villi drain into a plexus of lymphatics
in the submucosa and are joined by vessels from lymph spaces at the
Ileal branches Ileal branches are more numerous (around 8–12) bases of solitary lymphoid follicles; these lymphatic vessels have few, if
and slightly smaller in calibre than the jejunal branches. They arise from any, valves. Secondly, a coarse plexus of lymphatics also runs in the
the left and anterior aspects of the superior mesenteric artery. The length muscularis externa between the two muscle layers. The submucosal and
of the mesentery from the superior mesenteric artery to the mesenteric muscular networks share few connections but both communicate freely


border of the bowel is greater in the ileum, and the branches form with larger valved collecting lymphatics at the mesenteric border of the
between two and six arcades before giving rise to multiple straight arter- small intestine (Miller et al 2010). Mesenteric lymphatics pass between
ies that run directly towards the ileal wall (see Fig. 65.10B). These the layers of the mesentery and drain via a series of mesenteric lymph
branches run parallel in the mesentery and are distributed to both nodes concentrated around the regional mesenteric vessels. Individual
aspects of the ileum. They are shorter and thinner than their jejunal segments of small bowel have a relatively wide field of lymphatic drain-
counterparts, particularly in the distal ileum, and do not form such age, which makes radical surgical resection of draining lymph nodes
distinct parallel ‘leaves’ of vessels. The terminal ileal arcades are sup- difficult if the blood supply to the remaining unaffected small bowel is
plied by the ileal branch of the ileocolic artery and the last ileal branch to be preserved. Mesenteric nodes drain into superior mesenteric nodes
of the superior mesenteric artery (see Fig. 65.13). Few other vessels around the root of the superior mesenteric artery.
connect the ileocolic and superior mesenteric artery territories, which
makes surgical dissection of the ileocolic artery up to its origin relatively
simple. INNERVATION
Veins
The jejunum and ileum are innervated by parasympathetic and sympa-
Superior mesenteric vein thetic fibres via the superior mesenteric plexus (see Figs 59.2–59.4).
The superior mesenteric vein drains the small intestine, caecum, ascend- Preganglionic sympathetic axons originate from neurones in the
ing and transverse parts of the colon, and parts of the stomach and intermediolateral grey matter of the mid-thoracic spinal segments and
greater omentum (Fig. 65.16; see Fig. 59.8). It is formed in the mesen- travel in the greater and lesser splanchnic nerves to the coeliac and
tery of the small bowel by the union of tributaries from the terminal superior mesenteric ganglia, where they synapse. Postganglionic axons


Small intestine physiology
1131
56
RETPAHC
Superior mesenteric artery
A
A
Coeliac trunk
Splenic vein Superior mesenteric artery
Superior mesenteric artery Jejunal branches
B
B
Ileocolic artery Ileal branches
Inferior vena cava Abdominal aorta Left renal vein
Fig. 65.15 A, An ultrasound image through the origin of the superior
mesenteric artery, sagittal plane. B, An axial CT scan in an adult
demonstrating the relationship of the superior mesenteric artery to the left
renal vein. (Courtesy of Professor Mark Stringer, Christchurch Hospital,
New Zealand.) (C, continued online)
accompany the superior mesenteric artery into the mesentery and are
distributed along branches of the artery. The sympathetic nerves are
vasoconstrictor to the vasculature and inhibitory to the musculature of
the jejunum and ileum. Sympathetic neurotransmitters also have an
immunomodulatory role by influencing mucosa-associated lymphoid
tissue (Straub et al 2006). Preganglionic parasympathetic axons travel
in the vagus nerves and are secretomotor to the mucosa and motor
to the smooth muscle of the jejunum and ileum. Visceral afferents from
the small bowel, conveying pain and other gut sensations, travel with
the splanchnic and vagus nerves.
C
Referred pain
Fig. 65.14 The superior mesenteric artery and its branches. A, A digital In common with other structures derived from the midgut, the visceral
subtraction angiogram. B, A surface-shaded, volume-rendered CT sensation of pain arising from the jejunum or ileum is poorly local-
angiogram. C, A sagittal reformat of a multislice CT angiogram. ized, and is usually referred to the periumbilical region.
(A, Courtesy of Dr Adam Mitchell, Charing Cross Hospital, London.
B, Courtesy of Dr Nasir Khan, Chelsea and Westminster Hospital,
SMALL INTESTINE PHYSIOLOGY
London. C, Courtesy of GE Worldwide Medical Systems.)
The principal functions of the small intestine are fluid and electrolyte
haemostasis, digestion and absorption of nutrients, immunoregulation,
and secretion of hormones. The absorptive capacity of the small


Small intestine
1131.e1
56
RETPAHC
Falciform ligament Splenic vein Superior mesenteric artery
C
Inferior vena cava Abdominal aorta Left renal vein
Fig. 65.15 C, An axial ultrasound image showing the superior mesenteric
artery and the splenic vein.


SmAll inTESTinE
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nOiTCES
Right hepatic Portal Retrohepatic inferior Left hepatic Splenic
vein vein vena cava vein vein
Circular folds
Fig. 65.17 The internal aspect of a representative sample of the proximal
jejunum, showing circular folds.
V
P
V
SM
P
IIeocolic vein Superior mesenteric vein Ileal vein Jejunal vein
Fig. 65.16 The superior mesenteric vein and its branches, CT venogram,
coronal plane. (Courtesy of Dr Nasir Khan, Chelsea and Westminster
Hospital, London.)
intestine is greatly enhanced by the plicae circulares, villi and microvilli,
Fig. 65.18 A low-power micrograph showing several circular folds
which increase its surface area by factors of approximately 2, 7 and 13,
(arrows) in the wall of the ileum. The folds are covered with villi (V)
respectively (Helander and Fändriks 2014), yielding a total estimated
projecting into the lumen, and the submucosa (SM) extends into the core
surface area of almost 30 m2.
of each fold. Circular (innermost) and longitudinal smooth muscle layers
form the underlying muscularis externa. Large masses of lymphoid tissue
(Peyer’s patches, P) lie in the mucosa. (Courtesy of Mr Peter Helliwell and
FLUIDS AND ELECTROLYTES
the late Dr Joseph Mathew, Department of Histopathology, Royal
Cornwall Hospitals Trust, UK.)
Available with the Gray’s Anatomy e-book
duodenum (see Fig. 59.9). The submucosa contains aggregates of lym-
NUTRIENTS
phoid tissue, most numerous in the ileum.
Circular folds
Available with the Gray’s Anatomy e-book
Except in the first part of the duodenum, large circular folds of mucosa
(known as plicae circulares or valvulae conniventes) project into the
MOTILITY
lumen of the small intestine, orientated either transversely or slightly
obliquely to its long axis (see Figs. 65.8, 65.17; Fig 65.18). Unlike
gastric folds, they do not disappear during physiological distension of
Available with the Gray’s Anatomy e-book


the intestine. Most extend round half or two-thirds of the luminal cir-
cumference, some are complete circles, some bifurcate and join adjacent
folds, while others are spiral and extend one or more times round the
MICROSTRUCTURE
lumen. Larger folds are up to 8 mm deep but most are smaller than
this, and larger folds often alternate with smaller ones. Folds begin to
The intestinal wall is composed of mucosa, submucosa, muscularis appear 2.5–5 cm beyond the pylorus and are relatively large and close
externa and serosa or adventitia (see Fig. 59.9). The mucosa is thick and together in the distal duodenum and proximal jejunum. Beyond this
very vascular in the proximal small intestine, but thinner and less vas- point, they diminish in size and disappear almost completely in the
cular distally. In places, it is ridged by the underlying submucosa to terminal ileum, which therefore has a relatively thin wall. The circular
form circular folds, plicae circulares, which protrude into the lumen; folds increase the absorptive surface area and enhance mechanical seg-
mucosal finger- or leaf-like intestinal villi cover the whole surface (Fig. mentation in the small intestine. They are visible at endoscopy (see Fig.
65.17). There are numerous simple, tubular intestinal glands or crypts 65.3), during small bowel contrast studies (see Fig. 65.6), and some-
between the bases of the villi, and additional submucosal glands in the times on plain radiographs.


Small intestine
1132.e1
56
RETPAHC
The secretion and absorption of electrolytes and fluid are two essential muscle of the small intestine exhibits cyclical myoelectrical activity, the
functions of the small intestinal epithelium. In healthy adults, the so-called migrating motor complex (MMC). The frequency of these
gastrointestinal tract is capable of secreting 8–10 litres of fluid per day slow waves is regulated by interstitial cells of Cajal and is about 11/
in addition to the 1.5–2 litres ingested each day. To reduce this volume minute in the jejunum, decreasing along the length of the small intes-
to the typical ileocaecal daily flow of approximately 2 litres per day, tine to 7–8/minute in the ileum (Husebye 1999). When slow waves are
the small intestinal epithelium has a large number of transport mole- amplified beyond a threshold by neural, endocrine or paracrine stimu-
cules and regulatory proteins. Net fluid movement across the gastro- lation, action potentials are generated, resulting in smooth muscle
intestinal epithelium is primarily the result of the active transport of contraction. This spreads circumferentially at first and then propagates
Na+, Cl−, and HCO− ions. The jejunum can only absorb sodium across along the entire circumference of the bowel in an aboral direction,
3
a small sodium gradient and therefore does not normally absorb salt causing a ring-like contraction of the circular smooth muscle that
and water, but secretes it into the lumen. Most fluid and electrolyte progresses down the small bowel (with about 10% of contractions
absorption occurs in the ileum, which is able to absorb across a larger reaching the ileum).
sodium gradient (Nightingale and Spiller 2001). The MMC consists of three phases: phase I is a period of quiescence,
phase II is composed of irregular contractions, and phase III is a short
The mucosal brush border has an abundance of enzymes and pumps burst of phasic contractions at the maximum frequency of the slow
on its surface. Examples include the sodium-dependent glucose trans- waves for that part of the intestine. The velocity of migration is 5–10 cm/
porter protein (SGLT1) and the fructose transporter protein (GLUT5). minute in the jejunum, decreasing gradually along the small bowel to


Most carbohydrates are broken down by enzymes to monosaccharides about 1 cm/minute in the ileum. The MMC requires an intact enteric
before being transported across the mucosa. A deficiency of lactase on nervous system and does not depend on extrinsic innervation. Conse-
the mucosal brush border is relatively common and results in lactose quently, MMCs are present in the transplanted (denervated) small
intolerance. Proteins are broken down into single amino acids and bowel. However, there is evidence that extrinsic innervation modulates
small peptides. The peptidase gradient across the microvillus brush the MMC. For example, sleep causes the almost complete disappearance
border increases along the length of the small bowel, suggesting that of phase II. Phase III activity normally recovers in the proximal small
amino acids are absorbed more distally than carbohydrates. Dietary fats bowel within a few hours of major abdominal surgery but the pressure
are emulsified by biliary secretions and then broken down by pancreatic wave amplitude is temporarily reduced. Somatostatin and opioids
lipase into free fatty acids and phospholipids, which form micelles. The decrease phase II activity.
lipid components are then absorbed within the distal small bowel and After a meal, the cyclical fasting activity is abolished for several hours
converted into chylomicrons in the enterocytes before being secreted and replaced by irregular contractions that consist of both segmental
into the lymphatic system. Minerals such as calcium, magnesium, phos- ‘stationary’ contractions and propulsive contractions (Barnert 2007).
phorus and iron, and the water- and fat-soluble vitamins, are predomi- Assessment of small bowel motility can complement detailed micro-
nantly absorbed in the duodenum and proximal jejunum (Farrell 2002, scopic examination of full-thickness biopsy specimens of the intestine
Malik and Westergaard 2002). Vitamin B is absorbed in the terminal when characterizing conditions associated with intestinal dysmotility,
12
ileum after binding to intrinsic factor released by gastric parietal cells. helping to distinguish intestinal neuropathy from visceral myopathy.
The motility of the small intestine differs, depending on whether the
individual is fasting or postprandial. In the fasting state, the smooth


microstructure
1133
56
RETPAHC
the epithelium against pancreatic enzymes in the intestinal lumen. The
L
cell coat also contains a number of digestive enzymes as integral mem-
brane proteins. These include enzymes that degrade disaccharides and
oligopeptides prior to absorption.
IC
The luminal surface is an important barrier to diffusion. Nutrients
generally have to pass through enterocytes (transcellular absorption)
V before they reach the underlying lamina propria and its blood vessels
and lymphatics (lacteals). Classical epithelial junctional complexes
encircle the apical plasma membranes of adjacent enterocytes; their
tight junctions form an effective barrier to non-selective diffusion
between the gut lumen and the body as a whole. The lateral plasma
B membranes of enterocytes are highly folded, interdigitating with each
B other to form complex intercellular boundaries, anchored periodically
by desmosomes, and making contact at gap junctions. The lateral inter-
cellular space expands during active absorption and is an additional
conduit (supplementing transport across the basal cell surface) for the
passage of fluids, nutrients and other solutes to the vessels of the lamina
propria.
ME
Enterocytes have a lifespan after differentiation of about 5 days.
Their position on the villus wall reflects their stage in the life cycle; at
ME the tips of intestinal villi, they undergo programmed, apoptotic, cell
death and are shed from the epithelium. They are replaced at the base
of the villus by stem cell mitosis.
Goblet cells Goblet cells are most numerous in the distal small
Fig. 65.19 A low-power micrograph showing the wall of the duodenum,
intestine, increasing in number from the duodenum to their highest
with villi (V) projecting into the lumen (L); intestinal crypts (IC) of
density in the terminal ileum. They have elongated basal nuclei, an
Lieberkühn in the mucosa, seen mainly in transverse section; muscularis
apical region containing many membrane-bound mucinogen granules,
mucosae (arrows); submucosal seromucous (Brunner’s) glands (B); and


muscularis externa (ME). (Courtesy of Mr Peter Helliwell and the late and apical surfaces that bear a few short microvilli. Goblet cell mucins
Dr Joseph Mathew, Department of Histopathology, Royal Cornwall contribute to protection against microorganisms and toxins in the gut
Hospitals Trust, UK.) lumen, and also provide lubrication and mechanical protection from
the intestinal contents.
Microfold (M) cells Microfold cells are present where the epithe-
lium covers lymphoid aggregates in the intestinal wall. They are cuboi-
Intestinal villi
dal or flattened in shape and have long, widely spaced microfolds rather
than microvilli on their apical surfaces. They sample luminal antigens
Intestinal villi are highly vascular projections of the mucosal surface, by endocytosis, and transport antigen to lymphocytes lying within
just visible to the naked eye (see Fig. 65.18; Figs 65.19–65.21; Com- intercellular pockets formed by deep invaginations of the M-cell baso-
mentary 8.2). They cover the entire small intestinal mucosa, increase lateral plasma membranes. See Chapter 4 for details of antigen process-
the surface area of the lumen about seven-fold (Helander and Fändriks ing and presentation.
2014), and give it a velvety texture. Villi are large and numerous in the
duodenum and jejunum, and smaller and fewer in the ileum. In the Lymphocytes Intraepithelial lymphocytes are found in close asso-
first part of the duodenum, they appear as broad ridges, become tall ciation with M cells and also between the basolateral regions of entero-
and foliate in the distal duodenum and proximal jejunum, and then cytes and goblet cells. They are migratory cells derived from the
gradually shorten to a finger-like form in the distal jejunum and ileum. underlying lymphoid tissue and constitute an important means of
Villi vary in density from 10 to 40 per mm2 and from 0.5 to 1.0 mm in immune defence.
height (see Fig. 65.19). Infoldings of the mucosa dip down from the
base of the villi for a short distance into the lamina propria. These crypts intestinal glands or crypts


(of Lieberkühn) are most prominent in the proximal small intestine Intestinal glands or crypts (of Lieberkühn) are tubular pits that open
(see Fig. 65.20). into the lumen throughout the intestinal mucosa via small circular
apertures between the bases of the villi (see Fig. 65.20). Their thin walls
Mucosa are composed of columnar enterocytes supplemented by mucous
cells, Paneth cells, stem cells and neuroendocrine cells. They are sepa-
The mucosa consists of epithelium, lamina propria and muscularis rated by a basal lamina from a rich capillary plexus within the lamina
mucosae. propria.
Epithelium Enterocytes Enterocytes in the crypts secrete ions and alkaline fluid
A single-layered epithelium covers the intestinal villi (see Figs 65.20, to dilute chyme and aid absorption by structurally similar cells that
2.2C) and also lines the intestinal glands (crypts) that open between cover the villi.
the bases of villi. Two types of cell, enterocytes and goblet cells, cover
the surfaces of the villi, whereas microfold cells (M cells) are restricted Mucous cells The mucous cells in the crypts are similar to the goblet
to the dome epithelium that covers localized accumulations of lym- cells of the villi.
phoid tissues. These cell types are all in contact basally with a basal
lamina to which they adhere, and all are derived from a common stem Paneth cells Paneth cells are highly specialized epithelial cells of
cell in the intestinal crypts. the small intestine, involved in the coordination of many physiological
functions. They were first identified more than a century ago on the
Enterocytes Enterocytes are columnar absorptive cells, approxi- basis of their readily discernible secretory acidophilic granules that stain
mately 20 µm tall. They are the most numerous type of cell in the small strongly with eosin or phosphotungstic haematoxylin. Paneth cells are
intestinal lining, and are responsible for nutrient absorption. Their numerous in the deeper parts of the intestinal crypts. They synthesize
surfaces bear up to 3000 microvilli, which greatly increase the surface and secrete lysozyme, a highly specific antibacterial enzyme, other


area for absorption (Marsh and Swift 1969). Collectively, microvilli are defensive proteins (defensins) and tumour necrosis factor alpha (TNF-
visible by light microscopy as a brush border 1 µm thick; individual α), which help to protect the luminal surface. Recent studies have
microvilli can be resolved only by electron microscopy. Enterocyte shown that these antimicrobial molecules are key mediators of host–
nuclei are elongated vertically, mainly euchromatic and located just microbe interactions, including maintaining the homeostatic balance
below the centre of the cell. with colonizing microbiota and providing innate immune protection
The apical cell surface is resistant to protease attack because micro- from enteric pathogens (Clevers and Bevins 2013). In addition, Paneth
villi possess a specialized glycoprotein-rich surface coat (glycocalyx), cells secrete factors that help sustain and modulate epithelial stem and
which, with an overlying layer of mucus (Johansson et al 2011), protects progenitor cells in the crypts.


SmAll inTESTinE
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nOiTCES
Direction of cell migration
Absorptive
enterocyte
Goblet cell
Central lymphatic
vessel (lacteal)
within the
lamina propria
Orifices of crypts of Lieberkühn
Fibroblasts
Epithelium Lymphoid follicle
Lamina
propria
Mucosa
Muscularis
mucosae
Submucosa
Circular muscle
Muscularis externa
Longitudinal muscle
Serosa
Fig. 65.20 The architecture of an intestinal villus. The various layers and cells are not drawn to scale.
Stem cells Stem cells occur in a zone just above the basal region of
the crypts. Lineage tracing has demonstrated that Lgr5hi crypt base
columnar stem cells generate all cell types of the small intestinal epi-
thelium (Barker et al 2007). These cells divide rapidly in the crypt
compartment directly above the Paneth cells. Their progeny (transit-
amplifying cells) move upwards from this compartment on to the
surfaces of the villi while differentiating into goblet cells, tuft cells,
neuroendocrine cells and enterocytes. An individual cell takes only
4–5 days to reach the villus tip and undergo apoptosis. Paneth cells
escape this upward flow and migrate downwards instead to settle at the
base of the crypt, where they can remain for one month or more
(Clevers and Bevins 2013). Stem cell daughter cells and young transit-
amplifying cells retain their plasticity and are able to revert back to stem
cells if the crypt is damaged and existing stem cells are lost.
Neuroendocrine cells Several types of neuroendocrine cell are
scattered among the walls of the intestinal crypts, and less commonly
over the villi. They secrete bioactive peptides, such as gastrin, chole-
cystokinin and secretin, basally into the surrounding lamina propria.
Crypt neuroendocrine cells are derived from stem cells, which also give
Fig. 65.21 The endoscopic appearance of the terminal villi (magnified). rise to enterocytes and other epithelial elements.


1135
56
RETPAHC
Key references
Lamina propria the duodenojejunal junction. They are small, branched tubuloacinar
glands; each has several secretory acini lined by low columnar epithelial
The lamina propria is composed of connective tissue and provides
cells, which produce an alkaline (pH 9) mucoid secretion that effec-
mechanical support for the epithelium. It has a rich vascular plexus,
tively neutralizes acidic chyme from the stomach. Many neuroendo-
receives nutrients absorbed by the enterocytes, and forms the cores of
crine cells are present among the acinar cells.
the villi. It contains lymphoid tissue, fibroblasts and connective tissue
extracellular matrix fibres, smooth muscle cells, eosinophils, macro-
Muscularis externa
phages, mast cells, capillaries, lymphatic vessels and unmyelinated
nerve fibres. Plasma cells are numerous. Lymphocytes may be clustered
in solitary or aggregated follicles, some of which extend through the The muscularis externa consists of a thin, external, longitudinal layer
muscularis mucosae into the submucosa. and a thick, internal, circular layer of smooth muscle cells. It is thicker
in the proximal small intestine and is mainly responsible for the dif-
Villus core ferential appearance of proximal jejunum and distal ileum on CT
Each villus has a core of delicate connective tissue that contains a large, scanning.
blind-ending lymphatic vessel or lacteal (so called because of its content
of suspended chylomicrons, the droplets of apoprotein–lipid complex Interstitial cells of Cajal
elaborated by enterocytes from absorbed dietary fats). The core also
contains blood vessels, nerves and smooth muscle cells derived from
Interstitial cells of Cajal (ICC; p. 1043) are found throughout the entire
fine extensions of the muscularis mucosae. Each lacteal, usually single
length of the gastrointestinal tract, where they lie in close contact with
but occasionally double, starts in a closed dilated extremity near the tip
nerve terminals (Farrugia 2008). They originate from mesenchymal
of a villus, and extends through the core to the base of the villus, where
cells and may be considered to be specialized smooth muscle cells.


it joins a narrower lymphatic plexus in the deeper lamina propria. Its
However, whereas smooth muscle cells develop an extensive array of
wall is a single layer of endothelial cells. Smooth muscle cells surround
contractile elements, ICCs are fusiform cells with a large oval nucleus,
the lacteal throughout the villus and their contraction propels its con-
sparse cytoplasm, dendritic-like processes, few contractile elements, an
tents into the underlying lymphatic plexus. Capillaries within the core
abundance of endoplasmic reticulum and mitochondria, and charac-
are lined by fenestrated endothelium to facilitate the rapid intake of
teristic sets of channels in their membrane (Mostafa et al 2010). Distinct
nutrients diffusing from the covering absorptive epithelium.
networks are found in the myenteric plexus between the circular and
longitudinal muscle layers; ICCs contact each other and neighbouring
mucosa-associated lymphoid tissue
smooth muscle cells via gap junctions. Looser arrangements exist within
Mucosa-associated lymphoid tissue (MALT) consists of lymphoid fol-
the individual muscle layers and the submucosa of the gut. Isolated or
licles covered by follicle-associated intestinal epithelium, which includes
small groups of ICCs are also found in connective tissue septa and the
scattered M cells. MALT is found mainly in the lamina propria but
subserosal region. The cells are involved in the generation of pacemaker
sometimes expands into the submucosa; it is the source of B and T
signals, the propagation of electrical slow wave activity, neuromuscular
lymphocytes and other related cells involved in the immune defence of
transmission, and mechanosensation (Sanders et al 2014). Defective
the gut wall.
ICC function has been implicated in a wide range of intestinal motility
Like other masses of MALT (except lymph nodes), solitary and aggre-
disorders (Al-Shboul 2013).
gated lymphoid follicles are most prominent around the age of puberty,
after which they diminish in number and size, although many persist
Serosa
into old age. For further details of intestinal MALT and its function,
including Peyer’s patches, see Didierlaurent et al (2005).


Serosa is visceral peritoneum and consists of a subserous stratum of
Muscularis mucosae loose connective tissue covered by mesothelium. It covers the majority
The muscularis mucosae forms the base of the mucosa, and has external of the muscularis externa; the only exceptions are the part adjacent to
longitudinal and internal circular layers of smooth muscle cells. It mesenteric adipose tissue and the retroperitoneal portions of the duo-
follows the surface profiles of the circular folds and sends fine fascicles denum, both of which are covered mainly by a connective tissue adven-
of smooth muscle cells into the cores of the villi. titia rather than by serosa.
Submucosa Bonus e-book images
The submucosa is composed of loose connective tissue and contains
blood vessels, lymphatics and nerves. Its ridged elevations form the Fig. 65.7 A, Computed tomographic (CT) enterography. Coronal
cores of the circular folds. The geometry of its collagen and elastin slice showing small intestinal loops and superior mesenteric
fibres permits the considerable changes in transverse and longitudinal vessels. B, Magnetic resonance (MR) enterography. Coronal slice
dimensions that accompany peristalsis, whilst still providing adequate showing the small intestine and transverse colon.
support, elasticity and strength.
Fig. 65.11 A Meckel’s diverticulum containing ectopic mucosa at
Submucosal (Brunner’s) glands
its tip.
Submucosal glands are limited to the submucosa of the duodenum and
often referred to as ‘Brunner’s glands’ (see Fig. 65.19). Their ducts Fig. 65.12 A temporary loop ileostomy in an infant.
traverse the muscularis mucosae to enter the bases of the mucosal
crypts. They are largest and most numerous near the pylorus and form Fig. 65.15C An axial ultrasound image showing the superior
an almost complete layer in the proximal half of the descending duo- mesenteric artery and the splenic vein.
denum; thereafter, they gradually diminish in number and disappear at
KEY REFERENCES
Sanders KM, Ward SM, Koh SD 2014 Interstitial cells: regulators of smooth Teitelbaum EN, Vaziri K, Zettervall S et al 2013 Intraoperative small bowel


muscle function. Physiol Rev 94:859–907. length measurements and analysis of demographic predictors of
A review of the structural, functional and molecular features of interstitial increased length. Clin Anat Mar 20. doi: 10.1002/ca.22238.
cells of Cajal and their importance in smooth muscle function. Information on small bowel length in living adults, which varies with sex,
height and age.