ageing, cellular senescence, cancer and apoptosis
25
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Genetic mutations in integrins or integrin regulators have been asso- smooth muscle cells, in the intercalated discs of cardiac muscle cells
ciated with Glanzmann’s thrombasthenia (caused by mutations in and between glial cells and neurones. The junctions involve cadherins
integrin β3 subunit), the immunodeficiency disorder leukocyte adhe- attached indirectly to actin filaments on the inner side of the
sion deficiency types I and III (determined by mutations in integrin β2 membrane.
subunit and kindlin 3, respectively) and skin diseases (caused by muta-
tions in kindlin 1 and integrin α2, α6 and β3 subunits). Integrins are Desmosomes (maculae adherentes)
essential in the homing process, following the selectin phase, and are Desmosomes are limited, plaque-like areas of particularly strong inter-
also involved in tumour progression and metastasis. cellular contact. In epithelial cells, they may be located subjacent to the
tight junction and zonula adherens belts, forming collectively the epi-
Specialized intercellular junctions thelial apical junctional complex (see Fig. 1.19A). The intercellular gap
Specialized cell–cell junctions are the hallmark of all epithelial tissues. is approximately 25 nm; it is filled with electron-dense filamentous
There are two major categories: symmetric junctions and asymmetric material (the intercellular cadherins) running transversely across it and
junctions. Symmetric junctions may be subdivided into three types: is also marked by a series of densely staining bands (the cytoplasmic
tight junctions (also known as occluding junctions or zonulae occlu- dense plaques) running parallel to the cell surfaces. Adhesion is medi-
dentes); anchoring junctions (including zonulae adherentes, or belt ated by Ca2+-dependent cadherins, desmogleins and desmocollins.
desmosomes, and maculae adherentes, or spot desmosomes); and com- Within the cells on either side, each cytoplasmic dense plaque underlies


munication junctions, represented by gap junctions. Tight junctions and the plasma membrane and consists of the proteins plakophilin, desmo-
anchoring junctions are components of the epithelial apical junctional plakin and plakoglobin (γ-catenin), into which the ends of intermedi-
complex. Hemidesmosomes are asymmetric junctions (see Fig. 1.19). ate filaments are inserted. The type of intermediate filament depends
on the cell type, e.g. keratins are found in epithelia and desmin fila-
Tight junctions (occluding junctions, ments are found in cardiac muscle cells. Desmosomes form strong
zonulae occludentes) anchorage points, likened to spot-welds, between cells subject to
Tight junctions are the most apical component of the epithelial apical mechanical stress, e.g. in the prickle cell layer of the epidermis, where
junctional complex. The main functions of tight junctions are the regu- they are extremely numerous and large.
lation of the paracellular permeability of the epithelial layer and the
Hemidesmosomes
formation of an apical–basolateral intramembrane diffusion barrier,
the hallmark of epithelial cell polarity. Tight junctions form a continu- Hemidesmosomes are asymmetric anchoring junctions found between
ous belt (zonula) around the cell perimeter, near the apical domain of the basal side of epithelial cells and the associated basal lamina.
epithelial cells, and are connected to the actin cytoskeleton. At the site The latter is a component of the basement membrane and contains
of the tight junction, the plasma membranes of adjacent cells come into laminin, an integrin ligand. The other component of the basement
close contact, so that the space between them is obliterated. Freeze- membrane is the reticular lamina, a collagen-containing layer produced
fracture electron microscopy shows that the contact between these by fibroblasts that also contains fibronectin, another integrin ligand.
membranes is represented by branching and anastomosing sealing Hemidesmosomes resemble a single-sided desmosome, anchored on
strands of protein particles on the P (protoplasmic) face of the lipid one side to the plasma membrane, and on the other to the basal lamina


bilayer (Fig. 1.19A,B). A tight junction contains numerous proteins: and adjacent collagen fibrils (Fig. 1.19C). The plaque has distinct pro-
occludins and claudins, members of the tetraspanin family of proteins, teins not seen in the plaques of a zonula adherens or a macula adher-
containing four transmembrane domains, two loops and two cytoplas- ens: BPAG1 (bullous pemphigoid antigen 1), a member of the plakin
mic tails – occludins and tetraspanins provide the molecular basis for family, and BPAG2 (bullous pemphigoid antigen 2), which possesses
the formation of the branching and anastomosing strands seen in an extracellular collagenous domain. BPAG1 and BPAG2 were initially
freeze-fracture preparations; the afadin–nectin complex and junctional detected in patients with bullous pemphigoid, an autoimmune blister-
adhesion molecules (JAMs), each forming cis-homodimers and interact- ing disease. On the cytoplasmic side of the dense plaque there is a less
ing with each other through their extracellular domains (forming trans- dense plate into which keratin filaments are inserted, where they inter-
homodimers) – nectins and JAMs are members of the immunoglobulin act with the protein plectin associated with integrin α6β4. Hemidesmo-
superfamily, and the afadin component of the afadin–nectin complex somes use integrins and anchoring filaments (laminin 5) as their
interacts with F-actin; and cytosolic zonula occludens proteins 1, 2 and adhesion molecules anchored to the basal lamina, whereas desmo-
3 (ZO-1, ZO-2 and ZO-3). ZO-1 protein is associated with afadin and somes use cadherins.
the intracellular domain of JAMs. All three ZO proteins facilitate the
Focal adhesion plaques
reciprocal interaction of occludins, claudins and JAMs with F-actin.
Defects in paracellular magnesium permeability and reabsorption in Less highly structured attachments with a similar arrangement exist
kidneys occur when there is a mutation in claudin 16 and claudin 19 between many other cell types and their surrounding matrices, e.g.


(renal magnesium wasting). For further reading on claudins, see between smooth muscle cells and their matrix fibrils, and between the
Escudero-Esparza et al (2011). For further reading on JAMs, see Bazzoni ends of skeletal muscle cells and tendon fibres. The smaller, punctate
(2003). adhesions resemble focal adhesion plaques, which are regions of local
attachment between cells and the extracellular matrix. They are typically
anchoring junctions situated at or near the ends of actin filament bundles (stress fibres),
In contrast to tight junctions, zonulae adherentes and maculae adher- anchored through intermediary proteins to the cytoplasmic domains of
entes are characterized by the presence, along the cytosolic sides of the integrins. In turn, these are attached at their external ends to collagen
plasma membranes of adjacent epithelial cells, of symmetric dense or other filamentous structures in the extracellular matrix. They are
plaques connected to each other across the intercellular space by cad- usually short-lived; their formation and subsequent disruption are part
herins. They differ in that F-actin is associated with plaques in zonulae of the motile behaviour of migratory cells. See Geiger et al (2009) for
adherentes and intermediate filaments are linked to plaques in maculae further reading on focal adhesions.
adherentes.
Gap junctions (communicating junctions)
Zonula adherens (belt desmosome) Gap junctions resemble tight junctions in transverse section, but the
A zonula adherens is a continuous belt-like zone of adhesion parallel two apposed lipid bilayers are separated by an apparent gap of 3 nm,
and just basal to a tight junction and also encircling the apical perimeter which is bridged by a cluster of transmembrane channels (connexons).
of epithelial cells. Ca2+-dependent cell adhesion molecules (members Each connexon is formed by a ring of six connexin proteins whose
of the desmoglein and desmocollin families of cadherins) are key com- external surfaces meet those of the adjacent cell in the middle. A minute
ponents of a zonula adherens. In addition to the cadherin–catenin central pore links one cell to the next (Fig. 1.19D). Larger assemblies


complex, a zonula adherens also houses the afadin–nectin complex. of many thousands of channels are often packed in hexagonal arrays.
A specific component of a zonula adherens is a cytoplasmic dense Gap junctions occur between numerous cells, including hepatocytes
plaque attached to the cytosolic side of the plasma membrane. It con- and cardiac myocytes.
sists of desmoplakin, plakophilin and plakoglobin proteins (the latter
is also known as γ-catenin). A similar plaque is seen in a macula adhe-
AGEING, CELLULAR SENESCENCE, CANCER
rens or spot desmosome (see below).
AND APOPTOSIS
Fascia adherens
A fascia adherens is similar to a zonula adherens, but is more Ageing is a universal feature of biological organisms, defined by a
limited in extent and forms a strip or patch of adhesion, e.g. between gradual decline over time in cell and tissue function that often, but not


Basic structure and function of cells
25.e1
1
RETPaHC
Essentially, two molecules, cadherins and afadin, link to the actin
cytoskeleton. In cultured cells, nectins appear to initiate the formation
of a zonula adherens before the involvement of cadherins.


BaSIC STRuCTuRE aNd fuNCTION Of CEllS
26
1
NOITCES
always, decreases the longevity of an individual. The hallmarks of A Granzyme B pathway B FasL pathway
ageing are reviewed in López-Otín et al (2013).
Fas ligand
Cellular senescence is defined by an irreversible arrest in cell prolif- Granzyme B
eration when cells experience DNA damage at telomeres and a decrease
in mitogenic signalling. In contrast to reversibly arrested quiescent cells Death receptors
in G of the cell cycle, senescent growth arrest is irreversible; cells in this
0 Perforin
state cannot be stimulated to proliferate by known stimuli and cannot
be prompted to re-enter the cell cycle by physiological mechanisms. For Active caspase 8
further reading on senescence and the cell cycle, see Chandler and Peters FADD
(2013). Senescent cells can cause or foster degenerative diseases. In old Granzyme B Caspase 8
age, cellular senescence in humans determines typical pathologies, in-
BIDD
cluding atherosclerosis leading to stroke, osteoporosis, macular degen-
eration, cardiopulmonary and renal failure, and neurodegenerative
diseases such as Alzheimer’s and Parkinson’s disease. BAX–BAK
Senescent cells undergo changes in gene expression, which result in channels
the secretion of proinflammatory cytokines, growth factors and pro-
Mitochondrion
teases, activities that collectively define a senescence-associated secre-
tory phenotype capable of triggering angiogenesis, inflammatory
responses, stem cell renewal and differentiation, and which may also
determine resistance to cancer chemotherapy. Senescent cells can be C Cytochrome c pathway
Caspase 3
identified histochemically by their expression of either senescence-
Cytochrome c
associated β-galactosidase, a lysosomal marker which is overexpressed
in these cells, or the tumour suppressor protein p16INK4a, which pro- Apoptosome Caspase 7
motes the formation of senescence-associated chromatin. For further Caspase 9
reading on ageing, cellular senescence and cancer, see Campisi (2013).
Cellular senescence can be caused by a disruption of metabolic Caspase 3


signalling pathways, derived from mitogens and proliferation factors,
and the activation of tumour suppressors, combined with telomere
shortening and genomic damage. See Sahin and DePinho (2012) for Caspase 6 Caspase 2
further reading.
Cellular senescence suppresses tumorigenesis because cell prolifera-
tion is required for cancer development. However, senescent cells can Caspase 8 Caspase 10
stimulate the proliferation and malignant progression of adjacent pre-
malignant cells by the release of senescence-inducing oncogenic stimuli.
Cancer cells must harbour mutations to prevent telomere-dependent Fig . 1 .20 Caspase activation pathways during apoptosis . A, The
granzyme B extrinsic pathway activates caspase 8 and caspase 3
and oncogene-induced senescence, such as in the p53 and p16-
following entry of granzyme B across the plasma membrane pore-forming
retinoblastoma protein pathways. See López-Otín et al (2013) for
protein, perforin . This pathway is observed in cytotoxic T cells or natural
further reading on the pathogenesis of ageing.
killer cells for delivery of the protease granzyme B to target cells . B, The
Fas ligand (FasL) extrinsic pathway is initiated by binding of FasL to
Apoptosis clustered transmembrane death receptors that recruit adaptor proteins,
such as the Fas-associated death-domain protein (FADD) to their
intracellular domain, which in turn recruits and aggregates caspase 8
Cells die as a result of either tissue injury (necrosis) or the internal molecules, which become activated . Activated caspase 8 activates
activation of a ‘suicide’ programme (apoptosis) in response to extrinsic caspase 7 and caspase 3 . C, The cytochrome c intrinsic pathway starts
or intrinsic cues. Apoptosis (programmed cell death) is defined by the when granzyme B or activated caspase 8 causes the truncation by
controlled demolition of cellular constituents and the ultimate uptake proteolysis of the protein BIDD (BH3-interacting domain death agonist),
of apoptotic cell fragments by other cells to prevent immune responses. which penetrates a mitochondrion through BAX–BAK (BCL-2 associated


Some senescent cells become resistant to cell-death signalling, i.e. they X protein–BCL-2 antagonist killer) channel proteins on the outer
are apoptosis-resistant. In effect, senescence blocks growth of damaged mitochondrial membrane, causing the release of cytochrome c.
or stressed cells, whereas apoptosis quickly disposes of them. Apoptosis Cytochrome c enables the assembly of the apoptosome (consisting of
is a central mechanism controlling multicellular development. During seven molecules of apoptosis protease-activating factor-1 (APAF1) and
morphogenesis, apoptosis mediates activities such as the separation of seven molecules of caspase 9), which in turn activates caspase 3 and
the developing digits, and plays an important role in regulating the caspase 7 . Finally, the proteolytic activation cascade of caspase 6,
number of neurones in the nervous system (the majority of neurones caspase 2, caspase 8 and caspase 10 executes cell deconstruction .
die during development). Apoptosis also ensures that inappropriate
or inefficient T cells are eliminated in the thymus during clonal
enter the cell. The intrinsic mitochondrial route involves the release of
selection.
cytochrome c from the space between the inner and outer mitochon-
The morphological changes exhibited by necrotic cells are very dif-
drial membranes into the cytosol. Extrinsic and intrinsic pathways
ferent from those seen in apoptotic cells. Necrotic cells swell and sub-
work cooperatively in the subsequent activation of a family of initiator-
sequently rupture, and the resulting debris may induce an inflammatory
effector proteases, known as caspases (cysteine aspartic acid-specific
response. Apoptotic cells shrink, their nuclei and chromosomes frag-
proteases), which are present in healthy cells as inactive precursor
ment, forming apoptotic bodies, and their plasma membranes undergo
enzymes or zymogens. Activation of caspases 3, 6 and 7 mediates apop-
conformational changes that act as a signal to local phagocytes. The
tosis by initiating a cascade of degradative processes that target major
dead cells are removed rapidly, and as their intracellular contents are
constituents of the cell cytoskeleton, producing membrane blebbing, a
not released into the extracellular environment, inflammatory reactions


distinctive feature of apoptosis caused by cytosolic and nuclear frag-
are avoided; the apoptotic fragments also stimulate macrophages to
ments flowing into the developing apoptotic bodies. Caspase cleavage
release anti-inflammatory cytokines.
inactivates many systems that normally promote damage repair and
Apoptosis and cell proliferation are intimately coupled; several cell
support cell viability, and activates a number of proteins that promote
cycle regulators can influence both cell division and apoptosis. The
the death and disassembly of the cell. For further reading on apoptosis,
signals that trigger apoptosis include withdrawal of survival factors or
see Taylor et al (2008).
exposure to inappropriate proliferative stimuli. Three main routes to
the induction of apoptosis have been established (Fig. 1.20). Two, the
Fas ligand (FasL) pathway and the granzyme B pathway, are extrinsic,
Bonus e-book video
whereas the mitochondrial route is intrinsic. The Fas ligand (FasL)
pathway involves binding of FasL to death receptors on the plasma
membrane and recruitment of adaptor proteins, such as the Fas-associ-
ated death domain proteins, followed by the recruitment and activation Video 1 .1 Mitosis in a cell with fluorescently-labelled chromosomes
of caspase 8. The granzyme B pathway involves creation of a perforin and microtubules .
plasma membrane channel enabling the caspase-like granzyme B to


Basic structure and function of cells
26.e1
1
RETPaHC
The ends of the chromosomes, or telomeres, become shorter and
more dysfunctional with each DNA replication round. Telomere short-
ening has been shown to activate DNA damage responses, leading to
mitochondrial dysfunction (a decrease in production of ATP and an
increase in reactive oxygen species) and the activation of p53, which
induces growth arrest, apoptosis and senescence of stem cells and pro-
genitor cells. p53 interconnects with different longevity metabolic sig-
nalling pathways, including the insulin, insulin-like growth factor I
(IGFI) and mammalian target of rapamycin (mTOR) pathways, which
are known to regulate lifespan by increasing the expression of genes
involved in stress resistance and energy balance. Mutations in TERC (the
RNA component of telomerase) and TERT (the catalytic component of
telomerase) are found in patients with the premature ageing syndrome,
dyskeratosis congenita (poor growth of fingernails and toenails, skin
pigmentation and oral leukoplakia). Other important contributors to
cell senescence are dysregulated autophagy and lack of disposal of
misfolded proteins by the ubiquitin–26S proteasome machinery.
These responses are collectively designated telomere-initiated cellular
senescence.


27
1
RETPaHC
Key references
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defects in human disease, with a specific focus on the mechanisms mutant studies focused on keratin diversity required for cells to attune to
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CHAPTER
2
Integrating cells into tissues
Cells evolved as single, free-living organisms, but natural selection basal lamina, which is synthesized predominantly by the epithelial
favoured more complex communities of cells, multicellular organisms, cells. The basal lamina is described on page 34.
where groups of cells specialize during development to carry out specific Epithelia can usually regenerate when injured. Indeed, many epithe-
functions for the body as a whole. This allowed the emergence of larger lia continuously replace their cells to offset cell loss caused by mechani-
organisms with greater control over their internal environment and the cal abrasion (reviewed in Blanpain et al (2007)). Blood vessels do not
evolution of highly specialized organic structures such as the brain. penetrate typical epithelia and so cells receive their nutrition by diffu-
The human body contains more than 200 different cell types, sharing sion from capillaries of neighbouring connective tissues. This arrange-
the same genome but with different patterns of gene expression. ment limits the maximum thickness of living epithelial cell layers.
Some cells in the body are essentially migratory, but most exist as Epithelia, together with their supporting connective tissue, can often be
cellular aggregates in which individual cells carry out similar or closely removed surgically as one layer, which is collectively known as a mem-
related functions in a coordinated manner. These aggregates are termed brane. Where the surface of a membrane is moistened by mucous
tissues, and can be classified into a fairly small number of broad catego- glands it is called a mucous membrane or mucosa, whereas a similar
ries on the basis of their structure, function and molecular properties. layer of connective tissue covered by mesothelium is called a serous
On the basis of their structure, most tissues are divided into four major membrane or serosa.
types: epithelia, connective or supporting tissue, muscle and nervous
tissue. Epithelia are continuous layers of cells with little intercellular
space, which cover or line surfaces, or have been so derived. In connec- CLASSIFICATION
tive tissues, the cells are embedded in an extracellular matrix, which,
typically, forms a substantial and important component of the tissue. Epithelia can be classified as unilaminar (single-layered, simple), in


Muscle consists largely of specialized contractile cells. Nervous tissue which a single layer of cells rests on a basal lamina; or multilaminar,
consists of cells specialized for conducting and transmitting electrical in which the layer is more than one cell thick. The latter includes:
and chemical signals and the cells that support this activity. stratified squamous epithelia, in which flattened superficial cells are
There is molecular evidence that this structure-based scheme of clas- constantly replaced from the basal layers; urothelium (transitional epi-
sification has validity. Thus the intermediate filament proteins charac- thelium), which serves special functions in the urinary tract; and other
teristic of all epithelia are keratins (Pan et al 2012); those of connective multilaminar epithelia such as those lining the largest ducts of some
tissue are vimentins; those of muscle are desmins; and those of nervous exocrine glands, which, like urothelium, are replaced only very slowly
tissue are neurofilament and glial fibrillary acidic proteins. However, under normal conditions. Seminiferous epithelium is a specialized
cells such as myofibroblasts, neuroepithelial sensory receptors and multilaminar tissue found only in the testis.
ependymal cells of the central nervous system have features of more
than one tissue type. Despite its anomalies, the scheme is useful for Unilaminar (simple) epithelia
descriptive purposes; it is widely used and will be adopted here.
In this chapter, two of the major tissue categories, epithelia and
Unilaminar epithelia are further classified according to the shape of
general connective and supporting tissues, will be described. Special-
their cells, into squamous, cuboidal, columnar and pseudostratified
ized skeletal connective tissues, i.e. cartilage and bone, together with
types. Cell shape may, in some cases, be related to cell volume. Where
skeletal muscle, are described in detail in Chapter 5 as part of the mus-
little cytoplasm is present, there are generally few organelles and there-
culoskeletal system overview. Smooth muscle and cardiac muscle are
fore there is low metabolic activity and cells are squamous or low
described in Chapter 6. Nervous system tissues are described in Chapter
cuboidal. Highly active cells, e.g. secretory epithelia, contain abundant


3. Specialized defensive cells, which also form a migrant population
mitochondria and endoplasmic reticulum, and are typically tall cuboi-
within the general connective tissues, are considered in more detail in
dal or columnar. Unilaminar epithelia can also be subdivided into
Chapter 4, with blood, lymphoid tissues and haemopoiesis.
those that have special functions, such as those with cilia, numerous
microvilli, secretory vacuoles (in mucous and serous glandular cells) or
EPITHELIA sensory features. Myoepithelial cells, which are contractile, are found as
isolated cells associated with glandular structures, e.g. salivary and
mammary glands.
The term epithelium is applied to the layer or layers of cells that cover
the body surfaces or line the body cavities that open on to it. The fate Squamous epithelium
of embryonic epithelial populations is illustrated in Figure 12.3. Epi- Simple squamous epithelium is composed of flattened, tightly apposed,
thelia function generally as selective barriers that facilitate, or inhibit, polygonal cells (squames). This type of epithelium is described as tes-
the passage of substances across the surfaces they cover. In addition, sellated when the cells have complex, interlocking borders rather than
they may: protect underlying tissues against dehydration, chemical or straight boundaries. The cytoplasm may in places be only 0.1 µm thick
mechanical damage; synthesize and secrete products into the spaces and the nucleus usually bulges into the overlying space (Fig. 2.2A).
that they line; and function as sensory surfaces. In this respect, many These cells line the alveoli of the lungs, where their surface area is huge
features of nervous tissue can be regarded as those of a modified and cytoplasmic volume correspondingly large, and they also form the
epithelium and the two tissue types share an origin in embryonic outer capsular wall of renal corpuscles, the thin segments of the renal
ectoderm. tubules and various parts of the inner ear. Because it is so thin, simple
Epithelia (Fig. 2.1) are predominantly cellular and the little extracel- squamous epithelium allows rapid diffusion of gases and water across


lular material they possess is limited to the basal lamina. Intercellular its surface; it may also engage in active transport, as indicated by the
junctions, which are usually numerous, maintain the mechanical cohe- presence of numerous endocytic vesicles in these cells. Tight junctions
siveness of the epithelial sheet and contribute to its barrier functions. (occluding junctions, zonulae adherentes) between adjacent cells
A series of three intercellular junctions forms a typical epithelial junc- ensure that materials pass primarily through cells, rather than between
tional complex: in sequence from the apical surface, this consists of a them.
tight junctional zone, an adherent (intermediate) junctional zone and
Cuboidal and columnar epithelia
a region of discrete desmosome junctions. Epithelial cell shape is most
usually polygonal and partly determined by cytoplasmic features such Cuboidal and columnar epithelia consist of regular rows of cylindrical
as secretory granules. The basal surface of an epithelium lies in contact cells (Figs 2.2B, C). Cuboidal cells are approximately square in vertical
with a thin layer of filamentous protein and proteoglycan termed the section, whereas columnar cells are taller than their diameter, and both


Epithelia
29
2
RETPAHC
UNILAMINAR (SIMPLE) MULTILAMINAR
Squamous Stratified squamous
See also:
Mesothelium – lining body cavities
Endothelium – lining blood and lymphatic vessels
Non-keratinizing Keratinizing
Cuboidal Specializations Stratified cuboidal/columnar
Ciliated Secretory
Columnar Urothelial (transitional)
Without surface With microvilli Pseudostratified Relaxed Stretched
specialization (brush/striated border)
COMPLEX DERIVED STRUCTURES
• Multicellular – exocrine and endocrine glands • Nervous tissue – often classified separately, but retains many
• Sensory structures – e.g. taste buds characteristics of its epithelial origins
• Tooth germ • Seminiferous epithelium
Fig. 2.1 Classification of epithelial tissues and cells.
are polygonal when sectioned horizontally. Commonly, microvilli are Sensory epithelia
found on their free surfaces, which considerably increases the absorp-
Sensory epithelia are found in special sense organs of the olfactory,
tive area, e.g. in the epithelia of the small intestine (columnar cells with
gustatory and vestibulocochlear receptor systems. All of these contain
a striated border of very regular microvilli), the gallbladder (columnar
sensory cells surrounded by supportive non-receptor cells. Olfactory
cells with a brush border of microvilli); proximal convoluted tubules
receptors are modified neurones and their axons pass directly to the
of the kidney (large cuboidal to low columnar cells with brush borders);
brain, but the other types are specialized epithelial cells that synapse
and the epididymis (columnar cells with extremely long microvilli,
with terminals of afferent (and sometimes efferent) nerve fibres.
erroneously termed stereocilia).
Ciliated columnar epithelium lines most of the respiratory tract, Myoepithelial cells
except for the lower pharynx and vocal folds, and it is pseudostratified
(Fig. 2.2D) as far as the larger bronchioles; it also lines some of the
Myoepithelial cells, which are also sometimes termed basket cells, are


tympanic cavity and auditory tube; the uterine tube; and the efferent
fusiform or stellate in shape (Fig. 2.3), contain actin and myosin fila-
ductules of the testis. Submucosal mucous glands and mucosal goblet
ments, and contract when stimulated by nervous or endocrine signals.
cells secrete mucus on to the luminal surface of much of the respira-
They surround the secretory portions and ducts of some glands, e.g.
tory tract, and cilia sweep a layer of mucus, trapped dust particles and
mammary, lacrimal, salivary and sweat glands, and lie between the
so on from the lung towards the pharynx in the mucociliary rejection
basal lamina and the glandular or ductal epithelium. Their contraction
current, which clears the respiratory passages of inhaled particles. Cilia
assists the initial flow of secretion into larger conduits. Myoepithelial
in the uterine tube assist the passage of oocytes and fertilized ova to
cells are ultrastructurally similar to smooth muscle cells in the arrange-
the uterus.
ment of their actin and myosin, but differ from them because they
Some columnar cells are specialized for secretion, and aggregates of
originate, like the glandular cells, from embryonic ectoderm or endo-
such cells may be described as glandular tissue. Their apical domains
derm. They can be identified immunohistochemically on the basis of
typically contain mucus- or protein-filled (zymogen) vesicles, e.g.
the co-localization of myofilament proteins (which signify their con-
mucin-secreting and chief cells of the gastric epithelium. Where mucous
tractile function (Fig. 2.4)) and keratin intermediate filaments (which
cells lie among non-secretory cells, e.g. in the intestinal epithelium,
accords with their epithelial lineage).
their apical cytoplasm and its secretory contents often expand to
produce a characteristic cell shape, and they are known as goblet cells
Multilaminar (stratified) epithelia
(see Fig. 2.2D). For further details of glandular tissue, see page 32, and
for the characteristics of mucus, see page 40.


Multilaminar epithelia are found at surfaces subjected to mechanical
Pseudostratified epithelium
damage or other potentially harmful conditions. They can be divided
Pseudostratified epithelium is a single-layered (simple) columnar epi- into those that continue to replace their surface cells from deeper layers,
thelium in which nuclei lie at different levels in a vertical section (Fig. designated stratified squamous epithelia, and others in which replace-
2.2D). All cells are in contact with the basal lamina throughout their ment is extremely slow except after injury.
lifespan, but not all cells extend through the entire thickness of the
Stratified squamous epithelia
epithelium. Some constitute an immature basal cell layer of smaller
cells, which are often mitotic and able to replace damaged mature cells. Stratified squamous epithelia are multilayered tissues in which the
Migrating lymphocytes and mast cells within columnar epithelia may formation, maturation and loss of cells is continuous, although the
also give a similar, pseudostratified appearance because their nuclei are rates of these processes can change, e.g. after injury. New cells are
found at different depths. Much of the ciliated lining of the respiratory formed in the most basal layers by the mitotic division of stem cells
tract is of the pseudostratified type, and so is the sensory epithelium of and transit (or transient) amplifying cells. The daughter cells move
the olfactory area. more superficially, changing gradually from a cuboidal shape to a more


INTEgRATINg CEllS INTO TISSuES
30
1
NOITCES
U
RC
U
A B
C D
Fig. 2.2 A, Simple squamous epithelium lining the outer parietal layer (arrows) of a Bowman’s capsule in the renal corpuscle (RC), stained with the
trichrome, Martius Scarlet Blue (MSB). Oval epithelial nuclei project into the urinary space (U), within a highly attenuated cytoplasm. B, Simple cuboidal
epithelium lining a group of collecting ducts sectioned longitudinally in the renal medulla. The basement membranes are stained magenta with periodic–
acid Schiff (PAS) reagent. C, Simple columnar epithelium covering the tip (off field, right) of a villus in the ileum. Tall, columnar absorptive cells with oval,
vertically orientated nuclei bear a striated border of microvilli, just visible as a deeper-stained apical fringe. Numerous interspersed goblet cells are
present, with pale apical cytoplasm filled with mucinogen secretory granules and dark, flattened, basally situated nuclei. D, Ciliated columnar
pseudostratified epithelium in the respiratory tract, and interspersed goblet cells, with pale, mucinogen granule-filled apical cytoplasm. All human tissues.
(All human tissues, courtesy of Mr Peter Helliwell and the late Dr Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals Trust, UK.)
FF
FF
MM
MM
CC
CC
MM
Fig. 2.3 Stellate myoepithelial cells (M) wrapped around secretory acini in
the lactating mouse mammary gland, seen in the scanning electron
microscope after enzymatic depletion of extracellular matrix. Blood
Fig. 2.4 Myoepithelial cells (stained brown), in a human breast duct,
capillaries (C) and fibroblasts (F) are also indicated. (Courtesy of
demonstrated immunohistochemically using antibody to smooth muscle
Prof. Toshikazu Nagato, Fukuoka Dental College, Japan.)
actin. (Courtesy of Mr Peter Helliwell and the late Dr Joseph Mathew,
Department of Histopathology, Royal Cornwall Hospitals Trust, UK.)


Epithelia
31
2
RETPAHC
K
A B
C D
Fig. 2.5 A, Keratinized stratified squamous epithelium in thin skin. Pigmented melanocytes are seen in the basal layer and a few keratinocytes of the
prickle cell layer also contain melanin granules. The dead, keratinized layer (K) lacks nuclei. B, Non-keratinized stratified squamous epithelium of the
uterine ectocervix, stained with periodic–acid Schiff (PAS) reagent. The basement membrane (short arrows) and superficial squames, which retain their
nuclei, are PAS-positive; squames sloughing off the surface are indicated (long arrow). C, Stratified low columnar epithelium of an interlobular excretory
duct of the sublingual salivary gland. D, Urothelium (transitional epithelium) lining the relaxed urinary bladder. The most superficial cells have a thickened
plasma membrane as a result of the presence of intramembranous plaques, which give an eosinophilic appearance to the luminal surface (arrows). All
human tissues. (All human tissues, courtesy of Mr Peter Helliwell and the late Dr Joseph Mathew, Department of Histopathology, Royal Cornwall
Hospitals Trust, UK.)
flattened form, and are eventually shed from the surface as a highly general shape as are seen in the keratinized type, but they do not fill
flattened squame. Typically, the cells are held together by numerous completely with keratin or secrete glycolipid, and they retain their
desmosomes to form strong, contiguous cellular sheets that provide nuclei until they desquamate at the surface. In sites where considerable
protection to the underlying tissues against mechanical, microbial and abrasion occurs, e.g. parts of the buccal cavity, the epithelium is thicker
chemical damage. Stratified squamous epithelia may be broadly subdi- and its most superficial cells may partly keratinize, so that it is referred
vided into keratinized and non-keratinized types. to as parakeratinized, in contrast to the orthokeratinized state of fully
keratinized epithelium. Diets deficient in vitamin A may induce kerati-


Keratinized epithelium nization of such epithelia, and excessive doses may lead to its transfor-
Keratinized epithelium (Fig. 2.5A) is found at surfaces that are subject mation into mucus-secreting epithelium.
to drying or mechanical stresses, or are exposed to high levels of abra-
Stratified cuboidal and columnar epithelia
sion. These include the entire epidermis and the mucocutaneous junc-
tions of the lips, nostrils, distal anal canal, outer surface of the tympanic Two or more layers of cuboidal or low columnar cells (Fig. 2.5C) are
membrane and parts of the oral lining (gingivae, hard palate and fili- typical of the walls of the larger ducts of some exocrine glands, e.g. the
form papillae on the anterior part of the dorsal surface of the tongue). pancreas, salivary glands and the ducts of sweat glands, and they pre-
Their cells, keratinocytes, are described in more detail on page 141. A sumably provide more strength than a single layer. Parts of the male
distinguishing feature of keratinized epithelia is that cells of the super- urethra are also lined by stratified columnar epithelium. The layers are
ficial layer, the stratum corneum, are anucleate, dead, flattened squames not continually replaced by basal mitoses and there is no progression
that eventually flake off from the surface. In addition, the tough keratin of form from base to surface, but they can repair themselves if damaged.
intermediate filaments become firmly embedded in a matrix protein.
Urothelium (urinary or
This unusual combination of strongly coherent layers of living cells and
transitional epithelium)
more superficial strata made of plates of inert, mechanically robust
protein complexes, interleaved with water-resistant lipid, makes this Urothelium (Fig. 2.5D) is a specialized epithelium that lines much of
type of epithelium an efficient barrier against different types of injury, the urinary tract and prevents its rather toxic contents from damaging
microbial invasion and water loss. surrounding structures. It extends from the ends of the collecting ducts
of the kidneys, through the ureters and bladder, to the proximal portion


Non-keratinized epithelium of the urethra. In males it lines the urethra as far as the ejaculatory ducts,
Non-keratinized epithelium is present at surfaces that are subject to then becomes intermittent and is finally replaced by stratified columnar
abrasion but protected from drying (Fig. 2.5B). These include: the epithelium in the membranous urethra. In females it extends as far as
buccal cavity (except for the areas noted above); oropharynx and laryn- the urogenital membrane.
gopharynx; oesophagus; part of the anal canal; vagina; distal uterine The epithelium appears to be 4–6 cells thick and lines organs that
cervix; distal urethra; cornea; inner surfaces of the eyelids; and the undergo considerable distension and contraction. It can therefore
vestibule of the nasal cavities. Cells go through the same transitions in stretch greatly without losing its integrity. In stretching, the cells become


INTEgRATINg CEllS INTO TISSuES
32
1
NOITCES
flattened without altering their positions relative to each other, because secrete small amounts of protein by a merocrine mechanism, and have
they are firmly connected by numerous desmosomes. However, the been reclassified as merocrine glands.
urothelium appears to be reduced to only 2–3 cells thick. The epithe- In apocrine glands, some of the apical cytoplasm is pinched off with
lium is called transitional because of the apparent transition from a the contained secretions, which are stored in the cell as membrane-free
stratified cuboidal epithelium to a stratified squamous epithelium, droplets (see Fig. 2.6). The best-understood example of this is the secre-
which occurs as it is stretched to accommodate urine, particularly in tion of milk fat by mammary gland cells, in which a small amount of
the bladder. The basal cells are basophilic and contain many ribosomes; cytoplasm is incorporated into the plasma membrane-bound lipid
they are uninucleate (diploid), and cuboidal when relaxed. More api- globule as it is released from the cell. Larger amounts of cytoplasm are
cally, they form large binucleate or, more often, polyploid uninucleate included in secretions by specialized apocrine sweat glands in the axilla
cells. The surface cells are the largest and may even be octoploid; in the (Stoeckelhuber et al 2011) and anogenital regions of the body. In some
relaxed state they typically bulge into the lumen as dome-shaped cells tissues there is a combination of different types of secretion, e.g.
with a thickened, eosinophilic glycocalyx or cell coat. Their luminal mammary gland cells secrete milk fat by apocrine secretion and milk
surfaces are covered by a specialized plasma membrane in which protein, casein, by merocrine secretion.
plaques of intramembranous glycoprotein particles are embedded to In holocrine glands (see Fig. 2.6), e.g. sebaceous glands in the skin,
stiffen the membrane. When the epithelium is relaxed, the surface area the cells first fill with secretory products (lipid droplets or sebum, in


of the cells is reduced and the plaques are partially internalized by the this instance), after which the entire cell disintegrates to liberate the
hinge-like action of the more flexible interplaque membrane regions. accumulated mass of secretion into the adjacent duct or, more usually,
The plaques re-emerge on to the surface when it is stretched. hair follicle.
Normally, cell turnover is very slow; cell division is infrequent and
is restricted to the basal layer. However, when damaged, the epithelium Structural and functional classification
regenerates quite rapidly.
Seminiferous epithelium Exocrine glands are either unicellular or multicellular. The latter may
be in the form of simple sheets of secretory cells, e.g. the lining of the
Seminiferous epithelium is a highly specialized, complex stratified epi-
stomach, or may be structurally more complex and invaginated to a
thelium. It consists of a heterogeneous population of cells that form
variable degree. Such glands (see Fig. 2.6) may be simple units or their
the lineage of the spermatozoa (spermatogonia, spermatocytes, sper-
connection to the surface may be branched. Simple unbranched tubular
matids), together with supporting cells (Sertoli cells). It is described in
glands exist in the walls of many of the hollow viscera, e.g. the small
detail on page 1275.
intestine and uterus, whereas some simple glands have expanded, flask-
like ends (acini or alveoli). Such glands may consist entirely of secretory
GLANDS cells, or may have a blind-ending secretory portion that leads through
a non-secretory duct to the surface, in which case the ducts may modify
the secretions as they pass along them.
One of the features of many epithelia is their ability to alter the environ-
Glands with ducts may be branched (compound) and sometimes
ment facing their free surfaces by the directed transport of ions, water
form elaborate ductal trees. Such glands generally have acinar or alveo-
or macromolecules. This is particularly well demonstrated in glandular
lar secretory lobules, as in the exocrine pancreas, but the secretory units
tissue, in which the metabolism and structural organization of the cells


may alternatively be tubular or mixed tubulo-acinar. More than one
are specialized for the synthesis and secretion of macromolecules,
type of secretory cell may occur within a particular secretory unit, or
usually from the apical surface. Such cells may exist in isolation amongst
individual units may be specialized to just one type of secretion (e.g.
other non-secretory cells of an epithelium, e.g. goblet cells in the
serous acini of salivary glands).
absorptive lining of the small intestine, or may form highly coherent
Exocrine glands are also classified by their secretory products. Secre-
sheets of epithelium with a common secretory function, e.g. the mucous
tory cells in mucus-secreting or mucous glands have frothy cytoplasm
lining of the stomach and, in a highly invaginated structure, the complex
and basal, flattened nuclei. They stain deeply with metachromatic stains
salivary glands.
and periodic acid–Schiff (PAS) methods that detect carbohydrate resi-
Glands may be subdivided into exocrine glands and endocrine
dues. However, in general (i.e. non-specific) histological preparations,
glands. Exocrine glands secrete, usually via a duct, on to surfaces that
they are weakly stained because much of their content of water-rich
are continuous with the exterior of the body, including the alimentary
mucin has been extracted by the processing procedures. Secretory cells
tract, respiratory system, urinary and genital ducts and their derivatives,
in serous glands have centrally placed nuclei and eosinophilic secretory
and the skin. Endocrine glands are ductless and secrete hormones
storage granules in their cytoplasm. They secrete mainly glycoproteins
directly into interstitial fluid and thence the circulatory system, which
(including lysozyme) and digestive enzymes.
conveys them throughout the body to affect the activities of other cells.
Some glands are almost entirely mucous (e.g. the sublingual salivary
In addition to strictly epithelial glands, some tissues derived from the
gland), whereas others are mainly serous (e.g. the parotid salivary
nervous system, including the suprarenal medulla and neurohypophy-
gland). The submandibular gland is mixed, in that some lobules are


sis, are neurosecretory.
predominantly mucous and others serous. Mucous acini may share a
Paracrine glandular cells are similar to endocrine cells but their
lumen with clusters of serous cells (seen in routine preparations as
secretions diffuse locally to cellular targets in the immediate vicinity;
serous demilunes). Although this simple approach to classification is
many are classed as neuroendocrine cells because they secrete mole-
useful for general descriptive purposes, the diversity of molecules syn-
cules used elsewhere in the nervous system as neurotransmitters or
thesized and secreted by glands is such that complex mixtures often
neuromodulators. Modes of signalling by secretory cells are illustrated
exist within the same cell.
in Figure 1.6.
ENDOCRINE GLANDS
EXOCRINE GLANDS
Endocrine glands secrete directly into connective tissue interstitial fluid
Types of secretory process
and thence the circulation. Their cells are grouped around beds of capil-
laries or sinusoids, which typically are lined by fenestrated endothelia
The mechanism of secretion varies considerably. If the secretions are to allow the rapid passage of macromolecules through their walls.
initially packaged into membrane-bound vesicles, these are conveyed Endocrine cells may be arranged in clusters within vascular networks,
to the cell surface, where they are discharged. In merocrine secretion, in cords between parallel vascular channels or as hollow structures (fol-
which is by far the most common secretory mechanism, vesicle mem- licles) surrounding their stored secretions. In addition to the cells of
branes fuse with the plasma membrane to release their contents to the specialized ductless endocrine glands (e.g. pituitary, pineal, thyroid and
exterior (Fig. 2.6). Specialized transmembrane molecules in the secre- parathyroid), hormone-producing cells also form components of other
tory vesicle wall recognize marker proteins on the cytoplasmic side of organ systems. These include: the cells of the pancreatic islets; thymic
the plasma membrane and bind to them. This initiates interactions with epithelial cells; renin-secreting cells of the kidney juxtaglomerular appa-
other proteins that cause the fusion of the two membranes and the ratus; erythropoietin-secreting cells of the kidney; circumventricular


consequent release of the vesicle contents. The stimulus for secretion organs; interstitial testicular (Leydig) cells; interstitial follicular and
varies with the type of cell but often appears to involve a rise in intracel- luteal ovarian cells; and placental cells (in pregnancy). Some cardiac
lular calcium. Glands such as the simple sweat glands of the skin, where myocytes, particularly in the walls of the atria, also have endocrine
ions and water are actively transported from plasma as an exudate, were functions. These cells are all described in detail within the appropriate
once classified as eccrine glands. They are now known to synthesize and regional sections.


glands
33
2
RETPAHC
Mechanisms of secretion Arrangement of cells
A. Merocrine B. Apocrine C. Holocrine A. Unicellular B. Multicellular sheet
Structural classification of glands – Simple glands with unbranched ducts
A. Simple tubular without duct B. Simple tubular with duct C. Simple branched tubular D. Simple coiled tubular E. Simple acinar or alveolar
Structural classification of glands – Ductal branching pattern of complex glands
A. Branched tubular B. Branched acinar/alveolar C. Branched tubulo-acinar
Fig. 2.6 Classification of the different types of epithelial gland.
Isolated endocrine cells also exist scattered amongst other tissues as glands have a rich vascular supply and their blood flow is controlled
part of the dispersed (diffuse) neuroendocrine system, e.g. throughout by autonomic vasomotor nerves, which can thus modify glandular
the alimentary and respiratory tracts. Neuroendocrine cells are generally activity.
situated within a mucosal epithelium and their bases often rest on the Glandular activity may also be controlled directly by autonomic
basal lamina (see below). In response to an external stimulus, they secretomotor fibres, which may either form synapses on the bases of
secrete their product basally into interstitial fluid. A typical neuroendo- gland cells (e.g. in the suprarenal medulla) or release neuromediators
crine cell is shown in Figure 2.7. The secretory granules vary in shape, in the vicinity of the glands and reach them by diffusion. Alternatively,
size and ultrastructure according to cell type. Cells often take the name the autonomic nervous system may act indirectly on gland cells, e.g.
of the secretion they produce, e.g. gastrin-secreting G cells of the small on neuroendocrine G cells via histamine, released neurogenically
intestine. Neuroendocrine cells share many of their secretory products from another neuroendocrine cell in the gastric lining. Such paracrine
with chemical mediators in the nervous system. activities of neuroendocrine cells are also important in the respiratory
system.
Circulating hormones from the adenohypophysis stimulate syn-
CONTROL OF GLANDULAR SECRETION thesis and secretion by target cells in many endocrine glands. Such


signals, mostly detected by receptors at the cell surface and mediated
The activities of cells in the various tissue and organ systems of the body by second messenger systems, may increase the synthetic activity of
are tightly regulated by the coordinated activity of the endocrine and gland cells, and may cause them to discharge their secretions by exocy-
autonomic nervous systems. Endocrine (and paracrine) signals reach tosis. Secretions from certain exocrine glandular cells are expressed
target cells in interstitial fluid, often via blood plasma, and together rapidly from those glands by the contraction of associated myoepithe-
with autonomic nervous signals they ensure that the body responds to lial cells (see Figs 2.3, 2.4) that enclose the secretory units and smaller
normal physiological stimuli and adjusts to changes in the external ducts. Myoepithelial cells may be under direct neural control, as in the
environment. Hormone secretion is itself controlled in a number of salivary glands, or they may respond to circulating hormones, as in the
ways, e.g. by neural control, regulatory feedback loops or according to mammary gland, where they respond to the concentration of circulat-
various cyclical, rhythmical or pulsatile patterns of release. Endocrine ing oxytocin.


INTEgRATINg CEllS INTO TISSuES
34
1
NOITCES
Fig. 2.7 An electron
micrograph of a
neuroendocrine cell
between two absorptive
cells in the colon
K
(rat tissue). Dense
neurosecretory granules
are seen in the basal
cytoplasm, apposed
to the basal lamina K
(arrows). (Courtesy of
Michael Crowder MD.)
Fig. 2.8 The basal lamina as seen in an electron micrograph, underlying
the basal epithelial layer of human skin (see Fig. 7.3). The finely fibrillar
dense layer (long arrows) corresponds to the lamina densa, and fine
collagen fibrils (*) lie in the subjacent connective tissue. These contribute
to the appearance of the basement membrane in light microscope
preparations stained for carbohydrate-rich structures. The two cells seen
Feedback loops and endocrine axes in the upper field are basal keratinocytes (K), joined by desmosomes
(short arrow), with dense keratin filaments in their cytoplasm. (Courtesy of
J McMillan MD, St John’s Institute of Dermatology, St Thomas’ Hospital,
The pituitary gland, in particular the adenohypophysis, is often termed
London.)
the master gland because of its central role in endocrine physiological
processes. It provides the means by which the central nervous system
regulates and integrates, by non-neural mechanisms, the widespread plasma membrane proteins, e.g. keratinocyte hemidesmosomes are
functions of the body, including the activities of other endocrine glands anchored into the lamina densa in the basal lamina of the epidermis.
and, often indirectly, exocrine glands such as the breast. Regulatory The basal lamina is a delicate felt-like network composed largely of two
hormones from the adenohypophysis stimulate synthesis and secretion glycoprotein polymers, laminin and type IV collagen, which self-
in target cells of many endocrine glands; these glands therefore respond assemble into two-dimensional sheets interwoven with each other.
to, as well as generate, hormonal signals. Early embryonic basal lamina is formed only of the laminin polymer.
The hypothalamus and the adenohypophysis in the brain are central Two other molecules cross-link and stabilize the network: entactin


to most regulatory feedback loops within the endocrine system. Loops (nidogen) and perlecan (a large heparan sulphate proteoglycan).
can be either positive or negative, e.g. the hypothalamus stimulates Although all basal laminae have a similar form, their thickness and
release of follicle stimulating hormone (FSH) by the adenohypophysis, precise molecular composition vary between tissues and even within a
which in turn stimulates ovarian follicular maturation and secretion of tissue, e.g. between the crypts and villi of the small intestine. The iso-
oestradiol, which acts on breast and endometrial target tissues. Oestra- forms of laminin and collagen type IV differ in various tissues; thus
diol, in this case, also acts back on the adenohypophysis and hypotha- Schwann cells and muscle cells express laminin-2 (merosin) rather than
lamus to reinforce their function positively in a feedback loop. In the prototypical laminin-1. Laminin-5, although not itself a basal
contrast, hypothalamic and adenohypophysial stimulation of testicular lamina component, is found in the hemidesmosomes of the basal
production of testosterone, which acts on targets such as skeletal epidermis and links the basal lamina with epidermal transmembrane
muscle, is negatively regulated in a feedback loop generated by circulat- proteins, αβ integrin and collagen type XVII (BPAG2, bullous pem-
6 4
ing testosterone. Such negative feedback regulation is a widely utilized phigoid antigen 2, one of the targets of the autoimmune blistering skin
physiological mechanism. disease, bullous pemphigoid). The particular isoform of collagen type
IV in the basal lamina of different tissues is reflected in tissue-specific
disease patterns. Mutations in a collagen expressed by muscle and
BASEMENT MEMBRANE AND BASAL LAMINA
kidney glomeruli cause Alport’s syndrome, a form of renal failure. Renal
failure also occurs in Goodpasture’s syndrome, in which renal basal
There is a narrow layer of extracellular matrix, which stains strongly for lamina collagen is targeted by autoantibodies.
carbohydrates, at the interface between connective and other tissues, In Descemet’s membrane in the cornea, collagen type VIII replaces


e.g. between epithelia and their supporting connective tissues. In early collagen type IV in the much thickened (increasing with age, up to
histological texts this layer was termed the basement membrane. 10 µm,) endothelial basal lamina. The basal lamina of the neuromus-
As almost all of its components are synthesized by the epithelium or cular junction contains agrin, a heparan sulphate proteoglycan, which
other tissues, rather than the adjacent connective tissue, it will be dis- plays a part in the clustering of muscle acetylcholine receptors in the
cussed here. plasma membrane at these junctions.
Electron microscopy revealed that the basement membrane is com-
posed of two distinct components. A thin, finely fibrillar layer, the basal
lamina, is associated closely with the basal cell surface (Fig. 2.8). A RETICULAR LAMINA
variable reticular lamina of larger fibrils and glycosaminoglycans of the
extracellular matrix underlies this layer and is continuous with the con- The reticular lamina consists of a dense extracellular matrix that con-
nective tissue proper, although it is much reduced or largely absent in tains collagen. In skin, it contains fibrils of type VII collagen (anchoring
some tissues, e.g. surrounding muscle fibres, Schwann cells and capil- fibrils), which bind the lamina densa to the adjacent connective tissue.
lary endothelia. In other tissues, the basal lamina separates two layers The high concentration of proteoglycans in the reticular lamina is
of cells and there are no intervening typical connective tissue elements. responsible for the positive reaction of the entire basement membrane
This occurs in the thick basal lamina of the renal glomerular filter and to stains for carbohydrates, which is seen in sections prepared for light
the basal lamina of the thin portions of the lung interalveolar septa microscopy.
across which gases exchange between blood and air.
The basal lamina is usually about 80 nm thick, varying between 40
and 120 nm, and consists of a sheet-like fibrillar layer, the lamina densa FUNCTIONS OF BASAL LAMINA
(20–50 nm wide), separated from the plasma membrane of the cell it


supports by a narrow electron-lucent zone, the lamina lucida. The Basal laminae perform a number of important roles (Iozzo 2005). They
lamina lucida is absent from tissues prepared by rapid freezing and so form selectively permeable barriers (anionic filters) between adjacent
may be an artefact. In many tissues this zone is crossed by integral tissues, e.g. in the glomerular filter of the kidney; anchor epithelial and


Connective and supporting tissues
35
2
RETPAHC
Fig. 2.10 An electron
micrograph of a
fibroblast in human
connective tissue,
surrounded by bundles
of finely banded
C BV collagen fibrils (shown
at high magnification in
the insert), which they
secrete. (Courtesy
of Dr Bart Wagner,
Histopathology
Department, Sheffield
BV Teaching Hospitals,
UK.)
Fig. 2.9 General loose connective tissue (human), with bundles of
collagen fibres (C) within an amorphous ground substance, penetrated by
a neurovascular bundle of blood vessels (BV), lymphatics and nerves. A
small autonomic ganglion is arrowed. (Courtesy of Mr Peter Helliwell and
the late Dr Joseph Mathew, Department of Histopathology, Royal
Cornwall Hospitals Trust, UK.)
connective tissues, and so stabilize and orientate the tissue layers; may to demand. Embryologically, fibroblasts and adipocytes arise from mes-
exert instructive effects on adjacent tissues, and so determine their polar- enchymal stem cells, some of which may remain in the tissues to
ity, rate of cell division, cell survival, etc.; and regulate angiogenesis. In provide a source of replacement cells postnatally. As noted above, the
addition, they may act as pathways for the migration and pathfinding cells of haemopoietic origin migrate into the tissue from bone marrow
activities of growing cell processes, both in development and in tissue and lymphoid tissue.
repair, e.g. in guiding the outgrowth of axons and the re-establishment
of neuromuscular junctions during regeneration after injury in the Resident cells
peripheral nervous system. Changes in basal lamina thickness are often
associated with pathological conditions, e.g. the thickening of the Fibroblasts
glomerular basal lamina in glomerulonephritis and diabetes.
Fibroblasts are usually the most numerous resident cells. They are flat-
tened and irregular in outline, with extended processes, and in profile
CONNECTIVE AND SUPPORTING TISSUES they appear fusiform or spindle-shaped (Fig. 2.10; see also Fig. 2.12).
Fibroblasts synthesize most of the extracellular matrix of connective
tissue (see Fig. 2.10); accordingly, they have all the features typical of


The connective tissues are defined as those composed predominantly
cells active in the synthesis and secretion of proteins. Their nuclei are
of intercellular material, the extracellular matrix, which is secreted
relatively large and euchromatic, and possess prominent nucleoli. In
mainly by the connective tissue cells. The cells are therefore usually
young, highly active cells, the cytoplasm is abundant and basophilic
widely separated by their matrix, which is composed of fibrous proteins
(reflecting the high concentration of rough endoplasmic reticulum),
and a relatively amorphous ground substance (Fig. 2.9). Many of the
mitochondria are abundant and several sets of Golgi apparatus are
special properties of connective tissues are determined by the composi-
present. In old and relatively inactive fibroblasts (often termed fibro-
tion of the matrix, and their classification is also largely based on its
cytes), the cytoplasmic volume is reduced, the endoplasmic reticulum
characteristics. In some types of connective tissue, the cellular compo-
is sparse and the nucleus is flattened and heterochromatic.
nent eventually dominates the tissue, even though the tissue originally
Fibroblasts are usually adherent to the fibres of the matrix (collagen
has a high matrix : cell ratio, e.g. adipose tissue. Connective tissues are
and elastin), which they lay down. In some highly cellular structures,
derived from embryonic mesenchyme or, in the head region, largely
e.g. liver, kidney and spleen, and in most lymphoid tissue, fibroblasts
from neural crest.
and delicate collagenous fibres (type III collagen; reticular fibres) form
Connective tissues have several essential roles in the body. These may
fibrocellular networks, which are often called reticular tissue. The
be subdivided into structural roles, which largely reflect the special
fibroblasts may then be termed reticular cells or reticulocytes.
mechanical properties of the extracellular matrix components, and
Fibroblasts are particularly active during wound repair following
defensive roles, in which the cellular component has the dominant role.
traumatic injury or inflammation, when tissue mass is lost through cell
Connective tissues often also play important trophic and morphoge-
death. They proliferate and lay down a fibrous matrix that becomes
netic parts in organizing and influencing the growth and differentiation


invaded by numerous blood vessels (granulation tissue). Contraction
of surrounding tissues, e.g. in the development of glands from an epi-
of wounds is, at least in part, caused by the shortening of myofibrob-
thelial surface.
lasts, specialized contractile fibroblast-like cells (Hinz et al 2012) with
Structural connective tissues are divided into ordinary (or general)
properties similar to smooth muscle cells. It was thought that myofi-
types, which are widely distributed, and special skeletal types, i.e. car-
broblasts differentiated from fibroblasts (reviewed in McAnulty (2007))
tilage and bone, which are described in Chapter 5. A third type, haemo-
or their progenitor mesenchymal stem cells (see below) in granulation
lymphoid tissues, consists of peripheral blood cells, lymphoid tissues
tissue. However, recent evidence suggests that in wound healing and in
and their precursors; these tissues are described in Chapter 4. They are
many fibrotic disease processes, including hepatic cirrhosis, the myofi-
often grouped with other types of connective tissue because of their
broblast precursor is the vascular pericyte or a closely related cell
similar mesenchymal origins and because the various defensive cells of
(reviewed in Duffield (2012)). In cases where the specialized cells of
the blood also form part of a typical connective tissue cell population.
the damaged region cannot divide and regenerate functional tissue, e.g.
They reach connective tissues via the blood circulation and migrate into
cardiac muscle cells after infarction, connective tissue fibroblasts and
them through the endothelial walls of vessels.
their extracellular matrix fill the void to form a scar. An exception is the
central nervous system, where glial scars are formed after injury. Fibrob-
CELLS OF GENERAL CONNECTIVE TISSUES
last activity is influenced by various factors such as steroid hormone
concentration, dietary content and prevalent mechanical stresses. Col-
Cells of general connective tissues can be separated into the resident lagen formation is impaired in vitamin C deficiency.
cell population (fibroblasts, adipocytes, mesenchymal stem cells, etc.)
Adipocytes (lipocytes, fat cells)
and a population of migrant cells with various defensive functions


(macrophages, lymphocytes, mast cells, neutrophils and eosinophils), Adipocytes occur singly or in groups in many, but not all, connective
which may change in number and moderate their activities according tissues. They are numerous in adipose tissue (Fig. 2.11). Individually,


INTEgRATINg CEllS INTO TISSuES
36
1
NOITCES
M
BV
A L P
P
P
M
L
BV
A
BV
Fig. 2.12 Macrophages (M) in chronically inflamed human connective
tissue, showing prominent pigmented, haemosiderin-containing
Fig. 2.11 Adipose tissue (human, from a lymph node specimen). cytoplasmic granules derived from ingested erythrocytes. Many are
Adipocytes (A) are distended polygonal cells filled with lipid, which has multinucleate. Also seen are plasma cells (P), small lymphocytes (L) and
been extracted by the tissue processing. This leaves only the plasma other haemopoietic cells. (Courtesy of Mr Peter Helliwell and the late Dr
membranes with scant cytoplasm and nuclei (arrows), occasionally visible Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals
compressed against the cell periphery. Small blood vessels (BV) penetrate Trust, UK.)
the adipose tissue; larger vessels are seen on the right. (Courtesy of Mr
Peter Helliwell and the late Dr Joseph Mathew, Department of
Histopathology, Royal Cornwall Hospitals Trust, UK.)
nerve endings in adipose tissue is particularly important in this respect.
No new adipose tissue is thought to form after the immediate postnatal
period, and accumulation of body fat, as in obesity, is due to excessive
the cells are oval or spherical in shape, but when packed together they
accumulation of lipid in existing adipocytes, which become very large.
are polygonal. They vary in diameter, averaging 50 µm. Each cell con-
Conversely, weight loss results from the mobilization and metabolism
sists of a peripheral rim of cytoplasm, in which the nucleus is embed-
of lipid from adipocyte stores, with the consequent shrinkage of the
ded, surrounding a single large central globule of fat, which consists of
cells.
glycerol esters of oleic, palmitic and stearic acids. There is a small accu-
mulation of cytoplasm around the oval nucleus, which is typically Mesenchymal stem cells
compressed against the cell membrane by the lipid droplet, together
Mesenchymal stem cells are normally inconspicuous cells in connective


with the Golgi complex. Many cytoskeletal filaments, some endoplas-
tissues. They are derived from embryonic mesenchyme and are able to
mic reticulum and a few mitochondria lie around the lipid droplet,
differentiate into the mature cells of connective tissue during normal
which is in direct contact with the surrounding cytoplasm and not
growth and development, in the turnover of cells throughout life and,
enclosed within a membrane. In sections of tissue not specially treated
most conspicuously, in the repair of damaged tissues in wound healing.
to preserve lipids, the lipid droplet is usually dissolved out by the sol-
There is emerging evidence that, even in mature tissues, mesenchymal
vents used in routine preparations, so that only the nucleus and the
stem cells remain pluripotent and able to give rise to all the resident
peripheral rim of cytoplasm surrounding a central empty space remain.
cells of connective tissues in response to local signals and cues. The
Another form of adipose tissue, brown fat, occurs in the interscapu-
potential therapeutic use of mesenchymal stem cell-based therapy for
lar region of neonates, a location it shares with the classic brown fat of
a wide range of autoimmune disorders and degenerative diseases is
rodents. Brown fat is characterized by the presence of large cells, each
reflected in a burgeoning literature in the field of translational medi-
of which contains several separate droplets of fat (multilocular adipose
cine. (See, for example, Ankrum and Karp (2010), Jackson et al (2012)
tissue) rather than a single globule (typical of unilocular adipose tissue;
and Ren et al (2012)).
see above), and by mitochondria in which the cristae are unusually
large and numerous. White fat cells are specialized to store chemical
Migrant cells
energy, whereas the physiological role of brown adipose tissue (BAT)
cells is to metabolize fatty acids and generate heat; BAT cells uncouple
cellular respiration via the mitochondrial uncoupling protein UCP1. It Macrophages
had been thought that brown fat disappears during postnatal growth, Macrophages are numerous in connective tissues, where they are either
but significant deposits of UCP1-positive brown fat have been detected attached to matrix fibres or are motile and migratory (Fig. 2.12). They


by positron emission tomography (PET) scanning methods in adults, are relatively large cells, 15–20 µm in diameter, with indented and rela-
mainly in the supraclavicular region, in the neck and along the spine. tively heterochromatic nuclei and a prominent nucleolus. Their cyto-
Recent evidence suggests that these human UCP1-positive cells may not plasm is slightly basophilic, contains many lysosomes and typically has
be classic brown fat cells but a distinct type of thermogenic fat cell called a foamy appearance under the light microscope. Macrophages are
a beige fat cell, thought to be derived from precursor cells in white fat important phagocytes and form part of the mononuclear phagocyte
(Wu et al 2012). Such cells may represent an evolutionarily conserved system. They can engulf and digest particulate organic materials, such
cellular mechanism to provide flexibility in adaptive thermogenesis. as bacteria, and are able to clear dead or damaged cells from a tissue
It has long been recognized that adipose tissue is central to the too. They are also the source of a number of secreted cytokines that
control of energy balance and lipid homeostasis. There is a growing have profound effects on many other cell types. Macrophages are able
view that it may play a similarly important role as an endocrine organ, to proliferate in connective tissues to a limited extent, but are derived
secreting a class of peptides called adipokines (Trayhurn and Wood and replaced primarily from haemopoietic stem cells in the bone
2004), which may enter the blood via capillaries or lymph. Different marrow, which circulate in the blood as monocytes before migrating
types of adipose tissue display functional and regional heterogeneity through vessel walls into connective tissues, where they differentiate.
and differ in their involvement with disease processes (reviewed in Many properties of macrophages in general connective tissue are
Hassan et al (2012)). The mobilization of fat is under nervous or hor- similar to those of related cells in other sites. These include: circulating
monal control; noradrenaline (norepinephrine) released at sympathetic monocytes, from which they are derived; alveolar macrophages in the


Connective and supporting tissues
37
2
RETPAHC
lungs, which take up inhaled particles not cleared by the mucociliary CELLS OF SPECIALIZED CONNECTIVE TISSUES
rejection current; phagocytic cells in the lymph nodes, spleen and bone
marrow; Kupffer cells of the liver sinusoids; and microglial cells of the Skeletal tissues – namely, cartilage and bone – are generally classified
central nervous system. with the connective tissues, but their structure and functions are highly
specialized and they are described in Chapter 5. As with the general
Lymphocytes
connective tissues, these specialized types are characterized by their
Lymphocytes are normally present in small numbers; they are numer- extracellular matrix, which forms the major component of the tissues
ous in general connective tissue only in pathological states, when they and is responsible for their properties. The resident cells are different
migrate in from adjacent lymphoid tissue or from the circulation. The from those in general connective tissues. Cartilage is populated by
majority are small cells (6–8 µm) with highly heterochromatic nuclei chondroblasts, which synthesize the matrix, and by mature chondro-
but they enlarge when stimulated. Two major functional classes exist, cytes. Bone matrix is elaborated by osteoblasts. Their mature progeny,
termed B and T lymphocytes. B lymphocytes originate in the bone osteocytes, are embedded within the matrix, which they help to min-
marrow, then migrate to various lymphoid tissues, where they prolifer- eralize, turn over and maintain. A third cell type, the osteoclast, has a
ate. When antigenically stimulated, they undergo further mitotic divi- different lineage origin and is derived from haemopoietic tissue; osteo-
sions, then enlarge as they mature, commonly in general connective clasts are responsible for bone degradation and remodelling in collabo-
tissues, to form plasma cells that synthesize and secrete antibodies ration with osteoblasts.
(immunoglobulins). Mature plasma cells are rounded or ovoid, up to
15 µm across, and have an extensive rough endoplasmic reticulum.
Their nuclei are spherical, often eccentrically situated, and have a char- EXTRACELLULAR MATRIX
acteristic ‘clock-face’ configuration of heterochromatin (see Fig. 4.12)


that is regularly distributed in peripheral clumps. The prominent Golgi The term extracellular matrix is applied collectively to the extracellular
complex is visible with a light microscope as a pale region to one side components of connective and supporting tissues. Essentially, it con-
of the nucleus and the remaining cytoplasm is deeply basophilic sists of a system of insoluble protein fibres, adhesive glycoproteins and
because of the abundant rough endoplasmic reticulum. Mature plasma soluble complexes composed of carbohydrate polymers linked to
cells do not divide. protein molecules (proteoglycans and glycosaminoglycans), which
T lymphocytes originate from precursors in bone marrow haemo- bind water. The extracellular matrix distributes the mechanical stresses
poietic tissue but later migrate to the thymus, where they develop T-cell on tissues and also provides the structural environment of the cells
identity, before passing into the peripheral lymphoid system, where embedded in it, forming a framework to which they adhere and on
they continue to multiply. When antigenically stimulated, T cells which they can move (reviewed in Even-Ram and Yamada (2005) and
enlarge and their cytoplasm becomes filled with free polysome clusters. Wolf and Friedl (2011)). With the exception of bone matrix, it provides
The functions of T lymphocytes are numerous: different subsets recog- a highly hydrated medium, through which metabolites, gases and nutri-
nize and destroy virus-infected cells, tissue and organ grafts, or interact ents can diffuse freely between cells and the blood vessels traversing it
with B lymphocytes and several other defensive cell types. or, in the case of cartilage, passing nearby.
There are many complex interactions between connective tissue cells
Mast cells
and the extracellular matrix. The cells continually synthesize, secrete,
Mast cells are important defensive cells. They occur particularly in loose
modify and degrade extracellular matrix components, and respond to
connective tissues and in the fibrous capsules of certain organs such as
contact with the matrix in the regulation of cell metabolism, prolifera-
the liver, and are numerous around blood vessels. Mast cells are round
tion and motility. Degradation of the matrix is an important feature of
or oval, approximately 20 µm in diameter, with many filopodia extend-
embryonic development, morphogenesis, angiogenesis, tissue repair
ing from the cell surface. The nucleus is centrally placed and relatively


and remodelling (Mott and Werb 2004). Various types of proteinase are
small. The cytoplasm contains large numbers of prominent vesicles and
involved, principally metalloproteinases such as matrix metalloprotei-
a well-developed Golgi apparatus, but scant endoplasmic reticulum.
nases (MMPs), and those with a disintegrin and metalloproteinase
The vesicles have a high content of glycosaminoglycans and show a
domain (ADAMs) that include ADAMs with a thrombospondin domain
strongly positive reaction with the PAS stain for carbohydrates. They are
(ADAMTS). Tissue remodelling depends on the controlled degradation
membrane-bound, vary in size and shape (mean diameter 0.5 µm) and
of the extracellular matrix by secreted MMPs, regulated by their specific
also have a rather heterogeneous content of dense, lipid-containing
inhibitors, as occurs, for instance, during involution of the postpartum
material, which may be finely granular, lamellar or in the form of
uterus or during menstrual lysis and shedding of the endometrium
membranous whorls.
(Gaide Chevronnay et al 2012). In the process of matrix degradation,
The major granule components, many of them associated with
bioactive peptides are liberated that act as growth factors, cytokines and
inflammation (Frenzel and Hermine 2013), are the proteoglycan
other signalling molecules to change the behaviour of cells in the vicin-
heparin, histamine, tryptase, superoxide dismutase, aryl sulphatase,
ity. While precisely regulated under physiological conditions, patho-
β-hexosaminidase and various other enzymes, including chymase in
logically dysregulated extracellular matrix degradation is a cause of
connective tissue but not mucosal mast cells, together with chemotactic
many diseases, such as atherosclerosis, emphysema, osteoarthritis and
factors for neutrophil and eosinophil granulocytes. There are functional
diabetic vascular complications.
differences between mast cells found in different tissues.
The insoluble fibres are mainly of two types of structural pro-
Mast cells may be stimulated to release some or all of their contents,
tein: members of the collagen family, and elastin (Fig. 2.13). The


either by direct mechanical or chemical trauma, or after contact with
particular antigens to which the body has previously been exposed. The
consequences of granule release include alteration of capillary perme-
ability, smooth muscle contraction, and activation and attraction to the
locality of various other defensive cells. Responses to mast cell degranu-
lation may be localized, e.g. urticaria, or there may occasionally be a
generalized response to the release of large amounts of histamine into
the circulation (anaphylactic shock). Mast cells closely resemble
basophil granulocytes of the general circulation but are thought to
develop as distinct descendants of an earlier myeloid lineage precursor.
It is believed that they are generated in the bone marrow and circulate
to the tissues as immature basophil-like cells, migrating through the
capillary and venule walls to their final destination. For further reading,
see Bischoff (2007) and Collington et al (2011).
Granulocytes (polymorphonuclear
leukocytes)
Neutrophil and eosinophil granulocytes are immigrant cells from the
circulation. Relatively infrequent in normal connective tissues, their
numbers may increase dramatically in infected tissues, where they are
important components of cellular defence. Neutrophils are highly Fig. 2.13 Elastic fibres, seen as fine, dark, relatively straight fibres in a
phagocytic, especially towards bacteria. The functions of eosinophils are whole-mount preparation of mesentery, stained for elastin. The wavy pink
less well understood. These cells are described further in Chapter 4. bands are collagen bundles and oval grey nuclei are mainly of fibroblasts.


INTEgRATINg CEllS INTO TISSuES
38
1
NOITCES
interfibrillar matrix (ground substance) includes a number of adhesive
glycoproteins that perform a variety of functions in connective tissues,
including cell–matrix adhesion and matrix–cell signalling. These glyco-
proteins include fibronectin, laminin, tenascin and vitronectin, in
addition to a number of other less well characterized proteins. The
glycosaminoglycans of the interfibrillar matrix are, with one notable
exception, post-translationally modified proteoglycan molecules in H
which long polysaccharide side chains are added to short core proteins S
during transit through the secretory pathway between the rough endo- S
plasmic reticulum and the trans-Golgi network. The exception, the poly- H
meric disaccharide, hyaluronan, has no protein core and is synthesized
entirely by cell surface enzymes. For further reading on extracellular
matrix molecules, see Pollard et al (2008). Functional attributes of con-
nective tissues vary and depend on the abundance of its different com-
ponents. Collagen fibres resist tension, whereas elastin provides a
measure of resilience to deformation by stretching. The highly hydrated,
soluble polymers of the interfibrillar material (proteoglycans and gly-
cosaminoglycans, mainly hyaluronan) generally form a stiff gel resisting
compressive forces. Thus tissues that are specialized to resist tensile
forces (e.g. tendons) are rich in collagen fibrils; tissues that accommo- Fig. 2.14 Reticular fibres (type III collagen; reticulin demonstrated by
date changes in shape and volume (e.g. mesenteries) are rich in elastic silver-staining) in human liver, forming a delicate meshwork within the
space of Disse between hepatocytes (H), plasma membranes and the
fibres; and those that absorb compressive forces (e.g. cartilages) are rich
sinusoidal endothelia (S). (Courtesy of Mr Peter Helliwell and the late Dr
in glycosaminoglycans and proteoglycans. In bone, mineral crystals take
Joseph Mathew, Department of Histopathology, Royal Cornwall Hospitals
the place of most of the soluble polymers and endow the tissue with
Trust, UK.)


incompressible rigidity.
ency. Tendons, aponeuroses and ligaments are also highly ordered
Fibrillar matrix
tissues (Ch. 5).
Collagens Types II, III, V and XI collagens
Types II, III, V and XI collagens can also aggregate to form linear fibrils.
Collagens make up a very large proportion (approximately 30%) of all
Type II collagen occurs in extremely thin (10 nm), short fibrils in the
the proteins of the body. They consist of a wide range of related mol-
vitreous humour and in very thick fibrils in ageing human cartilage. The
ecules that have various roles in the organization and properties of
amino-acid sequence and banding pattern are very similar to those of
connective (and some other) tissues. The first collagen to be character-
type I collagen, as are the post-translational modifications of the triple
ized was type I, the most abundant of all the collagens and a constituent
helical protein molecule. The fine fibrils in the vitreous may fuse into
of the dermis, fasciae, bone, tendon, ligaments, blood vessels and the
thicker aggregates in older tissue.
sclera of the eyeball. The characteristic collagen of cartilage and the
Type III collagen is very widely distributed, particularly in young and
vitreous body of the eye, with a slightly different chemical composition,
repairing tissues. It usually co-localizes with type I collagen, and cova-
is type II, whereas type III is present in several tissues, including the
lent links between type I and type III collagen have been demonstrated.
dermis and blood vessels, and type IV is in basal lamina. The other
In skin, many fibrils are probably composites of type I and type III
types are widely distributed in various tissues. Five of the collagens,
collagens.
types I, II, III, V and XI, form fibrils; types IV, VIII and X form sheets or
meshworks; types VI, VII, IX, XII, XIV and XVIII have an anchoring or
Reticular fibres
linking role; and types XIII and XVII are transmembrane proteins.
Fine branching and anastomosing reticular fibres form the supporting
Biochemically, all collagens have a number of features in common.
mesh framework of many glands, including the liver (Fig. 2.14), the


Unlike most other proteins, they contain high levels of hydroxyproline
kidney and lymphoreticular tissue (lymph nodes, spleen, etc.). Classi-
and all are composed of three polypeptides that form triple helices and
cally, these fibres stained intensely with silver salts, although they are
are substantially modified post-translationally. After secretion, indi-
poorly stained using conventional histological techniques. They associ-
vidual molecules are further cross-linked to form stable polymers. Func-
ate with basal laminae and are often found in the neighbourhood of
tionally, collagens are structural proteins with considerable mechanical
collagen fibre bundles. Reticular fibres are formed principally of type
strength. Just a few of their distinguishing structural features are
III collagen.
described below. For further reading on the molecular structure and
functions of the collagens, see Pollard et al (2008). Elastin
Elastin is a 70 kDa protein, rich in the hydrophobic amino acids valine
Type I collagen
and alanine. Elastic fibrils, which also contain fibrillin, are highly cross-
Type I collagen is very widely distributed. It forms inextensible fibrils
linked via two elastin-specific amino acids, desmosine and iso-
in which collagen molecules (triple helices) are aligned side by side in
desmosine, which are formed extracellularly from lysine residues. They
a staggered fashion, with three-quarters of the length of each molecule
are less widely distributed than collagen, yellowish in colour, typically
in contact with neighbouring molecules. The fibril has well-marked
cross-linked and usually thinner (10–20 nm) than collagen fibrils. They
bands of charged and uncharged amino acids arranged across it; these
can be thick, e.g. in the ligamenta flava and ligamentum nuchae. Unlike
stain with heavy metals in a banding pattern that repeats every 65 nm
collagen type I, they show no banding pattern in the electron micro-
in longitudinal sections viewed in the electron microscope (see Fig. 2.10
scope. They stain poorly with routine histological stains but are stained
insert).
with orcein-containing preparations (see Fig. 2.13). They sometimes
Fibril diameters vary between tissues and with age. Developing