to achieve an accurate pathological evaluation in toxicity studies , it is particularly important to know the background histopathology , that is , to be familiar with pictures of incidental findings . as
abundant studies using rats are common in laboratories , toxicological pathologists know the background data of spontaneous lesion in rats and can visualize these histological figures easily . however , most pathologists do not have much experience with studies of beagles . therefore
, toxicological pathologists can not help but depend on the literature or textbooks when performing pathological evaluations in dog studies .
in veterinary pathology , many textbooks have been published , and information concerning disease pathology in dogs is readily available
. however , the descriptions of background lesions in beagles are insufficient in veterinary pathology textbooks due to the lack of understanding of the importance of publishing incidental findings , which are pathologically insignificant spontaneous lesions with no sign of disease .
the papers that have reported background data did not aim to show histological pictures exhaustively .
therefore , in this report , we provide pictures of spontaneous lesions in beagles that were detected in background data collection studies and ordinary toxicity studies in our laboratory .
the figures are grouped and arranged according to the cardiovascular , lymphoid , respiratory , alimentary , urinary , reproductive , endocrine , nervous and integumentary systems .
a total of 1188 beagles ( 605 males and 583 females ) were subjected to background data collection studies and ordinary toxicity studies conducted in our laboratory from 1995 to 2011 .
( former known as toyo beagle , 169 males and 152 females ) , covance research products inc .
, 268 males and 267 females ) , marshall bioresources ( 6 males and 6 females ) , beijing marshall biotechnology co. , ltd .
( 4 males and 4 females ) , nrd beagle ( 5 males and 5 females ) , csk research park inc .
( 5 males and 5 females ) and the institute for animal reproduction ( 2 males and 2 females ) .
the animals were housed individually in metal cages ( 700  800  700  mm , 780  880  780  mm or 900  1106  808  mm ) in conventional air - conditioned rooms at 19  c to 25  c with 35% to 75% relative humidity and a 12-hour light/12-hour dark cycle .
they were provided every day with 300  g or 250  g ( for marshall beagles ) of commercially available food ( ds , ds-5 or ds - a , oriental yeast co. , ltd . ) and were also allowed free access to drinking water .
the animals were cared for according to the principles outlined in the guides for the care and use of laboratory animals prepared by the japanese association for laboratory animal science and our institution .
organs fixed in 10% neutral phosphate - buffered formalin were embedded in paraffin , and sections were made and stained with hematoxylin and eosin ( he ) for microscopic examination .
we chose 155 typical findings or rare lesions from the background data of the beagles , and they are shown as follows .
heart : blood cyst in the atrioventricular valve . a blood cyst surrounded by endothelia occurs occasionally in the atrioventricular valve , although anatomically , the cardiac valves are nonvascular tissue .
1 . heart : blood cyst in the atrioventricular valve   heart : hemorrhage in the atrioventricular valve .
hemorrhage without inflammation or hemosiderosis occurs occasionally in the atrioventricular valve , suggesting that the change is agonal hemorrhage during the euthanasia and exsanguination .  
2 . heart : hemorrhage in the atrioventricular valve   heart : thickening of the intramural arterial wall .
3 . heart : thickening of the intramural arterial wall   heart : proliferation of the epicardial mesothelium .
focal papillary proliferation of single layered mesothelial cells occurs occasionally in the right or left atrial epicardium .
4 . heart : proliferation of the epicardial mesothelium   heart : hemorrhage in the endocardium .
slight focal hemorrhage in the endocardium and subendocardium occurs occasionally and is mainly considered an agonal change .
it is necessary to distinguish this agonal change from drug - induced hemorrhage by certain drugs such as minoxidil , theobromine and other vasodilating drugs .  
heart : hemorrhage in the endocardium   heart : focal inflammatory cell infiltration in the myocardium .
heart : focal inflammatory cell infiltration in the myocardium   heart : fatty infiltration in the myocardium .
812 ) the term of arteritis refers to various inflammatory changes of the arterial wall .
endarteritis , periarteritis and panarteritis are used to describe the affected part , and polyarteritis is used for multiple lesions .
various adjectives are added to describe the morphological characteristics such as acute arteritis , necrotizing arteritis , polyarteritis nodosa or granulomatous arteritis .
immunologic disorders or other latent infections are suggested to be the causes in beagles ; however , the detailed pathogenesis has not yet been clarified .
frequent sites of arteritis are the heart , spleen , pancreas , epididymis and spinal cord .   
the most frequent site of arteritis in beagles is the extramural atrial branch of the right coronary artery .
it is difficult to distinguish spontaneously occurring vasculitis from drug - induced vasculitis resulting from such things as minoxidil , theobromine and other vasodilating drugs .
arteritis is less frequently seen in the spleen as an isolated lesion or part of polyarteritis .
the change is characterized by fibrous thickening with inflammatory cell infiltration into the tunica intima and fibrinoid necrosis of the tunica media .   fig
the artery in the pancreas is one of the arteries with a high incidence of arteritis .  
the epididymal artery is one of the arteries with a high incidence of arteritis , often in the form of periarteritis .  
fig . 12 . artery : arteritis in the epididymis   aorta : mineralization in the aortic wall .
mineralization in the tunica media of the aorta is rare and focal in young beagles .  
erythrophagia is common in the lymphatic sinus undergoing blood absorption as an agonal change , especially in the mesenteric lymph node .  
the cause is unclear , but it is thought to be a nonspecific reactive change to some irritation such as inflammation , foreign body and parasite .
lymph node : granuloma   lymph node : increased number of pigment - laden macrophages .
brown pigment - laden macrophages accumulate predominantly in the medullary region , especially in the mandibular lymph node .
lymph node : increased number of pigment - laden macrophages   lymph node : foreign body granuloma .
macrophages accumulate and form multinucleated giant cells around foreign bodies such as hair fragments or parasites .
lymph node : foreign body granuloma   lymph node : increased number of foam cells in the sinuses .
the number of foam cells ( foamy macrophages ) occasionally increases in the lymphatic sinuses , especially in the mesenteric lymph node .  
lymph node : increased number of foam cells in the sinuses   thymus : involution ( physiological atrophy ) . a decrease in lymphoid cells from both the cortex and medulla with replacement by adipose tissues causes reduction in size and organ weight .
thymus : involution ( physiological atrophy )   thymus : proliferation of the thymic epithelium .
the change is characterized by proliferation of prominent tubular structures lined by the cuboidal epithelium .
solitary or multilocular cysts are associated with remnants of embryonic ducts connecting the parathyroid and thymus .
the cysts are covered by a cuboidal epithelium , partially ciliated , and contain the same eosinophilic proteinic substances as parathyroid cysts ( fig .
thymus : cyst   thymus : lymph follicle formation in the medulla . a germinal center is commonly formed in the medulla .
the change is known to be associated with myasthenia gravis in humans , whereas it is irrelevant and incidental in beagles .
vaccination is considered to be attributable to the change , so attention must be paid to the immunologic failure or overresponse if the follicle formation is severe .  
the aberration of splenic tissue separated from the original body and surrounding capsule occurs occasionally in the greater omentum and mesenterium as a congenital anomaly .  
an increase in hematopoietic cells , predominantly erythrocytic series , occurs occasionally in the red pulp under normal conditions .  
a focus consisting of relatively dense fibrous tissue or collagenous fibers impregnated with iron pigment and calcium salts is common usually in the splenic hilum and probably results from organization and scarring of the sites where a small perivascular hemorrhage occurred .  
spleen : necrosis of the splenic follicle   spleen : pigment deposition in the red pulp .
this is a common lesion that shows an increase in brown pigment - laden macrophages usually containing hemosiderin .  
spleen : pigment deposition in the red pulp   spleen : nodular hyperplasia ( lymphoid follicle ) .
nodular hyperplasia is essentially focal proliferation of various types of splenic components compressing slightly normal splenic tissues , although a large aggregation of lymphocytes is the main feature .
the nodule takes an appearance like a giant follicle without the normal structures of a lymph follicle such as a central artery , periarterial lymphoid sheath ( pals ) or germinal center .
spleen : nodular hyperplasia ( lymphoid follicle )   bone marrow : lymph follicle formation .
tonsilla : mineralization in the lymphoid tissue   tonsilla : neutrophilic migration in the mucosa .
larynx : focal inflammatory cell infiltration   larynx : erosion / ulcer of the mucosa .
larynx : erosion / ulcer of the mucosa   trachea : focal inflammatory cell infiltration .
trachea : focal inflammatory cell infiltration   trachea : hyperplasia of the mucosal epithelium in the tracheal bifurcation .
hyperplasia of the mucosal epithelium is frequently seen in the region near the bifurcation .  
trachea : hyperplasia of the mucosal epithelium in the tracheal bifurcation   trachea : squamous metaplasia in the tracheal bifurcation .
squamous metaplasia of the mucosal epithelium leading from a hyperplastic lesion occurs occasionally in the region near the bifurcation .
35 ) and squamous metaplasia because this region is most likely to receive the physiological irritant effects .  
trachea : squamous metaplasia in the tracheal bifurcation   lung : accumulation of foam cells in the alveolus .
focal accumulation of foam cells without other inflammatory cells is frequently seen in the alveoli .   fig .
lobar atelectasis ( stenosis of the bronchiole and alveoli ) and bronchial cartilage aplasia occur occasionally .
lung : focal hemorrhage in the alveolus   lung : inflammatory cell infiltration in the alveolus . inflammatory cell infiltration with / without hemorrhage occurs occasionally .  
lung : inflammatory cell infiltration in the alveolus   lung : fibrosis of the alveolar wall .
osseous metaplasia presumably caused by differentiation from fibroblasts to osteoblasts occurs occasionally and is similar to that seen in rats and mice .
thrombosis is frequently seen in the pulmonary vessels of beagles used in toxicity studies by intravenous injection .  
this change results from aspiration of stomach contents or food particles ( arrow ) .  
stenosis of the vascular lumen by a fibrous organization of thrombus containing hair fragments is rare .
foreign body granuloma caused by a hair fragment stuck on the tongue occurs occasionally in the lamina propria or muscle layer .  
tongue : foreign body granuloma   tongue : focal infiltration of macrophages in the muscle layer .
tongue : focal infiltration of macrophages in the muscle layer   esophagus : focal atrophy of the esophageal gland .
esophagus : focal atrophy of the esophageal gland   esophagus : hypertrophy of the esophageal gland .
esophagus : hypertrophy of the esophageal gland   esophagus : focal inflammatory cell infiltration in the lamina propria .
esophagus : focal inflammatory cell infiltration in the lamina propria   stomach : infection by helicobacter heilmannii in the gastric mucosa .
stomach : infection by helicobacter heilmannii in the gastric mucosa   stomach : mineralization in the lamina propria .
stomach : mineralization in the lamina propria   duodenum : hyperplasia of the lymphoid tissue .
hyperplasia of the lymphoid tissue occurs occasionally in not only the duodenum but other intestinal tracts .
dilatation of the crypt filled with / without cell debris or inflammatory cell infiltration is frequently seen .
the change most frequently occurs in the duodenum but also occurs in the other intestines .  
ectopic pancreatic tissue is frequently seen in the submucosa near the major duodenal papilla .  
focal lymphocytic infiltration , occasionally accompanied by lymph follicle formation , is common around the ducts or acini .  
focal fibrosis with atrophy and loss of the acini is frequently seen , especially in the parotid gland .   fig .
a small mineralized calculus occurs occasionally in a duct of a relatively large caliber .  
the mucous epithelium increases occasionally in the parotid gland , though the parotid gland of the beagles is composed mainly of a serous epithelium .   fig .
salivary gland : increase in the mucous gland   liver : accumulation of glycogen in hepatocytes .
the change is common in beagles and characterized by a clear appearance of hepatic cytoplasm after formalin fixation .
the quantity of glycogen in cytoplasm varies among individuals under normal conditions .   fig . 64 .
microgranuloma is accumulation of inflammatory cells , mainly macrophages , lymphocytes and a small number of neutrophils , and may be associated with minute necrosis of hepatocytes .
lipofuscin pigments are common in centrilobular hepatocytes of young beagles but are particularly prominent in older dogs .   fig .
a necrotic focus of hepatocytes with inflammatory cell infiltration occurs occasionally without any apparent cause .  
liver : pigment deposition in kupffer cells   liver : increased number of ito cells with lipid accumulation .
liver : increased number of ito cells with lipid accumulation   gall bladder : cystic mucinous hyperplasia of the mucosal epithelium . a cystic change or cystic hyperplasia of the mucosal epithelium with copious mucin production occurs occasionally .  
gall bladder : cystic mucinous hyperplasia of the mucosal epithelium   gall bladder : hyperplasia of the mucosal epithelium .
gall bladder : hyperplasia of the mucosal epithelium   gall bladder : hyperplasia of the lymphoid tissue .
prominent lymphoid tissues with / without lymph follicle formation are common in the lamina propria .  
irregularly shaped islets and small clusters of endocrine cells closely associate with ductules or intercalated ductules .
immature glomerulus is more frequently seen in the outer layer of the cortex , reflecting delayed maturation of the glomeruli in the outer cortex than those in the deeper layer .  
a lesion such as that resulting from collapse of the capillary loops due to the proliferation of mesangial cells and increase in matrix is called sclerosis .   fig . 79 .
a typical lesion of renal dysplasia consists of a scarring focus containing mesenchymal cells , immature renal tubules ( large arrows ) , small - sized glomeruli ( small arrows ) and dilatation of the collecting tubule ( arrowheads ) in the medulla to the cortex .
focal regenerative changes of damaged tubules are detected frequently as basophilic tubules with a high nuclear density in the cortex or outer stripe of the renal medulla .
kidney : regeneration of the tubular epithelium   kidney : focal inflammatory cell infiltration in the interstitium .
a solitary cast occurs occasionally in the distal segment or collecting tubule in the outer medulla , but it is not associated with chronic progressive nephropathy as in rats .  
focal interstitial fibrosis occurs occasionally and is accompanied by interstitial inflammation , pyelitis and infarction .
kidney : focal interstitial fibrosis   kidney : brown pigment deposition in the tubular epithelium .
kidney : brown pigment deposition in the tubular epithelium   kidney : fatty change of the proximal tubular epithelium .
tubules containing fine lipid droplets are frequently seen in the epithelia of the straight portion of the proximal tubules in most female dogs .  
kidney : fatty change of the proximal tubular epithelium   kidney : inclusion bodies in the collecting tubular epithelium .
kidney : inclusion bodies in the collecting tubular epithelium   kidney : mineralization in the medulla .
small mineralized calculus is frequently seen in the renal papillary collecting tubules .   fig .
kidney : mineralization in the medulla   kidney : focal hyperplasia of the pelvic epithelium .
focal hyperplasia of the transitional epithelium ( urothelium ) covering the papilla with inflammation occurs occasionally , but the lesion without inflammation , as in this case , is rare .  
focal lymphocytic infiltration including lymph follicle formation is frequently seen in the lamina propria of the renal pelvis .
kidney : pyelitis   kidney : pyelonephritis . a severe case of pelvic inflammation penetrating all mucosal regions and
this tubulointerstitial inflammation can be assumed to be accompanied by pyelitis because the lesion distributed along nephron is suggested to be an ascending spread of inflammatory lesions from the pelvis .  
kidney : interstitial nephritis   urinary bladder : round ligament of the urinary bladder . a fibromuscular band with mineralization is rarely attached to the urinary bladder .
only a small number of spermatids are present , whereas development up to the spermatocytes completes in the seminiferous tubules .   fig .
when seminiferous tubules sustain a permanent damage , the end - stage feature shows sertoli - cell - only tubules .  
testis : sertoli - cell - only tubule   testis : appearance of multinucleated giant cells .
this change is caused by disturbance of spermatozoa flow due to seminiferous tubular damage .  
cell debris of spermatocytes or spermatids occurs occasionally in the epididymal lumen in normal beagles .
an intraepithelial lumen is frequently seen and is lined by ciliated cells .   fig .
foreign body granuloma occurs occasionally in the interstitium and contains a mass of spermatozoa surrounded by macrophages and lymphocytes .  
epididymis : spermatic granuloma ( sperm granuloma )   epididymis : focal inflammatory cell infiltration in the interstitium .
the glandular epithelia are small and flat , and lumens are narrow because of no prostatic secretion .
maturation of the prostatic gland varies in young beagles aged 6 - 7 months .  
prostate : focal inflammatory cell infiltration cyclic changes in female reproductive system ( figs . 110114 ) .  
ovary ( containing postovulatory corpus luteum ) , uterus , vagina , mammary gland fig .
ovary ( containing functional corpus luteum ) , uterus , vagina , mammary gland fig .
ovary ( containing previously formed corpus luteum ) , uterus , vagina , mammary gland fig
the embryonic remnants of rete ovarii are frequently seen in the hilum of the ovary and occasionally undergo glandular hyperplasia .  
the luminal surface is lined by a single layer of flat or cuboidal ciliated epithelial cells .
cystic epithelial proliferation of the endometrium separated into two layers is formed in the uterus in beagles .
pituitary cysts are frequently seen in each lobe ( anterior , intermediate , posterior ) but are most often found in the anterior lobe .  
pituitary : focal inflammatory cell infiltration in the posterior lobe   thyroid ( parathyroid ) : ectopic thymic tissue .
the parathyroid , thyroid and thymic primordia derive from the same primordium , the pharyngeal pouches , during embryonic development .  
thyroid ( parathyroid ) : ectopic thymic tissue   thyroid : c - cell complex .
solid islands of c - cell - like clear cells are frequently seen in the thyroid tissue .
they are considered to be remnants of ultimobranchial bodies formed before differentiation into follicular cells and c - cells .  
the cysts are lined by a cuboidal epithelium that is often ciliated like a parathyroid cyst ( fig .
the duct is a remnant of the ultimobranchial body in the thyroid or parathyroid gland .
the follicles are focally small in size and contain a small amount of colloid inside with abundant connective tissue between each atrophic follicle .   fig .
papillary proliferation of the epithelial cells and an increase in small follicles are absent ; these may be the differential points from focal follicular hyperplasia .
this change is frequently seen and is characterized by lymphocytic infiltration often forming a prominent germinal center in the interstitium .
the follicles are destroyed and infiltrated also by lymphocytes , and residual follicles are lined with hypertrophic epithelial cells .
the cyst is lined by a cuboidal epithelium , often ciliated , and contains proteinous substance in the lumens .
this cyst is an embryonal remnant of the duct connecting the parathyroid - thymus tissue in the iii and iv pharyngeal pouches .  
the aberration of adrenocortical tissue separated from the original body and surrounding capsule is called accessory adrenocortical tissue , and it is common inside and outside of the adrenal capsule .  
adrenal : accessory adrenocortical tissue   adrenal : vacuolation of the cells of the zona glomerulosa .
accumulation of large lipid droplets containing cells is frequently seen in the zona glomerulosa in beagles .  
adrenal : vacuolation of the cells of the zona glomerulosa   adrenal : osseous metaplasia .
adrenal : osseous metaplasia   adrenal : vacuolation of the cells of the zona fasciculata .
various degrees of zonal vacuolation in the cells of the zona fasciculata occur occasionally , especially near the zona reticularis .  
adrenal : vacuolation of the cells of the zona fasciculata   adrenal : extramedullary hematopoiesis .
deposition of yellow - brown pigments occurs occasionally in the cytoplasm of cortical cells near the corticomedullary junction .
adrenal : pigment deposition in the corticomedullary junction   brain : focal inflammatory cell infiltration in the choroid plexus .
brain : focal inflammatory cell infiltration in the choroid plexus   brain : extramedullary hematopoiesis in the choroid plexus .
hematopoietic cells of granulocytic series with rare megakaryocytes ( arrow ) occasionally aggregate in the choroid plexus .  
brain : extramedullary hematopoiesis in the choroid plexus   brain : mineralization of the meninx .
fibrous thickening of the meninx may have resulted from chronic inflammation or circulatory disturbance and is very rare .  
brain : fibrous thickening of the meninx   brain : hamartoma in the callosal sulcus .
this rare lesion consists of choroid plexus components that are epithelia , fatty tissues , abundant collagen fibers and small vessels .
its function is thought to be protection of the peripheral nerve fibers from pressure damage .  
a cyst covered by epidermis with a skin appendage structure occurs occasionally in the dermis .
ulcer is rare as a secondary change associated with folliculitis or other skin lesions .  
the causes of this lesion may be congenital failures of optic fissure closure ( retinal coloboma ) , dysplasia of retinal structures or focal damage of the retina in the fetal developmental stage .  
eye : disarrangement of the retinal structures   eye : focal inflammatory cell infiltration in the conjunctiva .
eye : focal inflammatory cell infiltration in the conjunctiva   lacrimal gland : focal fibrosis .
focal fibrosis of the interstitium with lymphocytic infiltration and acinar atrophy occur occasionally in the lacrimal gland .  
in veterinary pathology , many textbooks have been published , and information concerning disease pathology in dogs is readily available . furthermore , there are some reports referring to spontaneous lesions of beagles in veterinary and toxicologic journals .
however , few publications showing pictures of spontaneous lesions exhaustively are available as a histopathology atlas .
therefore , we provided many pictures of spontaneous lesions in beagles that were detected in background data collection studies and ordinary toxicity studies in our laboratory .
1 ) was reported by takeda et al . , who showed that the endothelium surrounding the blood cysts revealed focal positive staining for the factor viii - related antigen .
there was no difference in the incidence of the lesion between male and female dogs or the right and left atria .
morphological similarities between spontaneous and drug - induced arterial lesions may make it difficult to evaluate a lesion in the toxicity studies of chemicals such as vasodilating agents . at an early stage ,
however , drug - induced arterial lesions are composed of medial necrosis and medial and adventitial hemorrhage with very few or no inflammatory cells ; these may be the differential points from spontaneous arteritis .
70 ) occurs spontaneously , but the change is also induced by long - term dosing of certain some steroids .
furthermore , we picked up the previous reports concerning spontaneous lesions in dogs , especially beagles , such as mineralization in the gastric lamina propria ( fig .
59 ) , pancreatic nesidioblastosis ( fig . 75 ) , renal cytoplasmic inclusion body in the collecting tubular epithelium ( fig .
117 ) , in the journal of toxicologic pathology . concerning the common lesions of beagles , we introduced reports of renal glomerular lipidosis ( fig .
these background lesions may not have an effect on the results of toxicity studies but should be taken into account along with their potential to influence safety assessment in drug administration .
   the purpose of our publication is to widely communicate the pictures of spontaneous findings occurring in beagles . 
 spontaneous arteritis occurs commonly in beagles . 
 frequent sites of arteritis are the heart , spleen , pancreas , epididymis and spinal cord . 
 morphological similarities between spontaneous and drug - induced arterial lesions may cause confusion when evaluating vascular toxicity of chemicals such as vasodilating agents . 
 focal and minimal inflammatory lesions are occasionally seen in the lung and may be associated with aspiration of food particles or of unknown causes . 
 a cystic change with copious mucin production occurs occasionally in the mucosal epithelium of the gall bladder . 
 nesidioblastosis is seen rarely in the pancreas of beagles . 
 c - cell complex and lymphocytic thyroiditis are common thyroid lesions . 
 spontaneous focal hypospermatogenesis and lobular sertoli - cell - only seminiferous tubules occurring frequently in beagles must be distinguished from drug - induced damage of the seminiferous tubules in toxicity studies . 
 the morphological differences of the female genital system in each cycle need to be understood ; therefore , we present the normal features of the cyclic changes of the female genital organs . 
 further , we provide more information on spontaneous findings in beagles for exact diagnoses in toxicity studies . 
Introduction
Materials and Methods
Result
Discussion