Aughey E., Frye F.L. Comparative veterinary histology with clinical correlates

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8.97

8.97 The pancreas of some snakes,

particularly many pythons, is

characterized by possessing

endocrine cells formed into giant

islets (often found at the edge of the

lobule) rather than into many small

nests of cells scattered randomly

throughout the parenchyma.

Illustrated is a section of the pancreas

from a regal (ball) python (Python

regius). H & E. ×125.

Reptilian, amphibian

and fish pancreas

Morphologically, the pancreas of teleost fish,

amphibians and reptiles is similar to that found in

mammals (8.95), but two major differences are

observed in some species. Many fish, and some

amphibians and reptiles, possess a pancreas that

has an intimate association, and admixture of cells,

with the spleen or liver (8.96). This combined

organ is termed a ‘splenopancreas’ or ‘hepato-

pancreas’, respectively, and the cells and tissues of

each organ receive blood from their respective

splenic, pancreatic or hepatic branches of the

splanchnic arteries and veins. Whereas the islet tis-

sue in most reptiles tends to be conventionally

arranged and evenly distributed, ‘giant’ islets of

Langerhans are characteristic in the pancreatic tis-

sues of some snakes, particularly members of the

family Boidae (pythons and boas); rather than

being scattered in a more or less random manner

throughout the pancreatic exocrine tissue, these

huge islets of endocrine cells tend to be localized in

specific areas of pancreatic tissue (8.97).

131

Digestive System

8.95 Pancreas of a salamander (Amphiuma tridactyla).

Exocrine pancreatic cells characterized by their fine

granular eosinophilic cytoplasm (on the left) extend a

finger-like isthmus into the islet of paler staining

endocrine cells bearing dense nuclei (on the right). The

islet cells are arranged into nest-like lobules that are

separated from each other and from the exocrine tissue

by thin strands of connective tissue that support small

blood vessels. H & E. ×250.

8.95

8.96 The splenopancreas of a milksnake (Lampropeltis

triangulum). The spleen is on the right, a large aggregate

of islet tissue is in the middle and a portion of the

exocrine pancreatic tissue is on the left. The islet displays

hyalinization. H & E. ×125.

8.96

Avian digestive system

The horny beak replaces functionally the lips and

teeth of mammals.

Oral cavity and oesophagus

The oral cavity is lined with stratified squamous

epithelium. The tongue is also lined with this type

of epithelium, with some keratinized areas. The

main mass of the tongue consists of striated muscle

and a small bar of cartilage or bone: the entoglos-

sal bone. There are no teeth. The glands in the lam-

ina propria of the oral cavity, tongue and pharynx

are simple-branched and mucus secreting.

The oesophagus is lined with stratified squamous

non-keratinized epithelium, with simple mucous

glands in the connective tissue lamina propria

(8.98). The muscularis externa consists of a thick

inner layer of circular and a thin outer layer of lon-

gitudinal smooth muscle. Lymphoid tissue accu-

mulates in the caudal oesophagus as the

oesophageal tonsil. The crop is an aglandular cau-

dal diverticulum situated two-thirds of the way

down the oesophagus (8.99). In the pigeon two lat-

eral glomerular sacs secrete crop milk.

Clinical correlates

Reptilian pancreas

Certain species of reptiles appear to have a

higher than expected incidence of some

tumours. In captivity, some lizards, especially

savanna monitors (Varanus exanthematicus),

seem to show a high incidence of adult-onset

diabetes mellitus and exocrine deficiency. No

evidence suggests that diabetes mellitus or

exocrine deficiency are as prevalent in wild

savanna monitors. Therefore, these disorders

seem to be artefacts of captive husbandry

(caused particularly by overfeeding and lack

of adequate exercise) resulting in spontaneous

acute and chronic pancreatitis with subse-

quent autodigestion of the pancreatic

parenchyma.

The migration of helminth larvae can also

induce pancreatitis with secondary fibrosis and

loss of secretory function both of exocrine and

of endocrine components.

132

8.98 Oesophagus (bird). (1) Stratified squamous non-

keratinizing epithelium. (2) Simple tubular mucosal

glands. (3) Lamina propria. H & E. ×62.5.

8.98

8.99 Crop (bird). (1) Stratified squamous non-keratinized

epithelium. (2) Lamina propria. (3) Muscularis mucosae.

(4) Muscularis externa. H & E. ×12.5.

8.99

Comparative Veterinary Histology with Clinical Correlates

8.101

8.100

Stomach

The stomach consists of the glandular proventricu-

lus and a muscular ventriculus. The gastric epithe-

lium of the proventriculus is simple columnar and

mucus secreting. A thin lamina propria separates

it from the lobules of the submucosal glands. These

glands form an almost continuous mass of tissue,

with adjacent lobules separated by fine strands of

connective tissue. Each gland lobule contains a cen-

tral cavity with straight secretory tubules radiating

to the interlobular connective tissue. An excretory

133

8.100 Proventriculus (bird). (1) Simple columnar mucus-secreting epithelium.

(2) Lamina propria. (3) Submucosal glands. (4) Muscularis externa. H & E. ×62.5.

8.101 Proventriculus (bird). (1) Simple columnar mucus-

secreting epithelium. (2) Lamina propria. (3) Submucosal

glands. (4) Muscularis externa. Masson’s trichrome. ×25.

8.102 Proventriculus (bird). The submucosal gland

lobules are separated by thin strands of connective tissue.

Masson’s trichrome. ×250.

8.102

duct drains onto the gastric mucosal surface. The

glands contain only one type of cell, which secretes

acid and pepsinogen, thus combining the functions

of both the chief and parietal cells of the mammal.

The muscularis externa is arranged as inner circu-

lar and outer longitudinal layers of smooth muscle

(8.100–8.102).

The ventriculus is the aglandular stomach or giz-

zard. The luminal surface is lined with secretory

product of the mucosal glands, which solidifies at the

surface to form a hard cuticle of koilin. The epithe-

lium is low columnar and continues within the

Digestive System

134

8.103 Ventriculus, gizzard (bird). (1) Cornified lining.

(2) Epithelium lining the gizzard. (3) Mucosal glands.

(4) Lamina propria. H & E. ×25.

8.103

8.104 Ventriculus, gizzard (bird). (1) Cornified lining.

(2) Lining epithelium. (3) Mucosal glands. H & E. ×125.

8.104

8.105 Duodenum (bird). (1) Simple

columnar epithelium. (2) Intestinal

mucosal glands. (3) Connective tissue

core of the villus. (4) Muscularis

mucosae. H & E. ×62.5.

8.105

simple straight tubular mucosal glands in the lamina

propria. A submucosa is present, and the muscularis

externa is a thick layer of smooth muscle (8.103 and

8.104). There is no muscularis mucosae.

Intestine

The small intestine is similar to that of mammals

but is more uniform throughout its length. Diffuse

lymphatic tissue is present in the lamina propria and

the submucosa, and a third layer of circular smooth

muscle may be present in the muscularis externa

(8.105–8.107).

The caeca are two blind sacs at the junction of the

small and large intestine and are of considerable size

in domestic birds. The epithelium is simple colum-

nar with mucous cells. Lymphatic tissue is particu-

larly abundant, forming the caecal tonsil in the nar-

row proximal part of the caecum (8.108 and 8.109).

Comparative Veterinary Histology with Clinical Correlates

8.108

135

Digestive System

8.106 Duodenum (bird). The simple

columnar epithelium lining the

duodenum has a striated border to

allow absorption and goblet cells.

Masson’s trichrome. ×125.

8.107 Duodenum (bird). The simple

columnar epithelium rests on a

lamina propria with numerous

lymphatic cells. Masson’s trichrome.

×250.

8.108 Caecum (bird). (1) Lumen of

the caecum. (2) Mucosa consists of a

columnar epithelium and a lamina

propria with extensive deposits of

lymphatic tissue. (3) Muscularis.

(4) Lumen of the duodenum.

Masson’s trichrome. ×12.5.

8.109 Caecum (bird). Dense masses

of lymphatic tissue fills the lamina

propria. H & E. ×12.5.

8.106

8.107

8.109

8.111 Cloaca (bird). (1) Simple columnar lining

epithelium. (2) Simple tubular glands. (3) Lamina

propria with lymphatic tissue. (4) Muscularis mucosae.

(5) Muscularis externa. Alcian blue/PAS. ×62.5.

8.111

The large intestine has the same histological

appearance as the caeca (8.110). The cloaca is lined

with tall columnar epithelium with a variable num-

ber of mucus-secreting cells. A vascular lamina pro-

pria separates it from the muscularis mucosae and

externa (8.111). (See Chapter 15 for the cloacal

bursa.)

The avian liver is very similar to the mammalian;

136

8.110 Rectum (bird). The lining epithelium is simple

columnar with goblet cells extending into the simple

tubular mucosal glands (arrowed). H & E. ×62.5.

8.110

8.112 Pancreas (bird). (1) Serous

units of the exocrine gland.

(2) Pancreatic islet, the endocrine

gland. H & E. ×125.

8.112

the connective tissue capsule extends into the gland

and divides it into lobes and lobules. The hepato-

cytes are arranged in rows, often two cells thick,

separated by sinusoids.

The avian exocrine pancreas is similar to the mam-

mallian, but has less interlobular connective tissue

(8.112). The endocrine pancreatic islets are of three

types: light (beta) islets, dark (alpha) islets and mixed.

Comparative Veterinary Histology with Clinical Correlates

9.3 Kidney cortex (dog). (1) Renal corpuscle.

(2) Uriniferous tubules. (3) Medullary ray. H & E. ×125.

9.3

9.2 Kidney cortex (dog). (1) Renal corpuscle.

(2) Uriniferous tubules. (3) Medullary ray. H & E. ×12.

9.2

The urinary system of mammals comprises the kid-

neys, renal pelvis, ureters, bladder and urethra.

Kidneys

The kidneys are highly vascularized organs that fil-

ter the blood and excrete waste materials, excess

water and electrolytes via the ureters to the bladder

as urine (see Appendix Figures A1–A7). The kidneys

have an endocrine function: they secrete erythropoi-

etin, which stimulates erythrocyte production in the

bone marrow, and renin, which helps to regulate

blood pressure. Each kidney is enclosed in a tough

connective tissue capsule extending into the

parenchyma and has two regions – the cortex and

the medulla (9.1–9.4). Smooth muscle may be pre-

sent in ruminants. The hilus is a deep fissure on the

medial border of the kidney and contains the renal

artery, the renal vein and the ureter.

137

9. URINARY SYSTEM

9.1 Kidney (horse). (1) Capsule. (2) Outer area of the cortex.

(3) Renal corpuscle. (4) Uriniferous tubules. H & E. ×62.5.

9.1

9.4 Kidney medulla (dog). (1) Blood vessels. (2) Collecting

tubules. (3) Ascending thin limb. (4) Descending thin limb.

H & E. ×125.

9.4

138

9.5 Kidney (horse). (1) Renal corpuscle with (2) capsular space (3) the urinary pole.

(4) Proximal convoluted tubule. (5) Distal convoluted tubule with (6) the macula

densa. H/PAS. ×250.

9.5

Nephron

The nephron is the functional unit of the kidney.

The major subdivisions are the renal corpuscle and

the uriniferous tubule.

Renal corpuscle

The blind end of the proximal tubule is indented

with a network of capillaries and supporting cells

to form a filtering system: the renal corpuscle. Each

renal corpuscle consists of a glomerulus and a

glomerular (Bowman’s) capsule. The outer layer of

the glomerular capsule is the capsular (parietal)

wall, which is separated from the glomerular (vis-

ceral) layer by the capsular (urinary) space (9.5).

The capillaries are lined with a fenestrated endothe-

lium resting on a basal lamina. The visceral epithe-

lial cells, or podocytes, closely invest the capillary

endothelium of the glomerulus and develop pri-

mary processes wrapped around each capillary.

These processes develop secondary foot processes

called pedicles. The foot processes of adjacent

podocytes interdigitate, resulting in the formation

of small gaps called slit pores (9.6). The podocyte

basal lamina is fused with the endothelial basal

lamina and blood passing through the capillary is

filtered through this common basal lamina into the

capsular space (9.6 and 9.7). Mesangial perivas-

cular cells are present between the endothelium

and the basal lamina. The capillaries of the

glomerulus are served by an afferent and an effer-

ent arteriole, entering and leaving the renal cor-

puscle at the vascular pole (9.8). At the opposite

pole is the capsular space, where the filtrate passes

into the proximal tubule at the urinary pole of the

renal corpuscle (9.8 and 9.9).

Comparative Veterinary Histology with Clinical Correlates

139

Urinary System

9.7 Kidney (dog). The renal corpuscle projects from the

surrounding tissue. Scanning electron micrograph. ×500.

9.7

9.6 Kidney (dog). (1) Part of the podocyte. (2) Secondary

foot processes, the pedicles – the filtration slit pores are

the spaces between the solid pedicles. Scanning electron

micrograph. ×1500.

9.6

9.8 Kidney (horse). The width of the

glomerular capsule is shown by the

line. (1) Renal corpuscle with

epithelial cells and mesangium.

(2) Vascular pole. (3) Capsular space.

(4) Distal convoluted tubule. (5) The

macula densa. (6) Proximal

convoluted tubule. H & E. ×250.

9.8

9.9 Kidney (horse). (1) Renal

corpuscle with epithelial cells and

mesangium. (2) Urinary pole opening

into (3) the proximal convoluted

tubule. (4) Distal convoluted tubule

and macula densa. H & E. ×250.

9.9

extends towards the medulla where the epithelium

changes abruptly to simple squamous. This part of

the tubule descends into the medulla as the thin

descending limb and bends sharply to return to the

cortex as the thick ascending limb, which was pre-

viously known as the loop of Henle. In the cortex

the epithelium becomes cuboidal or columnar and

forms the distal straight tubule and coils near the

glomerulus to become the distal convoluted tubule

(DCT). The DCT is shorter than the PCT, the

epithelium is cuboidal, the cytoplasm is paler and

there is no brush border.

Renal tubule

This nomenclature is based on the functional areas

of the renal tubule. The proximal convoluted tubule

(PCT) is long and lined with low columnar cells

with a basal nucleus. The cytoplasm is deeply

stained with eosin and the apical surface is a con-

tinuous brush border. The basal plasma membrane

is folded, with mitochondria in the cytoplasm giv-

ing a striated effect, and functions to increase the

surface for transport (cf. salivary glands striated

duct). The PCT is continued with the proximal

straight tubule. It is similar in appearance and

140

Comparative Veterinary Histology with Clinical Correlates

9.10 Kidney medulla (dog).

(1) Capillaries lined by endothelium.

(2) Collecting tubules lined by

cuboidal cells. (3) Ascending limb

lined by cuboidal epithelium.

(4) Descending limb lined by

squamous epithelium. H & E. ×125.

9.10

9.11 Kidney. Renal pelvis (sheep). The renal pelvis is lined

by urethelium resting on a vascular lamina propria. H & E.

×62.5.

9.11

The DCT approaches the glomerulus at the vas-

cular pole, where it thickens, and the cell nuclei of

the tubule wall become crowded together to form

the macula densa, part of the juxtaglomerular appa-

ratus. Juxtaglomerular cells are modified smooth

muscle cells in the walls of afferent arterioles close

to the glomerulus. The cells are epithelioid, contain

granules and produce renin (which plays a role in

the regulation of blood pressure) and angiotensin

(a vasoconstrictor and stimulus of aldosterone

secretion) (9.1–9.3 and 9.9).

Collecting tubule

The collecting tubule or duct (lined with poorly

staining cuboidal epithelium) is the terminal seg-

ment of the nephron, a continuation of the DCT

within the medulla (9.4 and 9.10), joining with oth-

ers to form straight ducts: the papillary ducts (of

Bellini). Here the epithelium becomes columnar, and

then becomes urethelium towards the opening into

the renal pelvis (9.11).