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

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101

8.10 Mixed salivary gland (dog).

(1) Interlobular duct lined by a

stratified columnar epithelium.

(2) Connective tissue stroma.

(3) Parasympathetic ganglion.

(4) Seromucous acini. H & E. ×125.

8.10

8.11 Mixed salivary gland (dog).

(1) Pale staining triangular mucous

cells with a basal nucleus. (2) Deeply

staining serous cells with a round

nucleus. Gomori’s trichrome. ×125.

8.11

8.13 Soft palate (ox). (1) Respiratory epithelium.

(2) Lymph nodule. (3) Bone. H & E. ×62.5.

8.13

8.12 Hard palate (ox). (1) Stratified squamous

epithelium. (2) Lamina propria. (3) Mucus-secreting

glands in the lamina propria. H & E. ×125.

8.12

Digestive System

Teeth

In the embryo teeth develop in the ectoderm as den-

tal papillae within the enamel organs (8.14). The

mesoderm invaginates each enamel organ into a bell

shape, with an inner enamel epithelium of

ameloblasts laying down enamel continuous with

the outer enamel epithelium and enclosing the stel-

late reticulum (8.15). The mesenchymal cells of the

papilla differentiate to become odontoblasts, the

dentine-forming cells (8.16) and cementoblasts, and

secrete cementum in a similar pattern to that of

bone. Enamel and dentine are involved with the cre-

ation of the crown; dentine and cementum are

involved in the root. The root is formed by an

extension of the enamel organ at the junction of the

inner and outer enamel epithelium: the root tubule

(8.17 and 8.18). The tooth is held in the develop-

ing mandible and maxilla by the periodontal mem-

brane of collagen fibres embedded in the cementum.

Temporary teeth develop first. The permanent teeth

are secondary offshoots on the lingual side of the

102

Comparative Veterinary Histology with Clinical Correlates

8.14 Developing tooth (cat embryo). (1) Oral epithelium.

(2) Enamel organ surrounded by mesoderm. H & E. ×12.5.

8.14

8.15 Developing tooth (cat embryo). (1) Mesenchymal

papilla with a layer of odontoblasts (arrowed). (2) Inner

enamel epithelium, continuous with (3) the outer enamel

epithelium. (4) Stellate reticulum. H & E. ×62.5.

8.15

103

8.16 Developing tooth (cat embryo). (1) Mesenchymal

dental papilla. (2) Ameloblasts in the inner enamel

epithelium. (3) Outer enamel epithelium. (4) Stellate

reticulum. H & E. ×62.5.

8.16

8.17 Developing tooth (cat embryo). (1) Mesenchymal

dental papilla lined by odontoblasts. (2) Inner enamel

epithelium, ameloblasts, turns to become (3) outer

enamel epithelium at the point of root development

(arrowed). H & E. ×125.

8.17

8.18 Developing tooth (cat). The ameloblasts are

tall columnar cells with a basal nucleus (arrowed).

H & E. ×125.

8.18

Digestive System

104

8.19 Diagram of an incisor.

8.19

Enamel

Dentine

Pulp cavity

Periodontal

membrane

Alveolar

bone

Root

Crown

8.20 Diagram of a molar.

8.20

Enamel

Cementum

Dentine

Pulp cavity

Enamel

Cementum

Dentine

Pulp cavity

Maxillary/

mandibular

bone

Root

Crown

8.21 Oropharynx. (1) Stratified

squamous epithelium. (2) Mucus-

secreting glands open onto the

surface (arrowed). H & E. ×62.5.

8.21

temporary teeth. The tooth is divided into a crown

and a root (8.19). In the carnivore, teeth cease to

grow after eruption and the ameloblast layer is lost:

brachydont teeth. In the horse, ruminant and pig,

teeth are much longer and continue to grow for all

or part of adult life: hypsodont teeth. In these the

dental sac covers the whole of the tooth before

eruption and cementum covers the entire tooth, pre-

venting loss of the ameloblasts and allowing con-

tinuing deposition of enamel and cementum and

thus allowing for the wear and tear in these species

(8.20), which include rodents and lagomorphs.

Oropharynx

This is a short junctional area between the oral cav-

ity and the alimentary canal with some mucus-

secreting glands (8.21).

Comparative Veterinary Histology with Clinical Correlates

8.22

Alimentary canal

The alimentary canal (digestive tract) is a muscular

tube extending from the oropharynx to the anus,

comprising the oesophagus, stomach, and small and

large intestines. Two large glands, the liver and pan-

creas, are also derived from the embryonic alimen-

tary canal. Each part of the canal has a specific

function and the histology reflects this (8.23). The

canal wall is derived from endoderm and splanchnic

mesoderm and consists of four layers:

• Tuna mucosa (mucous membrane), with epithe-

lial lining, supporting vascular lamina propria

and lymphatic cells. Mucosal glands, which are

derived from the epithelium, are variably present.

The outer muscularis mucosae (absent from the

mouth, pharynx, portions of the oesophagus and

rumen) is smooth muscle.

• Tela submucosa: a connective tissue layer with

lymphatic tissue and nerve plexi. Submucosal

glands may be present.

• Tunica muscularis: smooth muscle (except in the

oesophagus and the anus where the muscle is vol-

untary, striated muscle).

• Tunica adventitia/serosa (within the peritoneal

cavity) or adventitia (retroperitoneal).

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Digestive System

8.23 Diagram.TS. Alimentary

canal.

8.23

Extratubular glands

(liver and pancreas)

Serosa/adventitia

Mucosa

Muscularis

8.22 Plasmacytic gingivitis (dog). H & E. ×125.

Clinical correlates

Any level of the intestinal tract can be affected

by inflammation. Gingivitis, or inflammation of

the gums, is common in dogs and cats, often in

association with dental disease.

A gingival biopsy from a dog in which the gin-

gival epithelium is irregularly hyperplastic is

shown in 8.22. A dense inflammatory infiltrate

occupies the superficial submucosa and extends

into the mucosal epithelium. Plasma cells (mature

immunoglobulin-secreting cells) predominate in

the infiltrate. Their presence indicates a persistent

antigenic stimulus. The initiating disease may be

local or systemic.

Oesophagus

The oesophagus is lined with stratified squamous

epithelium, and both mucosal and submucosal

mucus- or seromucus-secreting glands may be pre-

sent. In ruminants the muscularis externa is skele-

tal muscle; in the pig and cat the distal part is

smooth muscle (8.24–8.27).

106

8.24 Oesophagus (cat). (1) Stratified squamous

keratinized epithelium. (2) Lamina propria. (3) LS vein.

(4) Inner layer of circular smooth muscle. (5) Outer layer

of longitudinal smooth muscle. H & E. LP.

8.24

8.25 Oesophagus (dog). (1) Stratified squamous

epithelium. (2) Mucus-secreting glands in the lamina

propria. (3) Inner layer of circular skeletal muscle. (4) Outer

layer of longitudinal skeletal muscle. H & E. ×12.5.

8.25

8.26 Oesophagus (cat). (1) Stratified squamous

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

(4) Submucosal mucous glands. (5) Muscularis externa.

Masson’s trichrome. ×12.5.

8.26

Comparative Veterinary Histology with Clinical Correlates

Stomach

The stomach mucosa may be non-glandular or glan-

dular in domestic animals. In the simple stomach of

the dog and cat, the mucosa is glandular. In the pig

and horse, there is a non-glandular (oesophageal)

region and a glandular region (8.28 and 8.29). In the

ruminant the non-glandular forestomach has three

compartments: the rumen, the reticulum and the

107

Digestive System

8.27 Oesophagus (dog). (1) Stratified squamous

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

(4) Submucosal mucous glands. (5) Striated muscle of the

muscularis externa. H & E. ×62.5.

8.27

8.28 Oesophageal/stomach junction (horse). The

epithelium changes abruptly from stratified squamous to

simple columnar at the junction (arrowed). H & E. ×62.5.

8.28

8.29 Oesophageal/stomach junction (horse). The

epithelium changes abruptly from stratified squamous to

simple columnar at the junction (arrowed). H & E. ×100.

8.29

omasum; the glandular stomach is a separate com-

partment: the abomasum (8.30–8.35).

The non-glandular stomach is lined with a strat-

ified squamous epithelium with some keratiniza-

tion. In the ruminant clear vacuolated cells in the

epithelium give it a distinctive appearance and allow

the transfer of water, electrolytes and short-chain

fatty acids. Muscularis mucosae is present in the

omasum and reticulum, but absent from the rumen.

The glandular mucosa of the stomach is folded

and lined with a simple columnar mucus-secreting

epithelium. The gastric pits, the foveoli, are surface

depressions continuous with the simple tubular gas-

tric glands (8.36). Three histological regions are

108

8.30 Rumen (sheep). (1) Stratified

squamous epithelium lines the

rumen. (2) Lamina propria. Masson’s

trichrome. ×12.5.

8.30

8.33 Omasum (sheep). The muscularis mucosae is present

in the long omasal folds (arrowed). H & E. ×12.5.

8.33

8.31 Rumen (goat). The lining epithelium is stratified

squamous. The lamina propria is loose connective tissue,

stained green. Gomori’s trichrome. ×12.5.

8.31

8.32 Reticular groove (goat). The mucosa is folded; this

allows stretching. The lining epithelium is stratified

squamous. H & E. ×12.5.

8.32

Comparative Veterinary Histology with Clinical Correlates

109

Digestive System

8.34 Abomasum. Fundic region (goat). (1) Mucosa.

(2) Muscularis mucosae. (3) Submucosa. (4) Muscularis

externa. Masson’s trichrome. ×25.

8.34

8.35 Abomasum. Pyloric region (goat). (1) The lining

epithelium is simple columnar and mucus secreting.

(2) Simple tubular mucus-secreting glands. (3) Muscularis

mucosae. (4) Submucosa. H & E. ×62.5.

8.35

8.36 Stomach (horse).

(1) Simple columnar

mucus-secreting

epithelium. (2) Gastric

pits. H & E. ×62.5.

8.36

8.37

110

8.37 Cardiac glands. Stomach

(horse). Simple columnar mucus-

secreting epithelium lines the

stomach (arrowed). (1) Parietal

(oxyntic) cell, deep red staining.

(2) Zymogen (chief) cell, basophilic

staining. (3) Lamina propria.

(4) Muscularis mucosae. H & E. ×125.

8.38 Cardiac glands. Stomach

(horse). (1) Parietal cell. (2) Zymogen

cell. (3) Lamina propria. H & E. ×125.

8.38

8.39 Fundic glands. Stomach (horse).

Simple mucus-secreting epithelium

extends into the gastric pits

(arrowed). (1) Parietal cell.

(2) Zymogen cell. H & E. ×250.

8.39

Comparative Veterinary Histology with Clinical Correlates