1. TONGUE


The tongue is a muscular organ covered by a mucous membrane. The epithelium is stratified squamous with varying thickness of the stratum corneum. It is thickest on the dorsal surface and thinnest on the ventral surface, where it may be nonkeratinized. The dorsal surface contains numerous papillae with a mechanical (filiform, conical and lenticular papillae) or gustatory function (fungiform, circumvallate and foliate papillae). The circumvallate papillae are surrounded by a moat-like trough or vallum and is level with the surface of the tongue; the epithelium on the papillary side of the moat contains many taste buds, and deep to the moat lie groups of gustatory serous glands. The lingual muscle mass is composed of longitudinally, transversely, and vertically arranged bundles of striated muscle. Adipose tissue can be identified scattered among the muscle fibers.

Filiform papillae (dog)



Filiform papilla (cat)



Conical and fungiform papillae



Circumvallate papillae
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Taste buds
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Gustatory serous glands

Lingual muscle fibers and adipose tissue

2. ESOPHAGUS

The mucosa is composed of three layers: a stratified squamous epithelium, a connective lamina propria, and a muscularis mucosae. The degree of keratinization of the stratified squamous epithelium varies with the species (usually nonkeratinized in carnivores, slightly keratinized in the pig, more so in the horse, and keratinized in ruminants and rodents). The muscularis mucosae contains only longitudinally oriented smooth muscle bundles.

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Rat





Ruminant

The submucosa is loose connective tissue and may contain mucous glands (depending on the species and anatomical location).

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The tunica muscularis consists of the inner circular muscle layer and the outer longitudinal muscle layer. They are striated or smooth muscle layers depending on the species and esophagic region.
The outer tunica is a tunica adventitia or serosa dependending on the esophagic region.

3. AVIAN CROP

The crop is an aglandular caudal diverticulum situated 2/3 of the way down the esophagus. It is lined by a stratified squamous nonkeratinized epithelium. The epithelium consists of numerous layers. Columbiforms have well-developed mucous glands.

4. RUMEN

The mucosa consists of a stratified squamous keratinized epithelium and a lamina propria There is not muscularis mucosae in any of the ruminal papillae. The submucosa is composed of loose connective tissue and blends into the lamina propria without any distinct line of demarcation. The tunica muscularis is composed of two layers (circular inner and longitudinal outer) of smooth muscle. The tunica serosa is composed of collagen and elastic connective tissue covered by a mesothelium.

5. RETICULUM

The histological structure is similar to that of the rumen, except for:
A well-developed band of smooth muscle (muscularis mucosae) can be identified in the upper part of the reticular folds (the taller folds that separate the mucosal surface into shallow compartments). The small folds or papillae do not have muscularis mucosae.

6. OMASUM

The histological structure is similar to that of the rumen and reticulum, except for:
The omasum is organized into numerous large longitudinal laminae. A muscularis mucosae forms a complete and thick layer just beneath the lamina propria of all laminae. In the large omasal laminae the inner circular layer of smooth muscle belonging to the tunica muscularis is continued into the omasal laminae.

7. AVIAN GIZZARD (MUSCULAR VENTRICULUS)

The luminal surface is lined with secretory product of the mucosal glands, which solidifies at the surface to form a hard cuticle of koilin (keratinoid substance). The epithelium is simple columnar and continues within the simple straight tubular mucosal glands in the lamina propria. A submucosa is present, and the tunica muscularis is a thick layer of smooth muscle. There is no muscularis mucosae.

8. AVIAN PROVENTRICULUS

The gastric epithelium of the proventriculus is simple columnar and mucous-secreting. A thin lamina propria separates it from the lobules of the submucosal glands. Each gland lobule contains a central cavity with straight secretory tubules radiating to the interlobular connective tissue. An excretory duct drains onto the gastric mucosal surface. The glands contain only one type of cell, which secretes acid and pepsinogen. The tunica muscularis is arranged as inner circular and outer longitudinal layers of smooth muscle.