4 Layers from Lumen to Outside:
1. Mucosa = epithelium + lamina propria (loose CT) + muscularis mucosae (thin smooth muscle).
2. Submucosa = denser CT + glands (in some organs) + vessels + Meissner's plexus.
3. Muscularis externa = inner circular + outer longitudinal smooth muscle (skeletal in upper oesophagus); Auerbach's plexus between them.
4. Serosa / Adventitia = outermost layer (peritoneum or fibrous CT).

Variations along GIT:
• Oesophagus: stratified squamous epithelium.
• Stomach: simple columnar with gastric pits + glands.
• Small intestine: simple columnar with villi + crypts; Peyer's patches.
• Large intestine: simple columnar; no villi; many goblet cells.

GASTROINTESTINAL TRACT — HISTOLOGICAL ORGANIZATION

Beyond the oral cavity, the entire GIT shares a 4-layer 'tube within a tube' design that varies in detail with each segment.

I. MUCOSA — innermost; 3 sub-layers:
• Epithelium — lines lumen; type varies (stratified squamous in oesophagus, simple columnar in stomach/intestine).
• Lamina propria — loose CT under epithelium; capillaries, lymphocytes, MALT (gut-associated lymphoid tissue).
• Muscularis mucosae — thin double layer of smooth muscle; produces local mucosal movement.

II. SUBMUCOSA:
• Dense CT with larger vessels, glands (oesophageal mucous, duodenal Brunner's), Meissner's plexus (parasympathetic).

III. MUSCULARIS EXTERNA:
• Two layers of smooth muscle: inner circular + outer longitudinal (3 layers in stomach: oblique + circular + longitudinal).
• Auerbach's (myenteric) plexus between these layers controls peristalsis.
• Cervical oesophagus muscle = SKELETAL (voluntary).
• Distal third of horse oesophagus + entire ox oesophagus = SKELETAL (allows rumination).

IV. SEROSA / ADVENTITIA:
• Serosa (visceral peritoneum) — covers organs in the peritoneal cavity (most of the gut).
• Adventitia (loose CT, no mesothelium) — where organ adheres to body wall (oesophagus in neck, retroperitoneal parts of duodenum/colon).

SPECIFIC HISTOLOGY:

OESOPHAGUS:
• Stratified squamous epithelium (keratinised in ruminants → tough, abrasive resistance).
• Submucosal mucous glands (in dog/cat).
• Muscle: Skeletal in upper part everywhere; mixed/skeletal in distal part of horse; SKELETAL throughout in ox/dog.

STOMACH (Glandular = Abomasum in ruminants):
• Simple columnar epithelium with mucous coat protecting from acid.
• Gastric pits open into gastric glands in lamina propria. Glands have:
– Mucous neck cells (mucus).
– Parietal cells (HCl + intrinsic factor) — large, eosinophilic.
– Chief cells (pepsinogen) — basophilic.
– Enteroendocrine cells (gastrin from G-cells, etc).
• 3 muscle layers (oblique + circular + longitudinal).

RUMINANT FORESTOMACHS (Rumen, Reticulum, Omasum):
• Stratified squamous KERATINIZED epithelium (no glands → 'aglandular').
• Rumen has tall papillae for VFA absorption.
• Reticulum has a honeycomb pattern.
• Omasum has many leaf-like laminae (psalterium).

SMALL INTESTINE:
• Highly absorptive — surface area increased ×600 by:
– Plicae circulares (folds, gross level).
– Villi (mucosal projections, microscopic).
– Microvilli (epithelial brush border, EM level).
• Lined by simple columnar epithelium (enterocytes + goblet cells).
• Crypts of Lieberkühn — tubular glands in lamina propria; contain Paneth cells (innate immunity, lysozyme), enteroendocrine cells, stem cells.
• Duodenum: Brunner's glands in submucosa secrete alkaline mucus (neutralise stomach acid).
• Jejunum: most numerous villi.
• Ileum: Peyer's patches = aggregates of lymphoid tissue in submucosa, covered by M-cells (sample antigens).

LARGE INTESTINE:
• Lined by simple columnar epithelium; NO villi.
• Many goblet cells (mucus for lubrication).
• Crypts (no Paneth cells in most species).
• Solitary lymphoid nodules in submucosa.
• Outer longitudinal muscle thickened into taeniae coli in horse → causing sacculations (haustra). Cattle have a complete longitudinal layer.