Avian Digestive System Anatomy

Oral Cavity

• Lining epithelium: keratinized stratified squamous
• Tongue: bundles of skeletal muscles, only found at the base of the tongue and floor of the oral cavity, with entoglossal bone

Esophagus

• Lining epithelium: thick non-keratinized stratified squamous
• Lamina propria: loose CT with large mucous glands
• Muscularis mucosae: longitudinally arranged smooth muscle
• Tunica submucosa: thin layer of loose CT
• Tunica muscularis: thick inner circular, thin outer longitudinal

Crop

• Sac-like diverticulum of esophagus, storage organ
• Histologically similar to esophagus, except glands restricted to area near its junction to esophagus

Proventriculus

• "Glandular stomach"
• Mucosa: macroscopic papillae with numerous microscopic folds (plicae)
• Proventricular glands present, adjacent cells in direct contact only on their basal half, only one cell type (oxynticopeptic cells) identifiable
• Lamina propria: thin
• Muscularis mucosae: interrupted by proventricular glands
• Tunica submucosa: contains lobules of compound tubular glands
• Tunica muscularis: typical
• Tunica serosa: typical

Ventriculus (Gizzard)

• "Muscular stomach"
• Lining: cuticle or koilin membrane, not a stratum corneum
• Lamina propria: loose CT
• Tunica submucosa: loose CT, no submucosal glands
• Tunica muscularis: thick layer of smooth muscle
• Tunica serosa: typical

Small Intestine

• Lamina propria and tunica submucosa: contain large amounts of diffuse and nodular lymphatic tissue
• Submucosal glands of the duodenum are generally absent
• Tunica muscularis: third layer of circular smooth muscle may be present
• Villi are present throughout, longest in the duodenum, but gradually shorten and thicken caudally

Cecum (Ceca)

• Two blind sacs attached to the junction of small intestine and large intestine
• Villi: at the orifice, diminish and flatten towards the caudal end
• Cecal tonsils (lymphatic nodule) present at the cecal orifice, infiltrate the lamina propria and submucosa

Cloaca

• 3 regions: coprodeum, urodeum, and proctodeum
• All 3 parts have similar structures, with villi and simple columnar epithelium with a variable number of mucus-secreting cells

Glands Associated with the Digestive System

Salivary Glands

• Made up of a series of secretory units
• Controlled by the autonomic nervous system, sympathetic stimulation results in thick and decreased secretion, while parasympathetic stimulation results in a thin and increased secretion

Major Salivary Glands

• Parotid: paired, located anterior and inferior to the external ear
• Submandibular: paired, located inferior to the mandible in the floor of the mouth
• Sublingual: aggregates of smaller glands located inferior to the tongue

Parotid Gland

• Predominantly serous glands separated by thin connective tissue septa
• Excretory duct: Stensen's duct

Mandibular (Submandibular/Maxillary) Gland

• Well-defined capsule
• Fairly prominent duct system
• Most animals: predominantly serous type, carnivores: predominantly mucous
• Excretory duct: Wharton's duct

Sublingual Gland

• Ill-defined capsule, but has more prominent connective tissue septa
• Mucous cells predominate
• Striated and intercalated ducts: well-developed in horse, ruminants, and pigs, poorly developed in carnivores
• Ducts: Bartholin duct (largest sublingual gland excretory duct) and Rivinus duct (minor sublingual duct)

Liver

• Largest gland in the body
• Capsule and stroma: each lobe covered by serosa (from peritoneum), connective tissue from the capsule extends into the liver lobes, forming interlobular connective tissue that surrounds individual liver lobules, supporting the vascular and bile duct systems
• Hepatic cords: rows of hepatocytes
• Sinusoids: lined by endothelial cells and stellate macrophage (Kupffer cells)

Liver Acinus

• 3 ways to describe in terms of the functional unit: classic lobule, portal lobule, and liver acinus
• Zone 1: closest to the short axis and the blood supply from penetrating branches of the portal vein and hepatic artery, corresponds to the periphery of the classic lobules
• Zone 2: in between 1 and 3, no sharp boundary
• Zone 3: farthest from the short axis and closest to the terminal central vein

Gallbladder

• Hollow pear-shaped organ attached to the liver of most animals
• Epithelium: tall columnar
• Lamina propria: blends with T.submucosa
• Muscularis mucosae: absent
• Tunica muscularis: circular bundles of smooth muscle fibers

Pancreas

• Encapsulated, lobulated, compound tubuloacinar gland
• Exocrine part: enzymes, e.g., amylase, lipase, trypsin
• Endocrine part: mainly insulin and glucagon