LIVER:
• Functional unit = hepatic lobule (hexagonal); central vein + portal triads at corners.
• Portal triad = bile duct + hepatic artery + portal vein branch.
• Hepatocytes arranged in cords; sinusoids between cords lined by Kupffer cells (macrophages).

PANCREAS:
• Exocrine (serous acini → digestive enzymes via duct system).
• Endocrine = scattered Islets of Langerhans (α — glucagon; β — insulin; δ — somatostatin).

SALIVARY GLANDS:
• Serous (parotid) — watery secretion; deep purple acinar cells.
• Mucous (sublingual) — pale mucus cells.
• Mixed (mandibular) — both, sometimes with serous demilunes on mucous acini.

ACCESSORY DIGESTIVE GLANDS

I. LIVER:
The largest gland; both exocrine (bile) and metabolic.

Three ways to view the liver microscopically:
1. Classic Hepatic Lobule (most common):
• Hexagonal; central vein in middle; portal triads (= bile duct + hepatic artery branch + portal vein branch + sometimes lymph vessel) at the corners.
• Hepatocytes arranged in radiating cords/plates of one cell thick (in adult).
• Between cords run sinusoids — wide capillaries lined by fenestrated endothelium + Kupffer cells (resident macrophages).
• Blood flow: portal triad → sinusoid → central vein → hepatic vein → CVC.
• Bile flow: hepatocyte → bile canaliculi (between hepatocytes) → bile ducts at portal triad → larger ducts → gall bladder/duodenum.
2. Portal Lobule: Triangular; portal triad at center; central veins at corners. Emphasises bile drainage.
3. Hepatic Acinus (of Rappaport): Diamond-shaped; physiologically meaningful — divides cells into zones 1 (periportal, most O₂), 2 (mid), 3 (centrilobular, least O₂). Zone 3 is most prone to ischemia + drug toxicity.

HEPATOCYTES:
• Polygonal cells with central round nucleus (often binucleate); abundant cytoplasm with mitochondria + RER + SER + Golgi + glycogen + lipid droplets.
• Functions: bile production, plasma protein synthesis (albumin, clotting factors), glycogen storage, drug metabolism, urea cycle.

SPACE OF DISSE: Narrow space between sinusoid endothelium and hepatocyte; contains Ito cells (hepatic stellate cells) — store vitamin A; activate to fibroblasts in chronic liver injury → cirrhosis.

GALL BLADDER:
• Simple columnar epithelium (no goblet cells); thin lamina propria; thin muscularis; serosa.
• ABSENT in horse.

II. PANCREAS:
Exocrine portion (~98% by weight):
• Serous acini — pyramidal cells with basophilic basal cytoplasm + apical eosinophilic zymogen granules (digestive proenzymes).
• Centroacinar cells — pale cells inside acinus; start of duct system. Unique to pancreas — makes alkaline (HCO₃⁻) fluid.
• Ducts: intercalated → intralobular → interlobular → main pancreatic duct.

Endocrine portion (~2%):
• Islets of Langerhans = pale clusters scattered among acini.
• Cell types (need special stains):
– α-cells (peripheral, ~20%) — Glucagon (raises blood glucose).
– β-cells (central, ~70%) — Insulin (lowers blood glucose).
– δ-cells (~5-10%) — Somatostatin (paracrine inhibitor).
– PP cells — Pancreatic polypeptide.

III. SALIVARY GLANDS:
Compound tubuloalveolar glands. Three types of secretory unit:
• Serous acinus: small spherical; cells pyramidal; deep blue-purple (basophilic) cytoplasm; central duct lumen narrow. Secretion: watery, with amylase, lysozyme, IgA. Example: Parotid gland.
• Mucous acinus / tubule: tubular shape; cells columnar; PALE cytoplasm (mucin extracted by histology processing); flattened basal nuclei. Secretion: thick, glycoprotein-rich. Example: Sublingual.
• Mixed acinus: mostly mucous, capped by a crescent of serous cells = 'serous demilune' (of Heidenhain). Example: Mandibular.

DUCT SYSTEM:
• Intercalated duct → striated (intralobular) duct → excretory duct.
• Striated ducts have basal striations (actually columns of mitochondria between basal infoldings) → modify Na⁺/K⁺ in saliva.