PANCREAS ANATOMY AND HISTOLOGY

Gross Anatomy

1. Location and Position
   • Retroperitoneal organ, 15–20 cm long, 75–100 g weight.
   • Oblique position: from duodenum (right) to splenic hilum (left).
   • Four Regions: head, neck, body, tail.
   • Head: resides in C-loop of duodenum, anterior to vena cava, right renal artery, both renal veins.
     • Uncinate process = posteromedial aspect, behind portal vein & SMA/SMV.
   • Neck: anterior to portal vein, at L1–L2 level.
   • Body: extends anterior to aorta (origin of SMA).
   • Tail: left end, near spleen, anterior to left kidney, over splenic hilum.
2. Peritoneal Cover
   • Anterior surface covered by peritoneum.
   • Base of transverse mesocolon attaches to inferior margin of body/tail.

Embryologic Origin

1. Fusion of Ventral & Dorsal Buds
   • Ventral bud → duct of Wirsung (main pancreatic duct), becomes inferior portion of head + uncinate process.
   • Dorsal bud → duct of Santorini (accessory), becomes body & tail.
2. Duct Anatomy
   • Main duct (Wirsung) joins common bile duct → ampulla of Vater → empties into duodenum (2nd part).
   • Sphincter of Oddi controls flow of bile & pancreatic juice at ampulla of Vater.

Blood Supply and Drainage

1. Arteries
   • Branches from celiac trunk (via splenic artery & common hepatic/gastroduodenal) and SMA.
   • Head & Uncinate: gastroduodenal artery branches, SMA branches.
   • Body & Tail: branches of splenic artery & inferior pancreatic artery (from SMA) → form arterial arcades.
2. Veins
   • Head region: anterior & posterior venous arcs → drain to portal & mesenteric veins.
   • Body & Tail: venous outflow → splenic vein → portal system.
3. Lymphatics
   • Dense lymphatic network draining to local/peripancreatic LNs.

Innervation

1. Sympathetic & Parasympathetic
   • Innervate acinar (exocrine) cells, islet (endocrine) cells, vasculature.
   • Parasympathetic → stimulates secretions (endo & exo).
   • Sympathetic → generally inhibitory.
2. Neurotransmitters & Peptides
   • Peptides/ amines (somatostatin, galanin, VIP, CGRP) released by pancreatic nerves.
   • Afferent sensory fibers → explain severe pain in pancreatitis.

Histologic Components

1. Tissue Distribution
   • ~85% exocrine, 2% endocrine, 10% extracellular matrix, 3–4% vessels & ducts.
2. Exocrine Portion
   • Acinar cells in clusters/lobules → secrete digestive enzymes into ducts → main pancreatic duct → duodenum.
   • Acinar cells: abundant RER, apical zymogen granules.
3. Endocrine Portion (Islets of Langerhans)
   • Scattered small clusters in the acini.
   • 3 main cell types:
     • β-cells (75%) → insulin
     • α-cells (20%) → glucagon
     • δ-cells (5%) → somatostatin
   • Other islet cells: δ2-cells (VIP), PP-cells (pancreatic polypeptide).

EXOCRINE FUNCTIONS OF THE PANCREAS

1. Pancreatic Juice
   • ~1 L/day, alkaline, isosmotic, rich in HCO₃⁻ & proteins.
   • Functions: neutralize gastric acid in duodenum & digest macronutrients.
2. Digestive Enzymes
   • Amylase: degrades starch, glycogen → maltose, dextrins.
   • Proteases (secreted as proenzymes): trypsinogen (→ trypsin by duodenal enterokinase), chymotrypsinogen, elastase, carboxypeptidases → degrade proteins to peptides/amino acids.
     • Pancreatic secretory trypsin inhibitor prevents autoproteolysis.
   • Lipolytic Enzymes: lipase, phospholipase A2, etc. → break down fats into FAs, glycerol, micelles.
3. Acinar Architecture
   • ~40 acinar cells/acinus, plus centroacinar cells at center near duct origin → secrete electrolytes, bicarbonate.
4. Regulation of Secretion
   • Basal rate ~2% of max; Meal-stimulated phases:
     • Cephalic (10%): via sight/smell of food, mediated by cholinergic input.
     • Gastric (10%): distention → gastrin + vagal reflex → moderate enzyme/bicarbonate secretion.
     • Intestinal (80%): acid/fat/protein in duodenum → secretin (↑ HCO₃⁻ secretion), CCK (enzyme secretion), synergy for maximum output.

NORMAL HISTOLOGY OF PANCREATIC ISLETS

1. Islet Distribution & Size
   • ~1 million islets, mostly in tail region. Size 40–300 µm.
   • Each islet ~3000 cells; total islet mass ~1 g.
2. Cell Types & Arrangement
   • β-cells: 75%, central in islet, produce insulin (Ponceau/Gomori stain: bluish-purple granules).
   • α-cells: 20%, peripheral in islet, produce glucagon (stain pink/red).
   • δ-cells: 5%, smaller, produce somatostatin.
   • PP-cells: produce pancreatic polypeptide (mainly in head).
3. Insulin
   • 56–AA peptide (two chains + disulfide bonds), discovered in 1920.
   • Regulated by plasma glucose, neural signals, paracrine factors.
   • Enhances glucose uptake, glycogen storage, protein synthesis; inhibits gluconeogenesis, lipolysis, ketogenesis.
4. Glucagon
   • 29–AA single chain, antagonizes insulin, promotes hepatic glycogenolysis & gluconeogenesis.
   • Inhibited by high glucose, GLP-1, insulin, somatostatin.
5. Somatostatin
   • 14 or 28–AA forms, generally inhibits hormone secretion (insulin, glucagon, GH, etc.).
6. Pancreatic Polypeptide (PP)
   • 36–AA hormone, inhibits gallbladder contraction, bile & pancreatic exocrine secretions; modulates insulin receptor.
7. Amylin (IAPP)
   • 37–AA co-secreted w/ insulin.
   • Slows gastric emptying, inhibits digestive secretions, suppresses glucagon release.