Describe the histology of the pancreas, exocrine and endocrine.

Dual Nature of the Pancreas

The Exocrine Pancreas in Detail

• Purely serous acini — no mucous cells at all, unlike the salivary glands
• Centro-acinar cells — pale cells within the lumen of the acinus, the beginning of the intercalated duct; found only in the pancreas and diagnostic of it
• NO striated ducts — which distinguishes it from the parotid, the other purely serous gland
• Duct system — intercalated → intralobular → interlobular → main duct; the intercalated cells secrete bicarbonate under secretin

The Islets of Langerhans

• Islets are more numerous in the tail of the pancreas
• They cannot be distinguished from one another on routine H and E; special stains or immunohistochemistry are needed
• Blood flows from the centre of the islet outward, so insulin reaches the alpha cells at high concentration and suppresses glucagon — an internal feedback built into the vascular arrangement

Protection Against Self-digestion

• Enzymes are secreted as inactive zymogens
• Trypsin inhibitor is packaged with them in the granules
• Activation occurs only in the duodenum, where enteropeptidase converts trypsinogen to trypsin, which then activates the rest
• The duct epithelium secretes bicarbonate, keeping the pH high and the enzymes inactive

Applied Aspects

• Acute pancreatitis — premature intrapancreatic activation of trypsin causes autodigestion, with fat necrosis and haemorrhage; gallstones and alcohol are the usual causes
• Serum amylase and lipase rise; lipase is more specific and stays raised longer
• Chronic pancreatitis — acini are replaced by fibrous tissue while the islets are relatively spared until late, which is why exocrine failure precedes diabetes
• Type 1 diabetes — autoimmune destruction of beta cells with lymphocytic insulitis; type 2 shows amyloid deposition in the islets
• Carcinoma of the head of the pancreas obstructs the bile duct and presents with painless obstructive jaundice and a palpable gall bladder (Courvoisier)