Digestive Glands

The salivary glands do something deceptively complex: they take roughly a litre of plasma-derived fluid each day, dress it up with digestive enzymes, lubricating mucins, antibodies and electrolytes, and deliver it to your mouth on demand. To get all of that into one organ they use a modular plan — acini (the secretory berries on the end of branches) connected to a tree of ducts that progressively modify what the acini just produced.

You meet three pairs of major glands and dozens of minor ones scattered through your lips, palate and tongue. The big three each pick a personality. The parotid is purely serous — watery, enzyme-rich, full of zymogen granules and famously dotted with adipocytes that lift out as clear holes on H&E. The submandibular is mixed but mostly serous, throwing crescent-shaped serous demilunes onto mucous acini. The sublingual flips that around — mostly mucous with a few serous caps.

Wrapping the acinus — squeezed between the secretory cells and the basement membrane — sits the myoepithelial cell, the “basket cell.” It has long branching arms full of actin and myosin and behaves like a hand cupped around a juice sac: when secretomotor nerves fire, it squeezes the acinus and helps fling its contents into the duct. Myoepithelial cells also coat the smallest intercalated ducts but disappear by the time you reach the striated duct.

Compound exocrine glands with serous, mucous or mixed acini, a duct system (intercalated → striated → excretory) and myoepithelial cells.

Saliva does not leave the acinus the same fluid it arrives at the mouth. The duct tree edits it. Intercalated ducts (simple cuboidal) collect raw secretion. Striated ducts (tall columnar with basal infoldings packed with mitochondria) actively pump Na⁺ back out of the saliva and dump K⁺ and HCO₃^- in, leaving the final saliva hypotonic and slightly alkaline — recall the basal membrane infoldings you saw in Unit 2 (epithelia). The excretory duct, lined initially by stratified columnar that turns into stratified squamous as it approaches the oral mucosa, just carries the finished product.

The pancreas is two organs jammed into one capsule. The bulk of the tissue is a vast serous exocrine gland that pours digestive enzymes into the duodenum; sprinkled through that exocrine sea are pale islands of endocrine cells — the islets of Langerhans — that quietly run blood-glucose homeostasis. Histologically you can usually tell which is which from across the room: exocrine acini are deep purple and grainy; islets look like pale pink ghosts because their cells secrete hormones straight into capillaries and so do not stockpile zymogen.

Each exocrine acinus is a cluster of serous cells with the same architecture as the parotid — basal RER making zymogen, apical granules waiting for cholecystokinin to release amylase, lipase, trypsinogen, chymotrypsinogen and the other proenzymes. The proenzymes part matters: the pancreas dares not activate trypsin inside itself, so it ships everything inactive and lets the duodenal brush border enzyme enterokinase fire the cascade outside the gland.

From there the duct system is intercalated → intralobular → interlobular → main pancreatic duct (of Wirsung). The duct cells — not the acinar cells — secrete the alkaline, bicarbonate-rich watery fluid that neutralises gastric acid in the duodenum. There are no striated ducts here. That is a reliable exam discriminator: striated duct = salivary; centroacinar cell = pancreas.

A mixed gland: a large exocrine part + scattered endocrine islets.

The islets themselves are tightly organised. Beta cells, which make insulin, dominate the centre — about 70% of the islet. Around the rim you find alpha cells producing glucagon (about 20%) and delta cells producing somatostatin (about 5%). PP cells and the rare epsilon (ghrelin) cells are scattered. The islet capillary network is fenestrated and the cells release hormones straight through it.

The liver is the body's central biochemical factory — it makes plasma proteins, stores glycogen, detoxifies xenobiotics, manufactures bile and filters incoming gut blood through a resident army of macrophages. To do all of that, its 1.5 kg of tissue is built from one repeating module so simple that exam questions revisit it every year: the hepatic lobule.

Anatomists have argued for a century about which way to draw that module. Three competing definitions exist — each emphasising a different function — and you need to know all three. The classic lobule centres on a central vein and is the easiest to see on slides. The portal lobule centres on a portal triad and follows bile drainage. The liver acinus of Rappaport centres on a terminal portal venule and explains why hepatocytes near the central vein die first in shock. We will build them up in that order.

Now zoom into the classic lobule. From the central vein, one-cell-thick plates of polygonal hepatocytes radiate outward like spokes on a wheel, branching and anastomosing as they go. Between every two plates runs a sinusoid — a wide, leaky capillary that carries mixed portal-venous and hepatic-arterial blood centripetally toward the central vein. At the six corners of the hexagon sit the portal triads: a branch of the hepatic artery, a branch of the portal vein and a bile duct, glued together in connective tissue.

11.3.1 — Sinusoid, Space of Disse & Bile Canaliculi

The sinusoid is the liver's signature capillary. It is deliberately leaky — discontinuous, with large fenestrations and almost no basal lamina — so that plasma can wash directly over the hepatocyte surface and exchange anything short of an erythrocyte. Between the sinusoidal endothelium and the hepatocyte microvilli sits a thin gap called the perisinusoidal space of Disse. That space is where the real metabolic traffic happens.

Three cell types share the sinusoidal wall and the space of Disse. Endothelial cells form the leaky lining. Kupffer cells, fixed macrophages of the mononuclear phagocyte system, sit inside the sinusoidal lumen and phagocytose old red cells, bacteria carried up from the gut, and immune complexes. Hepatic stellate cells of Ito live inside the space of Disse; in health they store vitamin A in lipid droplets, but in chronic injury they transform into myofibroblasts that lay down collagen — the engine of cirrhosis.

◆ Intuition — sinusoid as a river through a market

The sinusoid is a slow muddy river with deliberately broken banks. Plasma soaks out into the marketplace (space of Disse) where hepatocytes lean over their stalls and exchange goods. Kupffer cells are river police boats patrolling for invaders. Ito cells are the warehouse keepers on the bank — calm in peacetime, but they bring out the bricks and start building walls (fibrosis) when there is trouble.

Bile takes the opposite path. Between any two adjacent hepatocytes the apposed plasma membranes form a tiny groove sealed shut by tight junctions, with microvilli sticking into the lumen — that is the bile canaliculus. It is too small to see in routine H&E and is classically demonstrated by silver impregnation. Bile flows from these canaliculi centrifugally (outward, away from the central vein) to the lobule edge, drains into a transition channel called the canal of Hering, and empties into the bile duct of the portal triad.

• Perisinusoidal space (of Disse): between the sinusoidal endothelium and the hepatocyte; contains plasma, hepatocyte microvilli and hepatic stellate (Ito) cells that store vitamin A — site of metabolic exchange.
• Bile canaliculi: tiny channels formed by the indented plasma membranes of two adjacent hepatocytes, sealed by tight junctions, with microvilli; shown by silver impregnation. Bile flows away from the central vein toward the portal triad.
• Kupffer cells: sinusoidal macrophages (MPS) that phagocytose old erythrocytes & debris.

★ Rapid recall Sinusoid = discontinuous + fenestrated, no BL. Space of Disse = between endothelium and hepatocyte microvilli — holds Ito cells (vit A, fibrogenic in cirrhosis). Kupffer = sinusoidal lumen macrophages. Bile canaliculi = grooves between adjacent hepatocytes sealed by tight junctions, demonstrated by silver. Bile flows OUT to triad.

The gallbladder has one job: hold the bile the liver makes between meals, concentrate it 5–10 fold, and squeeze it into the duodenum when a fatty meal arrives. Its histology is shaped entirely around that storage-and-concentration brief. Importantly, the wall is unusual: it has no muscularis mucosae and no submucosa, so the textbook gut-tube plan you learnt in Unit 10 does not apply here. Going from lumen out you meet only mucosa → muscularis → perimuscular connective tissue → adventitia/serosa.

The mucosa is folded into low villus-like ridges when the bladder is empty and flattens out when full. It is lined by a simple columnar epithelium covered in apical microvilli, supported by mitochondria-rich cytoplasm. Those mitochondria power Na⁺/K⁺-ATPase pumps along the lateral membranes; sodium is pumped out into the lateral intercellular space, water follows by osmosis, and bile is concentrated as it sits in the lumen. There are no goblet cells and no glands — the gallbladder absorbs, it does not secrete.

A peculiar feature unique to the gallbladder is the Rokitansky–Aschoff sinus: a mucosal diverticulum that punches through the lamina propria and dives into the muscle layer. A few exist normally; they multiply and deepen in chronic cholecystitis, where they become little pockets in which bile, bacteria and even stones can lodge.

A pear-shaped musculomembranous sac that stores and concentrates bile (5–10×). Wall has four layers (no muscularis mucosae, no submucosa).