Histology - Liver & Pancreas

Study Notes

Histology - Liver & Pancreas

The liver is a solid organ composed of tightly packed hepatocytes, pink-staining plates.
The outer surface of the liver is covered by a collagenous tissue called Glisson's capsule.
A layer of mesothelial cells from the peritoneum overlies the Glisson's capsule.
The liver's structural integrity is maintained by a delicate meshwork of reticulin fibers.
These reticulin fibers are continuous with Glisson's capsule.
The hepatic lobule is the liver's structural unit, hexagonal in shape, centered on a terminal hepatic venule (central vein).
Portal tracts (portal triads) are positioned at the angles of the hepatic lobule and contain a terminal branch of the hepatic portal vein, a terminal hepatic artery branch, and bile ductules.
Lymphatics are also present in the portal tracts.
Bile ductules merge to form larger trabecular ducts, which drain via intrahepatic ducts into the right and left hepatic ducts, then the common hepatic duct and finally to the duodenum through the common bile duct.
Hepatocytes are arranged in anastomosing plates or double-layered rows radiating from the central vein.
Hepatic sinusoids are lined by a discontinuous, fenestrated endothelium.
The space of Disse is a narrow space separating the endothelium from the hepatocytes.
Kupffer cells, phagocytic cells, are scattered among the endothelial cells. They are part of the monocyte-macrophage defense system.
Hepatocytes are large, polyhedral cells with round nuclei and peripherally dispersed chromatin.
Binucleate cells are common in normal liver.
The layer of hepatocytes bordering the portal tract is known as the limiting plate.
Blood from the portal vein and hepatic artery branches flows away from the portal tract to the adjacent central veins.
In some species (like pigs), the hepatic lobule is outlined by fibrous tissue bands. In humans and most species, lobules are roughly hexagonal, and portal tracts surround the terminal hepatic venule.
The hepatic acinus is a more useful physiological model, roughly berry-shaped and centered on a portal tract.
The gall bladder is a muscular sac lined by a simple columnar epithelium with a capacity of about 100 mL in humans.

Pancreas

The exocrine pancreas forms the bulk of the gland and secretes enzyme-rich alkaline fluid into the duodenum via the pancreatic duct.
A thin collagenous capsule surrounds the pancreas, with delicate septa extending between the lobules.
Each pancreatic acinus comprises pyramid-shaped secretory acinar cells surrounding a central lumen.
Acinar cells drain into intercalated ducts.
Intercalated ducts empty into intralobular ducts.
The acinus contains two types of cells.
Acinar cells (typical protein-secreting, pyramidal-shaped cells) with basally located nuclei, surrounded by basophilic cytoplasm with rough endoplasmic reticulum. Their apices are packed with eosinophilic secretory granules containing proenzymes.
Centroacinar cells (pale nuclei and sparse pale-stained cytoplasm) are the terminal cells of the intercalated ducts.
The endocrine component (Islets of Langerhans) is scattered throughout the exocrine glandular tissue. The islets are composed of groups of up to 3000 secretory cells with a delicate capsule.
Endocrine cells in the islets are smaller and have pale, granular cytoplasm compared to the surrounding exocrine cells.
Islets contain several cell types each producing polypeptide hormones (e.g., insulin from beta cells, glucagon from alpha cells), impacting carbohydrate metabolism.
Other hormones from islet cells include somatostatin (from delta cells) and peptides (motilin, serotonin, substance P, VIP, pancreatic polypeptide, etc)