19 - Glandular GI

What is the pancreas, where is it located, and what are its functions?

The pancreas is a large retroperitoneal organ located behind the stomach. It has both endocrine and exocrine functions.

What is the structure of the stroma in the exocrine pancreas?

The stroma consists of subcapsular loose connective tissue and has incomplete septa, which lead to incomplete separation of lobules.

What is the parenchyma of the exocrine pancreas composed of, and what are its cytoplasmic features?

The parenchyma is a compound tubuloacinar gland composed entirely of serous cells. The cytoplasm is very basophilic due to a high rate of protein synthesis.

What are the main secretory products of the exocrine pancreas, and how are they secreted?

The exocrine pancreas secretes digestive enzymes including:

• Trypsinogen and chymotrypsinogen (proteases in zymogen form)

• Ribonucleases and deoxyribonucleases

• Pancreatic amylase

• Pancreatic lipase
  These enzymes are secreted in an alkaline sodium bicarbonate solution produced by duct cells.

What is the duct pattern and epithelial structure of the exocrine pancreas?

Intercalated ducts: simple squamous or low cuboidal epithelium
→ Intralobar ducts: cuboidal epithelium
→ Interlobar ducts: cuboidal epithelium

What are the general features of the endocrine pancreas (Islets of Langerhans)?

The endocrine pancreas contains approximately 1–2 million islets of Langerhans, totaling about 1 gram. The cells are arranged in clumps of anastomosing cords and are rich in fenestrated capillaries. Each islet is also innervated by autonomic fibers.

How are the cells of the islets of Langerhans arranged, and what kind of blood supply do they have?

Cells are arranged in clumps of anastomosing cords and have a rich supply of fenestrated capillaries.

What are the main cell types in the islets of Langerhans and their functions?

• α (Alpha) cells: release glucagon → raises blood glucose

• β (Beta) cells: release insulin → lowers blood glucose

• δ (Delta) cells: release somatostatin in small, local amounts → likely modulates insulin and glucagon release
  (Note: These cells are not distinguishable with routine stains.)

What are the main functions of the liver?

The liver is a major metabolic organ with many functions, including:

• Carbohydrate, lipid, and protein metabolism

• Drug detoxification

• Formation of albumins and clotting factors

• Storage of vitamins and minerals

• Removal of senescent red blood cells (RBCs)

• Bile production, among other roles

What are the structural and cytoplasmic features of hepatocytes?

• Shape: Polyhedral cells

• Nuclei: Central, spherical

• Arrangement: Organized in cords

• Cytoplasm:

  • Eosinophilic due to abundant smooth ER and mitochondria

  • Basophilic bodies represent rough ER

  • Glycogen granules visible with PAS stain

What functions do hepatocytes perform, and what are the exceptions?

• Functions:

  • Perform all major liver functions, including:

    • Metabolism

    • Detoxification

    • Protein synthesis

  Exceptions:

  • Do not store vitamin A

  • Do not phagocytose red blood cells (RBCs)

How are hepatocytes oriented within the liver?

Each hepatocyte has:

• At least one surface in contact with a sinusoid

• Another surface in contact with a bile canaliculus

What is the function of the central vein in the liver?

Central vein collects blood that has perfused through the liver sinusoids.

How are liver sinusoids arranged and where do they flow?

• Arrangement: Radially around the central vein

• Path: Run between cords of hepatocytes

What are the structural features of liver sinusoids?

• Lined with discontinuous endothelium and basal lamina

• Allow passage of all substances (except RBCs) into the space of Disse

What are Kupffer cells and what is their function?

• Kupffer cells are fixed macrophages found in liver sinusoids

• Function:

  • Phagocytize particulate matter

  • Remove senescent (old) red blood cells

What are stellate (Ito) cells and what is their role?

• Located in the space of Disse (not visible with routine stains)

• Function: Store vitamin A

What are portal triads and where are they located?

• Located at 3–6 corners of each liver lobule

• Contain:

  • Hepatic arteriole

  • Portal venule

  • Bile ductule

  • (Also may include lymphatic vessels and multiple arterioles)

What is the function of the portal vein in the liver?

• The portal vein brings nutrient-rich blood from the small intestine

• Both the portal venule and hepatic arteriole empty into sinusoids

What are bile canaliculi and how are they formed?

• Bile canaliculi are formed by grooves between adjacent hepatocytes

• Sealed by tight junctions

• Hepatocytes secrete bile directly into these canaliculi

How does bile drain from the liver?

Bile flows from canaliculi → bile ductules

What is the gallbladder and its main function?

• The gallbladder is an elongated organ located on the underside of the liver

• Function: Stores bile until it is needed in the small intestine

What are the key features of the gallbladder mucosa?

• Lined with simple columnar epithelium

• Contains microvilli, but not dense enough to appear striated

• No muscularis mucosa present

• Forms Rokitansky-Aschoff sinuses:

  • Deep mucosal folds that give a false glandular appearance

How does the gallbladder concentrate bile?

• Tight junctions near the lumen seal adjacent cells

• Epithelial cells actively pump sodium into the connective tissue

• Water follows osmotically, concentrating the bile

What are the characteristics and function of the gallbladder’s muscularis externa?

• Composed of thin sheets of smooth muscle

• Muscle fibers are arranged in an oblique spiral pattern

• Contracts in response to CCK (cholecystokinin) to release bile