exocrine pancreas

structure:

•non-existant capsule

•prominent CT around larger ducts

•serous cells in acini

•pale centroacinar cells inside acini mark beginning of duct system

•intercalated ducts, drain the exocrine, acini begin with centroacinar cells go a short distance outside acinus to join intralobular ducts, going from simple squamous to cuboidal epithelium

[image]

secretion:

•acinar cells-produce inactive enzymes (proenzymes, zymogens) stored in zymogen granules until stimulatd to release them

•enzymes include: lipases, amylases, proteases, and nucleases. Activated by enterokinases of the brush border glycocalyx on intestinal lumen.

•pancreatic duct-produce bicarbonate and fluid

Bicarbonate is secreted by way of an apical Cl-HCO3 exchanger that uses CFTR Cl- as a source of luminal Cl-.

Hormonal control:

•acinar cells - secrete constitutively at a low level; receptors for CCK and ACh, causing inc. in release and production of enzymes

•secretin- main stimulus for duct cells to produce their watery bicarbinate-rich secretion

Describe the structure and function of the endocrine pancreas. Name the component cells and describe their organization in the islet. Identify the islet's components in photogrpahs and diagrams. Describe the blood flow through the islet, relating it to the specific cell types.

[image]

endocrine pancreas

scattered throughout the pancreas (tail), both as single cells and in identifiable unencapsulated clusters (islets of Langerhans)

endocrine cell types:

A (alpha) cells: produce glucagon, which raises blood glucose level. Peripheral.

B (beta) cells: produce insulin, lowers blood glucose level. most common. centrally located.

D (delta) cells: produce somatostatin. 3rd most common. Peripheral

F (PP) cells: produce pancreatic polypeptide. throughout islet.

control:

•hormonal and neural (sympathetic & parasympatetic)

•gap junctions (between islet cells transmit signals from cell to cell)

blood supply:

•fenestrated capillaries perfuse A & D cells before reaching the central B cells;

•blood leaving islets then perfuse adjacent acinar cells (mini-portal system)

Describe the overall organization of the liver, including capsule, portal areas, stroma & parenchyma.

[image]

Liver organization

[image][image]•Capsule - Glisson's capsule, thin CT layer with mesothelium covers most of the liver except the bare area

•Stroma - type I collagen CT follows along the portal triad, and reticular fibers (type III collagen) form a network of support for the parenchymal cells

•Parenchyma- composed primarily of hepatocytes arranged in plates between sinusoids

-perisinusoidal space of Disse:space bet. sinusoidal endothelial cells and hepatocytes

-Kupffer cells: specialized macrophages located in the liver lining the walls of the sinusoids that form part of the reticuloendothelial system

-stellate cells (Ito cells): pericytes found in the perisinusoidal space of the liver; major cell type involved in liver fibrosis.

•Blood supply arrives parenchyma via portal vein & hepatic artery in the portal triad, and drains via the terminal hepatic venule (central vein)

•Bile exits the parenchyma via canaliculi into canals of Hering (intrahepatic ductules), entering bile ducts in portal areas

[image]

Describe and diagram the blood flow through the liver to the inferior vena cava.

•The portal vein carries blood from gut, spleen, and pancreas.

•hepatic artery carries oxygenated blood from celiac trunk

Both parallel each other as they branch repeatedly in the portal areas. Co-mingled venous & arterial blood enters the sinusoids. -> terminal hepatic venules (central veins) -> sublobular veins -> large hepatic veins -> IVC

Bile canaliculi (formed by facing surfaces of adjacent hepatocytes & are sealed by tight junctions) -> bile ductules (canals of Hering or cholangioles) -> smile bile ducts (portal areas) -> large hepatic duct + cystic duct (from gall bladder) -> common bile duct -> duodenal papilla

Note: small bile ducts have simple cuboidal epithelium, which then becomes simple columnar in larger ducts, like the lining of the epithelium of the gall bladder

Describe the classic liver lobule. Include characterisitc features of sinusoids, Kupffer cells, perisinusoidal spaces, central veins, and portal areas. Identify each of these structures in photos and diagrams.

[image]

[image]

[image]

classic liver lobule: Based on portal areas surrounding a terminal hepatic venule (central vein)

•portal areas: portal vein, hepatic artery, and bile duct define the edges of the classic lobule. CT with collagen type I are present there.

•sinusoids: lead from portal areas bet. the hepatocytes to the central veins, lined with discontinuous endothelium and local areas of fenestrations termed sieve plates.

•Kupffer cells (MPS members) line the sinusoids, that break-down old erthrocytes.

•perisinusoidal space (of Disse): space bet. the endothelium and sinusoid

•stellate cells (Ito): fat storing cells in space, store fat-soluble vitamin A in the cytoplasm and produce collagen fibers and extracellular matrix components for the perisinusoidal space around the central vein.

-in cirrhosis, the stellate cells transform into myofibroblasts and become the main collagen producing cells of the cirrhotic liver. 

-cytokines from hepatocytes, Kupffer cells, and lymphocytes stimulate production of type I collagen in stellate cells in the space of Disse, resulting in fibrosis that alters the blood flow in the sinusoids.

•plates of hepatocytes, lined by sinusoids, converge on a terminal hepatic venule (central vein)

liver acinus: based on zones of relative oxygenation of blood in sinusoids as opp. to portal areas surrounding a terminal hepatic venule (central vein) in classic lobule.

center: branches of the portal triad as opp. to central veins

edges: terminal hepatic venules (central veins) as opp. to portal triads

3 perfusion zones

Zone I: most oxygenated, since closest to blood supply from the hepatic artery

Zone II: intermediate

Zone III: cloest to the terminal hepatic venule (central vein), least oxygenated

Function of hepatocytes is correlated with zone type.

Describe the ultrastructural cytology of the hepatocyte, including canaliculi and sinusoidal aspects.

[image]

Hepatocytes: polygonal shape, allowing sheets or plates, with sinusoids on either side of plates/sheets

"apical": the portion of the cell facing the canaliculus; site of secretion of bile (bile salts, cholesterol, bilirubin, phospholipids, etc)

basolateral: portion facing sinusoidal space, site of uptake of materials from blood

rER present: => production of blood proteins (e.g. albumin, coagulation factors, etc.)

sER present: (1) glycogen synthesis and degredation, (2) bilirubin conjugation, (3) Phase I detoxification of a wide variety of substances, site of cytochrome P450 system redox reactions, (4) Phase II detoxification, sER as well as other cellular locations.

Golgi complexes: (1) lipoprotein secretion, (2) lysosome formation

↑Mitochondria: => high metabolic activity

↑Peroxisomes: => oxidation of excess fatty acids

↑lysosomes

lipid and glycogen can be stored, in cytoplasmic deposits without surrounding membranes

microvilli: face the perisinusoidal space, ↑surface area, and exchange for materials with the blood

bile canaliculi: course between hepatocytes, walls consist of plasma membranes of adjacent hepatocytes, tight junctions, lumen has small microvilli

Describe the functions of the liver, including its manufacturing, degradation & endocrine activites. When appropriate, correlate these activities with specific locations or oganelles. Correlate the locations of the major hepatic enzymes with their association with patterns of liver damage.

http://picasaweb.google.com/lh/photo/jyU20AgCsNYnttTnqSFifA?feat=directlink

http://picasaweb.google.com/lh/photo/jyU20AgCsNYnttTnqSFifA?feat=directlink

Functions of the liver:

•hematopoiesis in embryo

•blood destruction (salvages iron from old rbc's)

•degradation/detoxification of drug, toxins & waste products

•excretion of wastes

•exocrine secretion (bile salts, etc)

•endocrine (at least by modification of hormones)

•production of blood proteins & lipoproteins

•management of glucose, gluconeogenesis

•storage of fat, carbs, and other materials

[image]

Describe the structure and function of the gallbladder. Identify gallbladder mucosa in photographs and diagrams.

[image]

Gall bladder

Mucosa: irregular folds (villi on drugs)

-spiral valves at neck help control bile flow

-simple columnar epithelium, mucous secreting, rare goblet cells

-apical tight junctions (gradient inducing)

-loose CT lamina propria, typical loose CT rests directly on muscularis propria (no true muscularis mucosae or submucosa)

Muscularis: thin, regular (thickness inc. with irration, e.g. gallstones)

Adventitia (loose c.t.)- covered by serosa on peritoneal surfaces or by liver (bed of the gallbladder)

Function - concentrates bile about 10 times, adds mucus, stores bile; regulated by hormones and neural stimulation

note: cystic duct and common bile duct - epithelium is histologically similar to gall bladder.