Digestive System III

LiverGallbladderPancreas

Liver

occupies the upper right region of the abdominal cavity

Liver is the largest internal organ

is subdivided into two large lobes (right and left)two small lobes (quadrate and caudate)

is enclosed in a capsule (capsule of Glisson) is partially covered by the visceral peritoneum is associated with the gallbladder

Liver composition

Parenchyma - consists of epithelial cells - hepatocytes- arises from the endoderm

Stroma - consists of connective tissue trabeculae - contains blood and lymphatic vessels, nerves, and bile ducts- divides parenchyma into lobules

Connective tissue is poor-developed in the human liver

the human liver

connective tissue in the pig liver

Liver functions

Bile production – exocrine function

so-called “endocrine” functions

Liver is involved in many metabolic pathways

produces blood plasma proteins (albumins, prothrombin, fibrinogen,

nonimmune - and -globulins, etc.) stores the blood glucose as glycogen and

releases glucose back after glycogenolysis stores lipid-soluble vitamins – A, D, K synthesizes and stores cholesterol stores iron ions as ferritin or hemosiderin synthesizes urea from ammonium ions

Liver degrades drugs, toxins, alcohol, and hormones

Detoxication and degradation - occur in hepatocyte sER - include

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Liver blood supply

Liver receives blood from two blood vessels the hepatic artery the portal vein

the liver hilum or porta hepatis

Portal vein carries the venous blood

from the digestive tract, spleen, and pancreas

portal blood contains– nutrients and toxic materials – erythrocyte breakdown products– hormones

Blood vessels pass in connective tissue with bile ducts

Triad (portal area) includes interlobular– artery– vein (from v. porta)– bile duct

Interlobular vessels give rise to capillaries

capillaries (hepatic sinusoids) – pass trough lobule– empty into the central vein

Hepatic sinusoids carry mixed blood

hepatic sinusoids

Central vein is a terminal venule

receives blood from sinusoids

Central vein begins the vinous blood outflow

a central vein the sublobular veins the hepatic vein the inferior vena cava

Hepatic lobules

are the liver morphofunctional units

Hepatic lobule composition

hepatocytes arranged in plates hepatic sinusoids perisinusoidal spaces (spaces of Disse) bile capillaries (canaliculi) central veins

Classic hepatic lobule

is a hexagonal mass of tissue a central vein is at the center the triads are at the angles

hepatic plates radiate from the center to periphery sinusoids travel between plates blood flows from the periphery towards the center bile flows from the center towards the periphery

Portal lobule emphasizes the liver exocrine function

is a triangular mass of tissue the central veins are at the angles the triad is at the center blood flows from the center to the periphery bile flows from the periphery to the center

Liver acinus

is a diamond-shaped mass of tissue the short axis is between the triads the long axis is between the central veins

Plates of hepatocytes

consist of two anastomosing cell rows bile canaliculus is between the rows are surrounded by the blood sinusoids

delicate network of the reticular fibers supports plates

Hepatocytes

are large polygonal cells contain 1 or 2 nuclei with several nucleoli may be tetraploid

Hepatocytes are rich in organelles

rER and sER mitochondria Golgi apparatus peroxisomes

Hepatocyte inclusions

glycogen clumps

lipid droplets

Hepatocyte polarization

Vascular pole- faces blood sinusoids- is involved in the “endocrine” function (blood-hepatocyte exchange)

Biliar pole - faces bile canaliculi- is involved in the exocrine function (bile secretion)

Hepatocytes are capable for regeneration

Physiologic regenerationhepatocyte lifespan is about 5 monthshepatocytes can be renewed

Reparative regenerationoccurs when liver substance is lost after trauma or surgery

undifferentiated cells are in the periportal areas

Bile canaliculi

are intercellular gaps within the hepatic plates

Bile canaliculi lack their own wall

are isolated by the hepatocyte tight junctions

Bile canaliculi empty into the bile duct

bile duct is a part of the triad

Bile duct is lined with cuboidal epithelial cells

Hepatic blood capillaries – hepatic sinusoids

are sinusoidal fenestrated and discontinuous

are lined with fenestrated endothelial cells

Hepatic sinusoids contain Kupffer cells

Kupffer cells are hepatic macrophages

Kupffer cells

belong to the mononuclear phagocytotic system

arise from blood monocytes

pit-cell is a large granulated lymphocyte

Kupffer cell functions

degradation of damaged or senile erythrocytes conversation ferritin to hemosiderin phagocytosis of bacteria and antigens initiation of immune reactions

Perisinusoidal space (space of Disse)

is between vascular hepatocyte pole and sinusoid contains a few reticular fibers and amorphous substance

hepatocytes project microvilli into space of Disse

Space of Disse is a site of

exchange between blood and hepatocytes hepatic lymph origination hemopoiesis in the fetal period and

in cases of chronic anemia in the adult

Space of Disse contains lipocytes (stellate or Ito cells)

are located between hepatocytes lack contact with bile canaliculi contain numerous lipid droplets

Lipocyte functions

storage of vitamin A synthesis of reticular fibers and amorphous substance

In pathology- lose their lipids and differentiate into myofibroblast-like cells- synthesize collagen fibers resulting in liver fibrosis - may contract resulting in portal hypertension

Gallbladder

is attached to the visceral surface of the liver

Gallbladder functions

bile storage bile concentration bile release to the duodenum as required

Bile takes part in digestion

emulsifies lipids activates pancreatic lipases facilitates fatty acid absorption

Bile composition:waterelectrolytesphospholipidscholesterolbile acidsbile pigments (bilirubin)

Gallbladder wall structure

Mucosa Mascularis externa Adventitia/Serosa

Gallbladder mucosa

epithelium lamina propria

Gallbladder epithelium

simple columnar striated

brush border

Hormone cholecystokinin regulates the bile injection

is produced by enteroendocrine cells of the duodenum is released in response to a fatty chyme induce the gallbladder muscle contractions

Pancreas

is located behind the stomach is enclosed in a capsule and

partially covered with serosa is subdivided into

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Pancreas composition

Parenchyma - consists of epithelial cells - arises from the endoderm

Stroma - consists of connective tissue trabeculae - contains blood and lymphatic vessels, nerves, and ducts- divides parenchyma into lobules

Pancreas is mixed gland

Exocrine componentproduces pancreatic juice

Endocrine componentsecretes hormones

Exocrine pancreas

is compoundbranchedtubulo-alveolarserous (protein-producing) gland

main and accessory pancreatic ducts empty into the duodenum

Pancreatic secretory unit - acinus

is round with narrow lumen

Acinar cells

are pyramidal nuclei are round and centrally located broad basal portion is basophilic due to rER apical portion is acidophilic due to secretory (zymogen) granules

Acinar cell ultrastructure

zymogen granules in the apical pole

rER in the basal pole

Acinar cells secrete a variety of digestive enzymes

Trypsin, chymotrypsin, peptidases – digest proteins up to amino acids

Amylase – digests carbohydrates up to glucose

Lipases – digest lipids up to glycerol and fatty acids

Nucleases – digest nucleic acids up to nucleotides

Zymogen granules contain inactive enzymes

proenzymes are activated when reach the intestinal lumen

Exocrine pancreas duct system

Intercalated ducts Intralobular ducts Interlobular ducts

Main and accessory pancreatic ducts

Intercalated duct

is narrow tubule lined with squamous or cuboidal cells may begin within the acinus (compound acinus) its cells in the acinus are called centroacinar cells

Centroacinar cells exist in the compound acinus

but belong to the intercalated duct

Intercalated duct takes part in secretion

Adds watersodiumbicarbonate to the pancreatic juice

Intercalated duct secretion - neuyralizes acidic gastric chyme- establishes the optimal pH for pancreatic enzyme activity

Intralobular and interlobular ducts

are lined with simple cuboidal epithelium

Pancreas exocrine secretion hormonal regulation

Hormones of enteroendocrine cells from the duodenumpancreozymin stimulates acinar cells secretin stimulates intercalated duct cells

Pancreatic hormones insulin and VIP stimulate exocrine secretionglucagon and PP inhibit exocrine secretion

Pancreas exocrine secretion nervous regulation

parasympathetic fibers stimulate both acinar and duct cells

Endocrine pancreas

consists of pancreatic (Langerhans) islets

islets are scattered throughout the gland

Islet composition

endocrine cells fenestrated sinusoidal capillaries

Islet endocrine cell types

B cells A cells D cells D1cells PP cells

B cells constitute about 70% of the islet population

occupy the islet central portion secrete insulin

Insulin decreases blood glucose levels

Effects hepatocytes and skeletal muscles – stimulates uptake of glucose from circulation– activates glycogen synthesis

Effects the white adipose cells– stimulates uptake of glucose from circulation– stimulates glycerol synthesis– inhibits lipase activity

A cells constitute 15-20% of the islet population

occupy the islet periphery secrete glucagon

Glucagon increases blood glucose levels

Stimulates glucose release into the bloodstream glycogenolysis (breakdown of glycogen) gluconeogenesis (synthesis of glucose from amino acids) lipase and lipid mobilization in adipose cells

Insulin and glucagon are antagonists

D cells constitute about 5-10% of the islet population

occupy the islet periphery secrete stomatostatin

inhibits both insulin and glucagon secretioninhibits acinar cell

Minor cell types in pancreatic islets

D1 cells vasoactive intestinal peptide (VIP)– stimulates endocrine and exocrine pancreatic secretion– decreases blood pressure

PP cells pancreatic polypeptide– stimulates gastric chief cells– inhibits

bile secretionpancreatic juice productionintestinal motility

Regulation of the islet activity

Blood glucose levels increase stimulate insulin productiondecrease stimulate glucagon production

Nervous regulation parasympathetic fibers stimulate both insulin and glucagon secretionsympathetic fibers stimulate glucagon release but inhibit insulin secretion

The End

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