Histology of Accessory Digestive Organs

Bile Canaliculi

A continuous, polygonal, intercellular network of channels formed by the extracellular space between adjacent hepatocytes into which bile is secreted

Islet of Langerhans Cells

Alpha cells: located at periphery and secrete glucagon Beta cells: located in core and secrete insulin Delta cells: few located throughout and secrete somatostatin

Bile Resorption

Bile is resorbed by the ileum of the small intestine and carried to the liver via the portal vein where it gets reprocessed and secreted by hepatocytes

Portal Triad

Branches of the portal vein, hepatic artery, and bile ducts located within the connective tissue sheath at the apices of each liver lobule

Stellate Cells

Cells located within the Space of Disse that store vitamin A, fat, and produce reticular fibers (i.e. type III collagen)

Classifications of Liver Lobules

Classical lobule, portal lobule, and liver acinus

Bile Storage

Closure of the sphincter of Oddi causes bile to back flow from the common hepatic duct, through the cystic duct, and into the gall bladder for storage

Classical Lobule

Composed of plates of hepatocytes arranged radially in a hexagonal array around the central vein Connective tissue sheath separates the lobules and contains a portal triad and lymphatic vessels at each apex of the hexagon Emphasizes portal blood flow

Portal Lobule

Composed of three classic hepatic lobules centered around a bile duct to emphasize flow of bile from hepatocytes to the bile duct

Function of Gall Bladder

Concentrates and stores bile by removing water Cholecystokinin (CCK) released by enteroendocrine cells stimulates contraction of the gall bladder and expulsion of bile into the duodenum

Characteristics of Hepatocytes

Eosinophilic, often binucleate cells with microvilli, little or no basal lamina, but supported by reticular fiber network Abundance of smooth and rough ER, mitochondria, peroxisomes, and cytoplasmic glycogen granules and fat droplets High regeneration capacity

Sinusoidal Endothelial Cells

Fenestrated; gaps between adjacent cells; little or no basal lamina to allow direct and immediate contact between hepatocytes and blood plasma

Functions of Pancreatic Hormones

Glucagon: stimulates glucose synthesis and release Insulin: increases transport of glucose into cells Somatostatin: inhibits release of insulin and glucagon

Liver: Bile Flow

Hepatocytes --> Bile Canaliculi --> Bile Ducts --> R/L Hepatic Ducts --> Common Hepatic Duct

Mucosa: Gall Bladder

Highly folded mucosa consisting of simple columnar epithelium with microvilli and a highly vascularized lamina propria with no lymphatics Muscularis mucosae absent

Muscularis Externa: Gall Bladder

Indistinct inner longitudinal and outer circular smooth muscle layers

Kupffer Cells

Macrophages residing in the lumen of hepatic sinusoids that phagocytize particulate matter (e.g. decaying RBCs)

Liver: Afferent Blood Supply

Portal vein: deoxygenated, nutrient-rich, venous blood from the stomach, pancreas, spleen, and intestines (75%) Hepatic artery: oxygenated, arterial blood (25%) Blood flows from peripheral vessels, mixes in the hepatic sinusoids, and drain toward the central vein

Pancreatic Digestive Enzymes

Precursors of trypsin, chymotrypsin, carboxylpeptidases, amylases, lipases, and phospholipases

Function of Liver

Protein synthesis (e.g. albumin, prothrombin, and fibrinogen); nutrient storage (e.g. lipids, glycogen, and vitamin A); metabolic functions (e.g. gluconeogenesis and transamination); synthesis and recycling of bile; excretion of bilirubin; inactivation of drugs, toxins, and endogenous hormones

Function of Pancreas (Endocrine)

Secretion of carbohydrate metabolism regulating hormones by the islets of Langerhans directly into the bloodstream

Function of Pancreas (Exocrine)

Secretion of digestive enzyme precursors by pancreatic acinar cells and alkaline fluid (i.e. bicarbonate ions) by centroacinar cells and intercalated ducts into duct system Stimulated by cholecystokinin (CCK) and secretin, respectively

Outer Coverings: Gall Bladder

Serosa covers the anterior surface of the gall bladder, while adventitia joins the posterior surface with the capsule of the liver

Liver Acinus

Short axis runs between two adjacent portal triads, and long axis runs between the central veins of the classic lobules which share those portal triads Emphasizes flow of arterial blood (i.e. divided into three regions based on oxygen, nutrient, and toxin gradients)

Epithelium: Bile Ducts

Simple cuboidal epithelium

Liver: Efferent Drainage

Sinusoids --> Central Veins --> Sublobular Veins --> Hepatic Veins --> Inferior Vena Cava

Space of Disse

Space between endothelial cells and hepatocytes that collects lymph and houses stellate cells *Perisinusoidal space

Hepatic Sinusoid

Specialized capillaries running through the liver parenchyma Made up of endothelial cells, Kupffer cells, and Stellate (Ito) cells

Submucosa: Gall Bladder

Submucosa absent

What happens if the ileum can no longer resorb bile?

The bile passes into the large intestine, where it blocks water resorption and causes diarrhea

Liver Acinus: Zone 1 vs. Zone 3

Zone 1: region nearest to the terminal portal vein that receives the highest concentrations of oxygen, but also toxins (e.g. ethanol) Zone 3: region nearest to the central vein that receives the lowest concentration of toxins, but also oxygen (anoxia)