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)