GIT Glands

• Liver


  Largest gland in the body
• Liver

  • Covered by mesothelium of visceral peritoneum and by a thin capsule which divides the liver into lobules
  • Dual blood supply, receiving blood from the hepatic artery and hepatic portal vein
  • Blood exits the liver through the hepatic vein, while bile exits into the bile duct

  
• Hepatic portal vein

  75% of blood supply to the liver is oxygen poor blood with:

  • nutrient and toxic materials from the intestine,
  • blood cells and breakdown products of blood cells from spleen,
  • endocrine secretions of pancreas and entero-endocrine cells of GIT

  
• Hepatic artery

  25% of liver blood supply, carries oxygenated blood from the heart
• Blood from the hepatic portal vein and hepatic artery mix as they enter sinusoids and supply hepatocytes

  
• Functions of hepatocyte

  • Synthesis and secretion of plasma proteins (albumin, prothrombin, fibrinogen)
  • Uptake, storage, distribution of nutrients and vitamins
  • Maintains blood glucose level
  • Degradation and elimination of toxic substances and drugs (detoxification)
  • Stores and converts vitamins and iron
  • Endocrine-like functions - modify structure/function of many hormones
  • Amino acid deamination that results in production of urea
  • Detoxification & inactivation - various drugs and toxic substances can be inactivated by hepatocytes
  • Exocrine function - bile production
• Classic liver lobule

  Six-sided mass of tissue with portal triad at each corner surrounded by connective tissue stroma and liver parenchyma with stacks of anastomosing plates (one cell thick) of hepatocytes, extending from the central vein to the periphery of lobule
• Connective tissue stroma of classic liver lobule

  • Continuous with the fibrous capsule of Glisson, contains blood vessels, nerves, lymphatic vessels, and bile ducts
• Portal triad in classic liver lobule consists of three things

  An arteriole from hepatic artery, a venule from portal vein, and a bile duct
• Sinusoids of classic liver lobule

  Anastomosing sinusoidal capillaries, begin at the surface of the liver lobule and open into the central vein
• Hepatic vein

  Blood empties into the central vein, sub(inter) lobular veins, and then hepatic veins to the inferior vena cava
• Bile duct

  Bile canaliculi are grooves in neighbouring hepatocytes, bile produced in these cells drain into bile canaliculi and then into bile ducts
• Sinusoids

  • Endothelium - discontinuous capillaries with fenestrated endothelium
  • Kupffer cells - part of endothelial lining (macrophages) that breakdown aged/damaged red blood cells
  • Basal lamina - incomplete or absent
• Space of Disse

  The space between the wall of sinusoid and liver cell

  • Filled with blood plasma
  • Contains microvilli from liver cells and reticular fibres,
  • Stores stellate cells for lipids and vitamin A
• Hepatocytes

  • Polygonal cells with centrally placed round nucleus,
  • Have apical microvilli and supportive reticular fibres,
  • Process nutrients, toxins, and wastes as blood passes through the sinusoids
• Bile secretion

  Bile is produced in the hepatocytes and contains:

  • Bile salts/acids for digestion and absorption of lipids
  • Bile pigments detoxify metabolites of haemoglobin and carry to gut for disposal
  • Electrolytes maintain an isotonic condition, to be reabsorbed in the gut.
• Liver lobules

  • Classic liver lobules - characterized by the connective tissue septa that surround them
  • Portal lobule - characterized by the glandular tissue that is drained by a specific interlobular bile duct; exocrine function
  • Liver acinus - characterized by the glandular tissue that provides the best correlation between blood perfusion and metabolic support in liver pathology
• Medical conditions associated with zones of the parenchyma

  • Ischaemic necrosis in zone 3 around the central vein,
  • Bile stasis: ballooned hepatocytes around portal tract with bile in their cytoplasm in zone 1
• Gall bladder

  Pear shaped, blind sac attached to the visceral surface of the liver

  • Concentrates and stores bile received from the liver
  • Leads to a cystic duct through which it receives diluted bile from the hepatic duct.
• Histology of the gall bladder

  • Epithelium - absorptive cells (simple columnar)
  • Lamina propria - cellular with mucin-secreting glands
  • No muscularis mucosae & submucosa
  • Muscularis externa - randomly orientated
  • Mucosa forms deep crypts
  • Adventitia or serosa
• Pancreas

  Elongated gland (head, body and tail)

  • Thin capsule connective tissue surrounds it and extends into the gland dividing it into lobules
  • Lobules consist of connective tissue, parenchymal units, ducts, blood vessels and nerves.
  • Mixed exocrine and endocrine gland
• Exocrine pancreas

  Secretes pancreatic juice (digestive enzymes) into the duct system that delivers it to the duodenum
• Endocrine pancreas

  Secretes hormones that are released directly into adjacent capillaries
• Exocrine pancreas

  • Composed of cellular sacs (acinus) that are at the ends of ducts
  • Acinus consists of acinar cells which secrete digestive enzymes
  • Each acinus is drained by an intercalated duct and subsequently into a duct system
• Secretory units (acinar cells)

  • Serous cells that produce digestive enzymes precursors, with round basal nuclei,
  • Epithelium of acinar consist of pyramidal serous cells
  • Have a distinct basophilic cytoplasm in the basal region, and an acidophilic (eosinophilic) apical region with zymogen granules
• Centroacinar cells

  The initial cells of the intercalated duct, with a centrally placed flattened nucleus. They are not free-floating cells in the lumen of the acinus
• Duct system of the exocrine pancreas consists of:

  • Centroacinar cells,
  • Intercalated duct with simple squamous epithelium,
  • Intralobular collecting duct (non-striated) with high cuboidal to columnar epithelium,
  • Interlobular duct with columnar epithelium and extensive vascularized connective tissue,
  • Main pancreatic duct
• Endocrine pancreas

  Diffuse organ that secretes hormones,

  • Cells arranged in short irregular cords surrounded by a network of fenestrated capillaries,
  • Secretes glucagon (alpha cells), insulin (beta cells), and somatostatin (delta cells)
• Salivary glands

  Parotid, submandibular, and sublingual glands
• Secretory gland acinar

  • Capsule, septa, lobes and lobules,
  • Acini - serous, mucous or mixed,
  • Salivon - basic secretory unit,
  • Acinus (blind sac) - serous, mucous or both
• Duct system of the salivary glands

  • Intercalated duct - secrete bicarbonate and absorb chloride,
  • Striated duct (intralobular duct) - infoldings + mitochondria - reabsorption of fluid and electrolytes,
  • Excretory duct
• Differentiating between serous and mucous Secretory end piece of the salivary glands

  • Serous - alveolar (round) in shape, stain more intense, no lumen visible, nuclei: round, central
  • Mucinous - more tubular in shape, washed-out appearance, lumen visible, nuclei: base, flattened
• Parotid gland

  Serous gland (100%),

  • Numerous, long intercalated ducts
  • Striated ducts large and prominent
  • Adipose tissue
• Submandibular gland

  Mixed gland

  • Acini: serous, mucus or mixed,
  • Intercalated ducts less extensive than in parotid gland
• Sublingual gland

  Mixed mostly mucous (80%) secreting in humans

  • Mucous secretory units tubular, serous demilunes,
  • Intercalated and striated ducts short or absent
• Bile secretion

  Where adjacent hepatocytes join, small canals (bile canaliculi) form, from invaginations of the cell membrane of adjacent cells.