Which gastric cells have distinct cytoplasmic ultrastructure named 'tubulovesicular system'?
Ans. a (Oxyntic cells) (Ref. Ganong Physiology 22nd/pg. Chapter 26).It is H+-K+ ATPase in the apical membrane of the parietal cells that pumps H+ against a concentration gradient of this magnitude. The parietal cells are polarized, with an apical membrane facing the lumen of the gastric glands and a basolateral membrane in contact with the interstitial fluid. Canaliculi extend from the apical surface into the cell. At rest, the cells also contain abundant tubulovesicular structures with H+-K+ ATPase molecules in their walls.The K+ inside the vesicles is reduced to low levels, and the ATPase molecules are inactive. When the parietal cells are stimulated, the tubulovesicular structures move to the apical membrane and fuse with it, thus inserting many more H+-K+ ATPase molecules into the membrane. This increases their surface area and forms numerous microvilli, which project into the canaliculi. The ATPase molecules are now exposed to the K+ in the ECF, and H+- K+ exchange begins.GASTRIC GLANDS# The gastric epithelial lining consists of rugae that contain microscopic gastric pits, each branching into four or five gastric glands made up of highly specialized epithelial cells. Glands within the gastric cardia comprise <5% of the gastric gland area and contain mucous and endocrine cells. The 75% of gastric glands are found within the oxyntic mucosa and contain mucous neck, parietal, chief, endocrine, enterochromaffin, and enterochromaffin-like (ECL) cells. Pyloric glands contain mucous and endocrine cells (including gastrin cells) and are found in the antrum.# Gastric glands contain several types of cells that secrete different products:- goblet cells, which secrete mucus;- parietal cells (oxyntic), which secrete hydrochloric acid (HCl); Ghrelin is released mainly by the oxyntic cells.- chief (or zymogenic) cells, which secrete pepsinogen, an inactive form of the protein-digesting enzyme pepsin;- enterochromaffin-like cells, found in the stomach and intestine, which secrete histamine and 5-HT (serotonin);- G cells, which secrete the hormone gastrin into the blood;- D cells, which secrete the hormone somatostatin.- In addition to these products, the gastric mucosa (probably the parietal cells) secretes a polypeptide called intrinsic factor, which is required for the intestinal absorption of vitamin B12.Oxyntic/ PARIETAL CELLS# Parietal cell secretion is probably an isotonic solution of HCl that contains 150 mEq of Cl" and 150 meq of H+ per liter. Yet the pH of the cytoplasm of the parietal cells, like that of other cells, is 7.0-7.2.# It is H+-K+ ATPase in apical membrane of the parietal cells that pumps H+ against concentration gradient.# In pernicious anemia 90% of anti-parietal cell antibodies are against H+-K+ ATPase. Q# At rest, cells contain abundant "tubulovesicular structure" with H+-K+ ATPase in their walls.Q# When the parietal cells are stimulated, the tubulovesicular structures move to the apical membrane and fuse with it, thus inserting many more H+-K+ ATPase molecules into the membrane. These ATPase molecules are now exposed to the K+ in the ECF, and H+-K+ exchange begins.# The H+ that is extruded comes from H2C03, and H2C03 in turn is formed by the hydration of COv This latter reaction is catalyzed by carbonic anhydrase, and parietal cells are particularly rich in this enzyme.# The HC03 formed by dissociation of H2C03 is extruded by an antiport in the basolateral membrane of the parietal cells that exchanges HC03 for another anion, and since Cl is the most abundant anion in interstitial fluid, the exchange is mainly for Cl .# When gastric acid secretion is elevated after a meal, sufficient H+ may be secreted to raise the pH of systemic blood and make the urine alkaline (postprandial alkaline tide).# Acid secretion is stimulated by histamine via H, receptors and by Ach via M3 muscarinic receptors.# Prostaglandins, particularly those of the E series, inhibit acid secretion by activating Gi.Enterochromaffin-like cells (ECL)# Gastrin also acts by stimulating the secretion of histamine from ECL cells.# These are vesicle- and granule-containing cells that are the predominant endocrine cell type in the acid-secreting portion of the stomach. They have Ach receptors in addition to gastrin receptors.# They are inhibited by somatostatin.# They undergo hypertrophy when gastric acid secretion is suppressed for prolonged periods.Paneth (zymogen) cells# Reside at the base of crypts of intestinal lining .0# Are the only cell type to undergo downward migration from the proliferation zone.# They are called guardians of the gut cell hatchery.# They are zinc-rich cells which secrete defensins and other defensive proteins.0 Gastric secretions PurposeSource1.MucusLubricant, protects surface from H+Mucous cell2.Intrinsic factorVitamin B12 absorption (in small intestine)Parietal cell3.H+Kills bacteria, breaks down food, converts pepsinogenParietal cell4.PepsinogenBorken down to pepsin (a protease)Chief cell5.GastrinStimulated acid secretionG cell Cell typeSiteFunction1.Partietal/oxynticBody & fundusSecrete H+, IF, Blood group substances,(Superficial part) Contain carbonic anhydrase.2.Chief/peptic/ zymogenBody & fundusPepsinogen, pepsin(Deeper part)3.G-cellsPyloric antrumGastrin04.Neck cellsBody/fundus, cardiaMucin & HC03The chief cells that secrete pepsinogens, the inactive precursors of the pepsins in gastric juice, contain zymogen granules.
Oxyntic cells
Zymogen cells
Goblet cells
Enteroendocrine cells
Stomach