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Ultrastructural studies on the secretory mechanism of goblet cells in the rat jejunal epithelium.

Goblet cells in the jejunal epithelium of adult and suckling rats were studied with transmission electron microscope. The use of triple fixation, i.e. osmium-aldehyde-osmium has some advantages in the preservation of mucous substances and some cell organelles. Cytochemical demonstration of polysaccharide and glycoprotein by methenamine silver as well as detection of TPPase and acid phosphatase (AcPase) were performed. Accumulation of secretory substances and combination with sugar moieties occurred in the internal sacs of Golgi lamellae, where TPPase activity was recognized. No rigid lamella usually positive to AcPase was found in goblet cells, though it was easily found in the columnar absorptive cells. AcPase was positive in some small immature mucus droplets as well as lysosomes. As the differentiation of goblet cells advances, mucus droplets come into contact and fuse to each other. As a result of the close contact of two adjacent membranes of mucus droplets, a pentalaminar structure is first formed. Then the central dense lamina disappears and the membrane becomes trilaminar. Disappearance of one of the apposed unit membranes, leaving another unit membrane, may take place. Finally the single unit membrane left between the adjacent droplets is also ruptured and complete fusion occurs. The mechanism of extrusion of mucous substance is complicated. Some mucus droplets located at the most superficial area may be released by the mechanisms of exocytosis. More frequently, however, deeply located droplets fuse to each other and a huge vacuole is made in the apical cytoplasm. The substance filling the vacuole contains debris of cytoplasmic matrix and droplet membranes along with mucous secretory substance. These substances derived from different sources are expelled into the lumen at the same time. This may be classified into the apocrine mechanism, because a part of the cytoplasm and membrane are lost. The typical apocrine secretion, i.e. pinching off of cytoplasmic projection containing mucus droplets, was also found in the goblet cells. Thus, two or three different mechanisms of secretion discharge may possibly occur simultaneously in goblet cells.