Secretory cell granules

Figure 46-6. Flow of membrane proteins from the endoplasmic reticulum (ER) to the cell surface. Horizontal arrows denote steps that have been proposed to be signal independent and thus represent bulkflow. The open vertical arrows in the boxes denote retention of proteins that are resident in the membranes of the organelle indicated. The open vertical arrows outside the boxes indicate signal-mediated transport to lysosomes and secretory storage granules. (Reproduced, with permission, from Pfeffer SR, Rothman JE Biosynthetic protein transport and sorting by the endoplasmic reticulum and Golgi. Annu Rev Biochem 1987 56 829.)...

Fig. 4. An apoptotic peripheral blood eosinophil observed under EM. The apoptotic cell displays the characteristic condensation of nuclear chromatin into large, electron-dense masses surrounding the central, relatively electron-lucent nuclear matrix. The swelling and breakage of plasma and perinuclear membranes and the release of the contents of granules from swollen, enlarged, electron-lucent granule containers within secretory cells are also observed. A single osmiophilic lipid body is found in the cytoplasm (x 18,500) (D6). Reproduced with permission from Dvorak, A. M., Images in clinical medicine, an apoptotic eosinophil. N. Engl. J. Med. 340, 437 (1999).

Insulin composes I % of pancreatic tissue, and secretory protein granules contain about 10% insulin. These granules fuse with the cell membrane with simultaneous liberation of equimolar amounts of insulin and the C-peptidc. Insulin enters the portal vein, and about S0% is removed in its first passage through the liver. The plasma half-life of insulin is approximately 4 minutes, compared with 30 minutes for the C-pcptidc. 

It has been known for 20 years that secretion of granule bound materials generally requires the presence of calcium in the medium (9). If calcium is omitted from the fluid perfusing a secretory tissue, evoked secretion is abolished or at least diminished. However, not all secretory cells handle calcium in the same way and more recent work emphasizes the role of calcium in membrane stabilization, the influence of magnesium ion on calcium mediated events, and the effect of stimulation frequency on calcium metabolism. 

After calcium enters secretory cells, how does it trigger secretion This question cannot be answered with certainty, but a few pertinent theories have been formulated. First, calcium may activate certain cytoplasmic enzymes such as protein carboxyl methylase, thereby causing methylation of carboxyl groups on granule membrane surfaces (25). This change in granule membrane surfaces may promote the interaction between granule membrane and plasma membrane and initiate the secretory process. 

Numerous ultrastructural and histochemical studies have now demonstrated conclusively that many of the epithelial cells in the thymus have the characteristics of secretory cells (Singh, 1981). Cortical as well as medullary thymic epithelial cells may exhibit membrane-bound, electron-dense granules, which are also found in a variety of endocrine organs. Epithelial cells containing membrane-bound, electron-dense granules have now been identified in avian (Frazier, 1973 Hakason et al., 1974 Kendall and Frazier,... 

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