Secretory Epithelia
The pancreas is a combined exocrine and endocrine gland, localized close to the posterior abdominal wall and composed of four anatomic components – a head, a neck, a body, and a tail.
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lls (RER). They define the “ergastoplasm” domain of the cells (cf. Fig. 19) and are the sites of synthesis of the pancreatic enzymes. After co-translational folding, release into the RER-lumen, and passing quality controls, the newly synthesized enzymes are exported out of the ER and taken up into the Golgi apparatus. The Golgi apparatus stacks (Golgi) are in typical supranuclear position. In the Golgi stacks, the newly synthesized enzymes are modified and, at the trans Golgi side, packed into secretory vesicles (for details cf. Fig. 34). The immature secretory vesicles or condensing vacuoles are sites of further processing and condensation of the enzymes; they are intermediates that develop to mature zymogen granules (ZG), containing the densely packed, mostly still inactive precursors of the digestive enzymes. The numerous zymogen granules accumulated in the apical cytoplasm of the cells are the intracellular storage compartments of the proenzymes. The proenzymes are released into the acinar lumen upon specific stimulation after dietary intake. Situated closely side by side, the acini comprise the major portion of the pancreatic parenchyma. Acini are surrounded by fine connective tissue leading blood capillaries, lymphatics, and unmyelinated nerve fibers (cf. Fig. 148).
References Palade G (1975) Intracellular aspects of the process of protein synthesis. Science 189:347 Schmidt K, Dartsch H, Linder D, Kern H-F, Kleene R (2000) A submembranous matrix of proteoglycans on zymogen granule membranes is involved in granule formation in rat pancreatic acinar cells. J Cell Sci 113:2233
Magnification: ×6,200 M. Pavelka, J. Roth, Functional Ultrastructure: Atlas of Tissue Biology and Pathology, Third Edition, DOI 10.1007/978-3-7091-1830-6_14, © Springer-Verlag Vienna 2015
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Secretory Epithelia
ACINAR CENTER: ACINAR AND CENTROACINAR CELLS The narrow ductuli in the center of the acini build up the initiation of the secretory duct system. Small intercalated canaliculi (intercalated ducts) export the secretion out of the acini and lead to the intralobular excretory ducts, which converge to form interlobular ducts. These, by anastomoses, further build up the main large pancreatic duct. The micrograph shows details of the center of an acinus. The secretory acinar cells are joined both to each other and to adjacent centroacinar cells (CAC) by junctional complexes (asterisk), composed of a zone of tight junctions, a zone of adhering junctions, and a circle of spot desmosomes (cf. Fig. 98). The apical tight junctions prevent leakage of pancreatic proenzymes from the acinar lumen (AL) into the intercellular spaces. All parts of the secretory system are displayed in the secretory acinar cells shown in the micrograph: rough endoplasmic reticulum in the basal and lateral parts of the cells, Golgi apparatus (Golgi), condensing vacuoles (CV), and zymogen granules (ZG) dominating in the apical cytoplasm. The increased electron densities of the zymogen granules compared with the condensing vacuoles reflect the
