The Fine Structure of the Gastric Exocrine Cells of the Purple Sea Urchin, Strongylocentrotus purpuratus

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Abstract
The gastric exocrine cells, found in the epithelium lining the 1st two-fifths of the stomach of the purple sea urchin, have the characteristic fine structure of protein exporting cells. The basal cytoplasm is packed with a highly-developed, rough endoplasmic reticulum and the apical cytoplasm contains membrane-bound secretory granules. The secretory granules are perhaps formed in dilated portions of the Golgi cisternae. The fine structural similarity of this sea urchin cell type to mammalian zymogen cells lends support to the idea that the gastric exocrine cells are the source of sea urchin digestive enzymes.