Microscopic study of the pyloric caeca of the starfish Marthasterias glacialis (Echinodermata): Finding of endocrine cells

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Abstract
The ultrastructure of five epithelial cellular types is described. (1) Absorptive-storing cells possess a striated border and accumulate lipids and glycogen. They also contain an apical multivesicular complex and numerous membrane-bound PAS-positive granules of unknown significance. (2) Zymogenic cells contain numerous, large secretory granules. These cells also possess microvilli and accumulate lipid droplets. (3) Current-producing cells contain bundles of thick and thin myofilaments and extend from the basal lamina to the lumen. (4) Mucous cells display the characteristic features of such secretory cells. (5) In addition to these four cellular types, already observed under the light microscope by other authors, endocrine cells have also been identified by both light and electron microscope. They contain variable amounts of secretory granules of diverse sizes and electron densities. Immunocytochemical studies reveal the presence of cells immunoreactive to somatostatin, glucagon, and pancreatic polypeptide. Intraepithelial nerve fibers in contact with endocrine cells are also present. As far as we know, this appears to be the first description of enteroendocrine cells in the phylum Echinodermata.