Fine structure of the intestinal epithelium of the colonial ascidian Botryllus schlosseri

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Abstract
In the intestine of the filtering zooids of B. schlossen three segments can be distinguished. In the intermediate segment the epithelium, which is encrusted by the ampullae of the pyloric gland, shows marked aspects of alteration. In the proximal and distal segments, ciliated mucous, vacuolated and endocrine-like cells are recognizable. Ciliated mucous cells, widely distributed along the intestine, possess the apical region filled with numerous mucous granules, which are extruded with merocrine modality. Variations in morphology of the granules are visible especially between cells of different regions. Vacuolated cells appear involved in absorptive function. They are characterized by developed microvilli, numerous apical small vesicles and great supranuclear vacuoles containing heterogeneous material. The vacuolated cells of the proximal segment resemble the gastric vacuolated cells of B. schlossen. The vacuolated cells of the distal segment show many morphological similarities with protein absorbing cells of various animals for the presence of a giant vacuole and an apical network of vesicles and tubules with fuzzy coating on the luminal face. The intestinal endocrine-like cells are rare and characterized by strongly electron dense granules distributed in all the cytoplasm, but predominantly in the basal region.