Fine structure of the gastric epithelium of the ascidian botryllus schlosseri vacuolated and zymogenic cells

Summary Vacuolated and zymogenic cells, which are two of five cell types identified by electron microscopy in gastric epithelium of B. schlosseri, are described in detail. The vacuolated cells are characterized by one, or a few, supranuclear vacuoles containing myelin figures. A peculiar Golgi apparatus is consistently found at the base of the vacuoles; it consists of cisternae frequently containing small vesicles and tubules of constant diameter and/or a strong electron-opaque material. A variety of vesicles and multivesicular bodies are visible in the apical cytoplasm below long ribbon-like microvilli. The se findings suggest that the vacuolated cells are involved in absorptive and perhaps secretory activity. The zymogenic cells are characterized by a highly developed RER, numerous apical secretory granules and a well developed supranuclear Golgi apparatus. At the apical end, autophagosomes are frequently encountered, some of which contain also zymogen granules. Both cell types contain numerous lipid droplets, which are interpreted as an energy reserve available for the cells and for the entire colony during the change of generation. Correlation between structure and function in both cell types is discussed by taking into account the peculiar life cycle of B. schlosseri, as well as previously reported data on similar cells in other ascidians.We would like to dedicate this work to Prof. Giuseppe Reverberi on the occasion of his 70th birthday.The authors are indebted to Profs. A. Sabbadin and G. Mazzocchi for their most helpful suggestions and advice. We would also like to thank Mr. G. Tognon for technical assistance and the staff of the Stazione Idrobiologica di Chioggia for their assistance in collecting material. — This research was supported by a grant of C.N.R., contract from the Istituto di Biologia del Mare, Venezia, No. 7100396/04115542 and with the E.M. facilities of C. N. R. contract No. 70.01798.04.115.876.