The fine structure of pinacocytes in the marine sponge Microciona prolifera (Ellis and Solander)

Summary The primitive nature of the sponge epithelium was revealed primarily by the mode of intercellular attachment. The zonula adhaerens (terminal bar) and macula adhaerens (desmosome) which occur in most metazoa are totally lacking in the exo- and endopinacoderms of Micoricona prolifera. Although a careful search was made for examples of the zonula occludens (tight junction complex), none were seen. There is still the possibility that they occur, but if so, they occupy only a small portion of the junctional area.Both exo- and endopinacoderms are single-layered and composed of cells whose thickness is usually 0.5–2 . The similarity of architecture of the endopinacoderm to that of capillary endothelium suggests that the flattened shape of the endopinacocytes is a specialization for rapid diffusion between the water channels and the mesenchyme. Exopinacocytes are distinguished by the location of their nucleus in a club-shaped projection extending below the level of the exopinacoderm and by the mat of fibrous material which usually covers their flattened external surface. The presence of phagosomes in both the endo- and exopinacocytes suggests a nutritive role for these cells.Contrary to the reports of Wilson and Penney (1930) the pinacoderms are not syncytial. Reasons for their failure to see cell borders are discussed.This investigation was supported by N. I. H. Biomedical Support Grant 5 S05 FR0708802 to the University of Tennessee and by a Faculty Research Grant from the University of Tennessee.The author is indebted to Prof. Tracy L. Simpson for his suggestions and critical evaluation of this study.