Ependymal cells of the filum terminale in fish (Poecilia sphenops) adapted to freshwater and saltwater: electron microscopic study.

The ependymal lining of the central canal of the filum terminale and spinal cord in the vicinity of the caudal neurosecretory system in P. sphenops was examined in this study. Two general cell types based on shape and location were observed in the ependymal lining: cuboidal ependyma located in dorsal aspects of the filum terminal and columnar to pseudostratified ependymal cells found in ventrolateral and ventral aspects of the filum terminale. Comparison of the ependymal lining was made in animals adapted to saltwater and freshwater. In animals adapted to saltwater there was an increase in the basal infolding of the cell membrane of the dorsal cuboidal ependyma. Infolding of the basal cell membrane is a phenomenon shared by cells known to participate in transport of electrolytes. Since a possible functional relationship between the ependyma of the third ventricle and median eminence has been shown, in future studies on the osmoregulatory function of the caudal neurosecretory system the ependymal lining of the central canal in this region should be considered.