RADIAL SECRETORY GLIA CONSERVED IN THE POSTNATAL VERTEBRATE BRAIN - ASTUDY IN THE RAT

Secretory glial cells in the roof of the last diencephalic prosomer, e pendymocytes and hypendymocytes, form the subcommissural organ. The ce lls of this complex were labelled immunocytochemically, using an antis erum against their specific secretory products. The study aims at the characterization of this cell type in the rat as an anatomical model s ituation. Radially oriented secretory glial cells remain after birth b ehind the posterior commissure in the mesencephalic aqueduct. At about postnatal day 10, the cell bodies descend into the conventional epend yma and at postnatal day 25 they assume a compact, rounded appearance. The secretory product they release is involved in the formation of Re issner's fiber. This differentiation in phenotype is not accompanied b y a change of the intermediate filament expression. In the adult rat t hese cells had been labelled immunopositive for cytokeratins 8 and 18 as well as vimentin but not for glial fibrillary acidic protein. DiI-m arking from the third ventricle and from the dorsal surface of the bra in shows that the basal processes of ependymocytes and hypendymocytes project to the external and internal glial limiting membrane, respecti vely, through the commissural fiber bundles. Also the subependymal loc ated hypendymocytes have apical processes with contacts to the cerebro spinal fluid. When this secretory cell population is studied with resp ect to cyto-architectonical changes during ontogeny the results lead t o a new understanding of the subcommissural cells. They are not specia lized ependymal cells in a regionally restricted and secondary differe ntiated ependymal area, but rather descendants of an ontogenetically a ncient, specific type of radial glia. Characteristic features for all subcommissural cells are that they: (1) appear very early during ontog eny, (2) are derived from a radial oriented glial cell type, (3) carry at least one kinocilium, (4) possess an original intermediate filamen t pattern, (5) release a secretory product.