GFAP-immunopositive structures in spiny dogfish, Squalus acanthias, and little skate, Raia erinacea, brains: differences have evolutionary implications

GFAP expression patterns were compared between the brains of a spiny dogfis h (Squalus acanthias) and a little skate (Raia erinacea). After anesthesia, the animals were perfused with paraformaldehyde. Serial vibratome sections were immunostained against GFAP using the avidin-biotin method. Spiny dogf ish brain contained mainly uniformly-distributed, radially arranged ependym oglia. From GFAP distribution, the layered organization in both the telence phalon and the tectum were visible. In the cerebellum, the molecular and gr anular layers displayed conspicuously different glial structures; in the fo rmer a Bergmann glia-like population was found. No true astrocytes (i.e., s tellate-shaped cells) were found. Radial glial endfeet lined all meningeal surfaces. Radial fibers also seemed to form endfeet and en passant contacts on the vessels. Plexuses of fine perivascular glial fibers also contribute d to the perivascular glia. Compared with spiny dogfish brain, GFAP express ion in the little skate brain was confined. Radial glia were limited to a f ew areas, e.g., segments of the ventricular surface of the telencephalon, a nd the midline of the diencephalon and mesencephalon. Scarce astrocytes occ urred in every brain part, but only the optic chiasm, and the junction of t he tegmentum and optic tectum contained large numbers of astrocytes, Astroc ytes formed the meningeal glia limitans and the perivascular glia. No GFAP- immunopositive Bergmann glia-like structure was found. Astrocytes seen in t he little skate were clearly different from the mammalian and avian ones; t hey had a different process system - extra large forms were frequently seen , and the meningeal and perivascular cells were spread along the surface in stead of forming endfeet by processes. The differences between Squalus and Raia astroglia were much like those found between reptiles versus mammals a nd birds. It suggests independent and parallel glial evolutionary processes in amniotes and chondrichthyans, seemingly correlated with the thickening of the brain wall, and the growing complexity of the brain. There is no str ict correlation, however, between the replacement of radial ependymoglia wi th astrocytes, and the local thickness of the brain wall.