An investigation of astroglial morphology in torpedo and scyliorhinus

The distribution and morphology of GFAP-immunoreactive cells was investigated in two elasmobranch species, Scyliorhinus canicula and Torpedo marmorata, in an attempt to distinguish between Horstmann's (1954) hypothesis that the presence of cells resembling mammalian astrocytes is a function of the thickness of the ventricular walls, and Cajal's (1911) hypothesis that astrocytes are a phylogenetic novelty found only in birds and mammals. Two types of GFAP-reactive elements were observed, but the distribution of these differed markedly between the two species. In Scyliorhinus, radial glial cells were predominant and astrocytes relatively rare. In Torpedo, on the other hand, a species in which the ventricles are atrophied and the ventricular walls extremely thick, the overwhelming majority of GFAP-labelled structures strongly resembled astrocytes; occasionally, GFAP-positive cells were observed in the ependyma of the spinal cord. These findings, together with previous results obtained by others in hagfish, provide strong evidence in favour of Horstmann's hypothesis.