The present study describes by means of immunohistochemistry the comparative distribution of glial fibrillary acidic protein (GFAP)-positive cells in the forebrain and midbrain of three species of lizards: Eumeces algeriensis, Scincoidae; Agama impalearis, Agamidae; Tarentola mauritanica, Gekkonidae. In the species studied, the different types and proportions of glial cells expressing GFAP showed considerable variation. These cells include radial glia, oval cells, tanycytes, ependymocytes, glia limitans, and astrocytes. In Eumeces, astrocytes are particularly abundant and their processes form numerous perivascular end-feet; in addition well-differentiated ependymal cells and glia limitans express GFAP. These mature glial features are concordant with the relatively advanced phylogenetic level of Eumeces. In Tarentola, relatively few GFAP-expressing glial cells are observed, consisting mainly of radial glia and tanycytes. These features indicate a relatively immature state of the glial cell populations in this species. In Agama, GFAP-immunostained cells are confined to the periventricular and subpial brain areas; the ventricular lining contains numerous GFAP-immunopositive tanycytes and well-differentiated glia limitans. This pattern indicates that the glial cell profile in Agama exhibits characteristics intermediate between Eumeces and Tarentola, a feature which is discordant with the relatively primitive phylogenetic level of Agamidae compared to Gekkonidae. Together, the results of the present study provide novel data on the characterization of GFAP-expressing cell populations in different species of lizards. We suggest that the different glial patterns observed in the lizard brain correlates with developmental and functional aspects.

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Many studies have emphasized species differences in the serotoninergic innervation and phenotypic characteristics of the subcommissural organ in mammals. The post-natal distribution patterns of serotonin-containing fibers, the onset of gamma-aminobutyric acid uptake, and glial markers have been studied in the subcommissural organ of the semi-desertic rodent, Meriones shawi, by using immunohistochemical and autoradiographic techniques. Abundant serotoninergic fibers can be observed in the subcommissural organ of the newborn Meriones, some of them running among the ependymocytes and reaching the apical part of this organ. During the first 2 post-natal weeks of development, the subcommissural organ displays a progressive increase of serotonin fiber density throughout the organ, including the apical part. The existence of a dense serotonin-containing basal plexus concomitantly with a high apical innervation in this organ is a specific characteristic of Meriones. Ependymocytes of this organ have the ability to take up gamma-aminobutyric acid at birth. This uptake decreases and completely disappears from the 2nd week. The reappearance of gamma-aminobutyric acid accumulation in ependymocytes of the adult subcommissural organ after destruction of the serotonin innervation by a neurotoxin (5-7 dihydroxytryptamine) suggests an inhibitory effect of the serotonin innervation on this accumulation. Immunohistochemical studies of the phenotype of the ependymocytes with respect to glial markers during ontogeny show the transitory expression of glial fibrillary acidic protein, the presence of vimentin and the absence of S100 protein expression. No correlation has been found between the serotonin innervation and the expression of the glial markers.

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