Astrocytes in the guinea pig, horse, and monkey retina: Their occurrence coincides with the presence of blood vessels

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Abstract
In the present study the distribution of astrocytes in the nerve fiber layer (NFL) has been studied in the sparsely vascularized retinae of the guinea pig and horse and in the richly vascularized retina of the Old World monkey (Cercopithecus aethiops) using immunocytochemical methods. In the guinea pig retina glial fibrillary acidic protein (GFAP)‐positive astrocytes could not be detected. They were found, however, in the myelinated region of the optic nerve. The optic nerve head and a small retinal region immediately adjacent to it contained few vimentin‐positive astrocytes. Histological sections confirmed the restriction of astrocytes to a small retinal region and showed that this is also the only retinal area that is vascularized. Astrocytes showing GFAP and vimentin immunoreactivity were absent from most of the horse retina. They were found only in a narrow zone close to the optic disc, which is also the only region of the horse retina that is vascularized. Thus, as in the rabbit retina (Schnitzer: J. Comp. Neurol. 240:128–142, 1985), in the guinea pig and horse retina astrocytes are not present ubiquitously in the NFL but coexist with blood vessels. In the monkey retina, GFAP‐positive astrocytes were found ubiquitously in the NFL. Astrocytes were absent from the avascular foveal region only. It is suggested that the concurrence of retinal astrocytes and intraretinal vascularization may be a feature common to many, if not all, mammalian species.