Glial cells in the bird retina: immunochemical detection

The avian retina is remarkably different from its mammalian counterpart in macroglial cell appearance. First, it is completely devoid of astrocytes. Thus, Müller cells constitute the only astrocytic-like cell population in avian retinae, whereas mammalian retinae also contain astrocytes in close association with blood vessels. Second, axons in the optic nerve layer of the retina of birds are myelinated, unlike those found in most mammalian species, with the exception of the rabbit, in which the medullary rays of the retina are myelinated by oligodendrocytes. Recent studies have revealed evidence that bird retinae contain a large number of oligodendrocytes, but which glial cell type myelinates axons intraretinally is still controversial. Apart from macroglial appearance, microglia in the bird retina show a very similar pattern of distribution to that of mammalian counterparts. This article reviews the existing data, including our new observations, and discusses the issues that remain to be resolved.