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. T hus, Muller cells constitute the only astrocytic-like cell population in av ian retinae, whereas mammalian retinae also contain astrocytes in close ass ociation with blood vessels. Second, axons in the optic nerve layer of the retina of birds are myelinated, unlike those found in most mammalian specie s, with the exception of the rabbit, in which the medullary rays of the ret ina are myelinated by oligodendrocytes. Recent studies have revealed eviden ce that bird retinae contain a large number of oligodendrocytes, but which glial cell type myelinates axone intraretinally is still controversial. Apa rt from macroglial appearance, microglia in the bird retina show a very sim ilar pattern of distribution to that of mammalian counterparts. This articl e reviews the existing data, including our new observations, and discusses the issues that remain to be resolved. (C) 2000 Wiley-Liss, Inc.