A discrete group of melanin containing cells are coincident with a major reorganization of retinal ganglion cell axons in the optic nerve ofXenopus

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Abstract
Further investigations into the abrupt reorganization of axons in the proximal optic nerve of the frogXenopus are presented. This reorganization reverses the radial, age-related organization of the projection. At the site of axon divergence a discrete group of melanin-producing astrocytes occurs, which are coincident with the reorganization. These cells are identified as astrocytes by their ultrastructure, and by stainingin vitro with an antibody to glial fibrillary acidic protein. In an attempt to see if this reorganization occurs in the absence of pigmented astrocytes, the periodic albino mutant ofXenopus was examined. In this albino a similar reorganization of the retinal axons occurs in the optic nerve, but there is also a population of pigment-containing astrocytes. Unlike the pigment of retinal pigment epithelium, which is lost during metamorphosis in these albinos, the pigmented astrocytes of the optic nerve retain their pigment throughout life. A second approach, used to provide support for the involvement of these pigmented astrocytes in the reorganization, was to study the development of the projection. In early development there is a period prior to gliogenesis in the optic nerve (Stage 47/48) when no pigmented cells are present. At these early stages of development there is no evidence for a reorganization of the retinal axons in the optic nerve. The possible role of these melanin-containing astrocytes in axon guidance is discussed.