Identification of oligodendrocyte precursors in the myelinated streak of the adult rabbit retina in vivo.

Whole-mount techniques have been applied to the myelinated axons of the rabbit retina in order to study oligodendrocytes in the developing and adult central nervous system in vivo. Retinas from rabbits on embryonic day (E) 26 to postnatal day (P) 50 and from adults were subjected to immunohistochemistry with antibodies to glial fibrillary acidic protein (GFAP), vimentin, glycolipid O4, myelin basic protein (MBP), galactocerebroside (Gal-C), 4D6, and Griffonia simplicifolia isolectin, markers chosen for their specificity for astrocytes, cells of the oligodendrocyte lineage, Müller cells, and microglia. The first glial cells labelled within the myelinated streak (MS) were vimentin+ astrocytes. These cells were first apparent near the optic disc at E26. Their numbers and distribution increased markedly between E26 and E30, but between E29 and P3, vimentin expression was replaced by GFAP expression. Between P3 and P4, O4+, Gal-C-, MBP oligodendrocyte precursor cells, O4+, Gal-C-, MBP- pre-oligodendrocytes, and O4+, Gal-C-, MBP- immature oligodendrocytes appeared in low numbers in the region adjacent to the optic nerve head. O4-/+, Gal-C+, MBP+ mature oligodendrocytes appeared soon after at P4 to P5. With age, the outer extent of Gal-C and MBP immunoreactivity expanded, with positive cells forming a continuous sheath around nerve fibre bundles. The sequence of gliogenesis in the (MS) of the rabbit retina thus appears similar to that in the rat optic nerve. In adult rabbits, a large population of O4+ cells was distributed across the MS, the outer limit of the cells coinciding with the outer limit of retinal vessels. These O4+ cells could be classified into three stages of differentiation on the basis of expression of developmental markers and morphology: O4+, Gal-C-, MBP- oligodendrocyte precursor cells with a bipolar, unipolar, or simple morphology; O4+, Gal-C-, MBP- pre-oligodendrocytes with a complex multipolar morphology; and O4+, Gal-C+, MBP- immature oligodendrocytes with a complex multipolar morphology. The final stage of differentiation was characterized as O4-, Gal-C+, MBP+ mature oligodendrocytes with multiple parallel processes aligned along nerve fibre bundles. These results provide in vivo evidence for the existence of a substantial population of oligodendrocyte precursor cells and non-myelin-producing immature oligodendrocytes in the MS of the adult rabbit retina. This preparation makes possible in situ patch clamping and determination of other functional responses of these cell types in vivo.