Contribution of O4+ oligodendrocyte precursors and astrocytes to the glial ensheathment of vessels in the rabbit myelinated streak.

Abstract

The barrier properties and glial ensheathment of blood vessels in the retinal myelinated streak of adult New Zealand White rabbits were characterized at the ultrastructural level by intravascular injection of horseradish peroxidase (HRP) and immuno-electron microscopy with monoclonal antibody O4 and antibodies to glial fibrillary acidic protein (GFAP). Vessels within the myelinated streak did not leak HRP, and they exhibited tight junctions between adjacent endothelial cells. However, unlike their adult counterparts, the retinal blood vessels at postnatal day 18 exhibited substantial endocytotic activity. Both GFAP+ astrocytes and O4+ cells were evident surrounding the preretinal blood vessels of the myelinated streak. Furthermore, O4+ cells exhibited features indicative of high synthetic activity, including a large proportion of extended chromatin and prominent nucleoli within the nucleus, as well as a well-developed Golgi apparatus and numerous mitochondria in the cytoplasm. O4+ cells also exhibited variable quantities of heterochromatin, indicative of early stages of cellular differentiation. These observations are consistent with previous data showing that O4+ cells in the myelinated streak include oligodendrocyte precursor cells, pre-oligodendrocytes, and immature oligodendrocytes (Morcos Y, Chan-Ling T. Glia 21:163-182, 1997). The present data indicate that the preretinal vessels of the myelinated streak possess barrier properties typical of microvasculature in the central nervous system, and that both O4+ cells and astrocytes contribute to the glial ensheathment of these vessels. These vessels thus differ markedly from the leaky preretinal vessels associated with pathological conditions such as retinopathy of prematurity.