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

The barrier properties and glial ensheathment of blood vessels in the retin al myelinated streak of adult New Zealand White rabbits were characterized at the ultrastructural level by intravascular injection of horseradish pero xidase (HRP) and immuno-electron microscopy with monoclonal antibody O4 and antibodies to glial fibrillary acidic protein (GFAP). Vessels within the m yelinated streak did not leak HRP, and they exhibited tight junctions betwe en adjacent endothelial cells. However, unlike their adult counterparts, th e retinal blood vessels at postnatal day 18 exhibited substantial endocytot ic activity. Both GFAP(+) astrocytes and O4(+) cells were evident surroundi ng 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 mitochon dria in the cytoplasm. O4(+) cells also exhibited variable quantities of he terochromatin, indicative of early stages of cellular differentiation. Thes e observations are consistent with previous data showing that O4(+) cells i n the myelinated streak include oligodendrocyte precursor cells, pre-oligod endrocytes, and immature oligodendrocytes (Morcos Y, Chan-Ling T. Glia 21:1 63-182, 1997). The present data indicate that the preretinal vessels of the myelinated streak possess barrier properties typical of microvasculature i n the central nervous system, and that both O4(+) cells and astrocytes cont ribute to the glial ensheathment of these vessels. These vessels thus diffe r markedly from the leaky preretinal vessels associated with pathological c onditions such as retinopathy of prematurity. (C) 1999 Wiley-Liss, Inc.