DEVELOPMENT OF THE HUMAN RETINAL VASCULATURE - CELLULAR RELATIONS ANDVEGF EXPRESSION

We have investigated the relationships of the cellular constituents of the retinal vasculature-astrocytes, microglia and pericytes-to the di fferentiating endothelium in human fetal retina. The vascular endothel ium was stained using NADPH-diaphorase histochemistry in 12 human feta l retinae ranging in gestational age from 15-22 weeks. Specimens were double labeled using antibodies against glial fibrillary acid protein, alpha smooth muscle actin, or major histocompatibility complex class II antigens to label astrocytes, contractile cells and microglia, resp ectively. In addition, specimens of 12, 14, 16 and 20 weeks gestation were hybridized in situ for VEGF expression. In retinal wholemounts th e vascularized area comprised four lobes that converged on the optic d isc, The vascular network was more dense in the temporal lobes than in the nasal lobes, and different growth patterns were evident. Astrocyt es were distributed in two layers-one associated with the optic axons and a deeper layer associated with the developing vessels. In retinae younger than 20 weeks, astrocytes in the deep layer were only loosely associated with the developing vessels and extended as far as 150 mu m ahead of the most peripheral vessels. A closer register between retin al vessels and the distribution of astrocytes was evident in the nasal region of retinas older than 20 weeks, In situ hybridization demonstr ated expression of VEGF mRNA in the vascular laver superficial to the ganglion cell layer, at the margins of the vascularized zone. Differen ces were evident in the density of astrocyte coverage of developing ve ssels and in the extent of VEGF expression in different regions of the retina; the relationship of these differences to differentiation grad ients in the neural retina is discussed. Intensely immunoreactive micr oglia were observed in the vascular layer, associated with the vascula r endothelium as far as the most peripheral loops, but not beyond. Alp ha smooth muscle actin-containing cells covered the proximal parts of large arteries, but not corresponding veins; they were absent from art erial side-arm branches, as well as the newly formed and small diamete r vessels in the age range studied. The results suggest that microglia , contractile cells and astrocytes have distinct temporo-spatial relat ionships to the differentiating vascular endothelium in human retinas and that VEGF expression at the vascular front, presumably by astrocyt es, is associated with the spread of the retinal vasculature, as descr ibed in other species. (C) 1997 Academic Press Limited.