Vascular precursors in developing human retina

PURPOSE. Prior investigation has demonstrated that angioblasts are present in the inner retinas of human embryos and fetuses and that they differentiate and organize to form the primordial retinal vasculature. The purpose of this study was to characterize these angioblasts further and examine ligands that might control their migration and differentiation. METHODS. Immunohistochemistry was used to localize stroma-derived factor- 1 (SDF-1), its receptor CXCR4, stem cell factor (SCF), and its receptor c-Kit on sections obtained from human eyes at from 6 to 23 weeks' gestation (WG). Coexpression of CD39 (marker for retinal angioblasts and endothelial cells) and CXCR4 or c-Kit was investigated by confocal microscopy. RESULTS. SDF-1 was prominent in inner retina with the greatest reaction product near the internal limiting membrane (ILM). SCF immunoreactivity was also confined to the inner retina and increased significantly between 7 and 12 WG. The level of both ligands declined by 22 WG. A layer of CXCR4(+) and c-Kit(+) precursors, some of which coexpressed CD39, existed in the inner retina from 7 to 12 WG. With migration, c-Kit was downregulated, whereas CD39(+) cells continued to express CXCR4 as they formed cords. With canalization, CXCR4 expression was downregulated. CONCLUSIONS. Embryonic human retina has a pool of precursors (CXCR4(+) and c- Kit(+)) that enlarged centrifugally during fetal development. From this pool emerges angioblasts, which migrate anteriorly into the nerve fiber layer where SDF-1 and SCF levels are highest. c- Kit expression declines with apparent migration, and CXCR4 expression declines with canalization of new vessels. Both SCF and SDF-1 are associated with the differentiation of retinal precursors into angioblasts and their migration to sites of vessel assembly.