VEGF expression by ganglion cells in central retina before formation of the foveal depression in monkey retina: evidence of developmental hypoxia.

In macaque monkeys the foveal depression forms between fetal day (Fd) 105 and birth (Fd 172 of gestation). Before this, the incipient fovea is identified by a photoreceptor layer comprising cones almost exclusively, a multilayered ganglion cell layer (GCL), and a "domed" profile. Vessels are absent from the central retina until late in development, leading to the suggestion that the GCL in the incipient fovea may be transitorily hypoxic. Vascular endothelial growth factor (VEGF), expressed by both glial and neuronal cells and mediated by the hypoxia-inducible transcription factor (HIF)-1, is the principal factor involved in blood vessel growth in the retina. We examined VEGF expression in macaque retinas between Fd 85 and 4 months postnatal. Digoxygenin-labeled riboprobes were generated from a partial-length human cDNA polymerase chain reaction fragment, detected using fluorescence confocal microscopy, and quantified using Scion Image. High levels of VEGF mRNA were detected in astrocytes associated with developing vessels. We also detected strong expression of VEGF mRNA in the GCL at the incipient fovea prior to Fd 105, with peak labeling in the incipient fovea that declined with distance in nasal and temporal directions. By Fd 152 peak labeling was in two bands associated with development of the inner nuclear layer (INL) capillary plexus: in the inner INL where Müller and amacrine cell somas are located, and in the outer INL where horizontal cells are found. The findings suggest that at the incipient fovea the GCL is hypoxic, supporting the hypothesis that the adaptive significance of the fovea centralis is in ensuring adequate oxygen supply to neuronal elements initially located within the avascular region.