Growth hormone-induced neuroprotection in the neural retina during chick embryogenesis

Recent studies have established that retinal ganglion cells (RGCs) of the neural retina are extrapituitary sites of growth hormone (GH) production and action in early chick embryos prior to the ontogeny of pituitary somatotrophs. The presence of GH in axons of the RGCs is, however, restricted to the period when the retinofugal neurons project to and synapse with visual centers within the brain, suggesting roles for GH in axonal growth or guidance. Autocrine and/or paracrine actions of retinal GH are also neuroprotective for RGCs during developmental waves of apoptosis that characterize RGC differentiation. The anti-apoptotic actions of retinal GH use signaling mechanisms that are common to other established neurotrophins (e.g., brain-derived growth factor, insulin-like growth factor-1, transforming growth factor beta-1). Retinal GH is therefore a novel neurotrophin in the visual system during chick embryogenesis.