Growth hormone localization in the neural retina and retinal pigmented epithelium of embryonic chicks

It is well-established that growth hormone (GH) is present in the brain, spinal cord, and peripheral nerves of embryonic chicks, prior to the differentiation of pituitary somatotrophs, but its presence and distribution in retinal tissues is controversial. The possible presence of GH and GH mRNA in retinal tissues of early embryos has therefore been further evaluated. A 466-bp fragment of the pituitary GH cDNA, derived from a portion of exon 3 and spanning exons 4 and 5, was amplified by RT-PCR from reverse-transcribed mRNA from the pituitary glands of juvenile chicks and from the whole eye, neural retina, and retinal pigmented epithelium (RPE) of embryonic-day (ED) 9 chick embryos. In ED 9 embryos, GH immunoreactivity was demonstrated in the choroid and neural retina, in which it was particularly abundant in a layer of cells with the location and morphological appearance of retinal ganglion cells. GH immunoreactivity was also present in tissue sections of the RPE that were bleached to remove the melanin pigment. The intense GH staining in the RPE of ED 9 embryos was also revealed using a fluorescein-labeled GH antibody and confocal microscopy. At the ultrastructural level, GH detected by immunogold electron microscopy was present in the cytoplasm of RPE and neural retinal cells of ED 9 embryos. Although not associated with secretory granules, GH in the RPE was particularly associated with the membranes of the melanin granules. These results demonstrate that the neural retina and RPE are extrapituitary sites of GH production in early chick embryos, prior to the differentiation of the pituitary gland.