Retinal Growth Hormone in the Chick Embryo

The presence of GH and GH mRNA in the eyes of embryonic chicks is controversial and has, therefore, been further examined. In this study, GH cDNAs identical in size and sequence to the full-length pituitary transcript were generated by RT-PCR from mRNA extracted from the neural retinas of embryonic day (ED) 7 chick embryo eyes. GH immunoreactivity in the neural retina of embryos was primarily associated with proteins of 15 and 16 kDa, whereas only trace amounts of monomer (22-25 kDa) GH, the most abundant form in the pituitary gland, were present. GH immunoreactivity was also present in the vitreous humor, although this was associated only with the 15-kDa protein. After hatch, retinal proteins with GH immunoreactivity of 15 and 16 kDa were present neonatally but not after 42 d of age. The GH immunoreactivity in the neural retina of ED8 embryos was widespread, although GH staining was particularly abundant in retinal ganglion cells (RGCs). Full-length GH mRNA was similarly located, by in situ hybridization, throughout the neural retina and concentrated in cells in the RGC layer. The neural retina is also a site of GH action because 10-6 m chicken GH greatly increased (4- to 5-fold) the content of IGF-1 mRNA in 48-h cultured ED8 neural retinas. These results demonstrate the presence of full-length GH mRNA in the neural retina of chick embryos, in which GH immunoreactivity is primarily associated with RGCs and submonomer GH proteins of 15-16 kDa. These results also demonstrate GH action in the neural retina of embryos and suggest hitherto unsuspected roles for GH in retinal development and/or ocular function.