Localization of p27kip1 in the developing avian retina: sustained expression in the mature tissue.

p27kip1 is a cyclin/CDK inhibitor that is expressed in cells that exit cell cycle and turn post-mitotic. Here, we characterized the expression and localization of p27kip1 during the development of the chick embryo retina. Expression of p27kip1 in this tissue begins at embryonic day 5 (E5), increasing as development proceeds. In contrast to the expression in the developing rat retina that markedly decreases after postnatal day 6, expression of p27kip1 in the chick retina decreases only slightly ( approximately 30%) after E12. Thereafter, it remains highly expressed in the tissue. p27kip1 expression increases in an orderly succession. By E5, immunoreactivity was observed over beta-tubulin III (TUJ-1) positive cell bodies located in the prospective Ganglion Cell Layer. By E7, p27kip1 was also detected over elongated cell nuclei located in the inner and outer portions of the Neuroblastic Layer and over cell bodies in the middle of the Inner Plexiform Layer. By E12, besides labeled cell bodies, labeled processes from amacrine cells and from cells at the GCL in the IPL were identified. In retinas from post-hatched chicken, immunoreactivity was observed over cell bodies located at all nuclear layers. Several differentiated ChAT positive cholinergic cells were labeled for p27kip1. Our data suggest that, as in the retina of other species, p27kip1 is expressed in cells that are exiting cell cycle and differentiating in the early developing chick embryo retina. However, as opposed to rodents and amphibians, neuronal expression of p27kip1 is sustained in the adult chick retina, indicating that its expression is differently regulated during development in this specie.