RETINOIC ACID AND DEVELOPMENT OF THE RETINA

Retinoic acid (RA) is known as the activating trigger for a large numb er of processes in developing and mature vertebrates, and it plays a p ivotal role in eye development. We present here a brief review of the RA system in general, and we summarize the evidence for a determining role of RA in the embryonic eye. The earliest and most significant ocu lar feature influenced by RA is the dorso-ventral axis. A lasting diff erential expression of different RA generating enzymes along the retin al dorso-ventral axis then creates very high endogenous RA levels, as well as a ventro-dorsal RA gradient, features that are likely to direc t morphogenesis along this axis in the embryonic eye. RA is also likel y to play a significant role in the function of the mature eye, as som e of the chromophore released from photo-bleached rhodopsin is convert ed to RA, a mechanism for light to directly influence gene expression. The pivotal role of RA in eye morphogenesis may represent a developme ntal correlate of an evolutionary origin of RA-mediated transcriptiona l regulation from retinoid usage in vision. (C) 1997 Elsevier Science Ltd.