Distribution of the cellular retinoic acid binding protein CRABP-I in the developing chick optic tectum.

Vitamin A is a major morphogen for the visual system. Most of its effects are mediated by retinoic acid (RA), whose developmental functions include pattern formation, neuronal differentiation and possibly axonal guidance. Although RA has been suggested to regulate development of the retina and its central projection, little is known about the distribution of retinoid receptors and binding proteins in the optic tectum, which in birds is the direct target of most retinofugal axons. We investigated the spatial and temporal distribution of the cellular retinoic acid binding protein-I (CRABP-I) in the chick midbrain. While the precise role of CRABP-I is still unknown, this is an intracellular transport protein for RA, which tends to be expressed in cells that are responsive to retinoic acid. Our data show immunoreactivity of CRABP-I in the tectal anlage at E2.5 and during the entire period of embryonic development. It was found in differentiating neurons of the generative zone, in migrating cells of the prospective stratum griseum et fibrosum superficiale and in mature neurons in this layer. In addition, we detected retinoid receptors RARalpha, RARbeta, RXRalpha, RXRbeta and RXRgamma in the developing tectum. Cell culture experiments demonstrate CRABP-I expression in a subpopulation of tectal neurons as they differentiate in vitro. These results are consistent with a regulatory role of RA in tectal neurogenesis and physiology.