Microenvironmental regulation of visual pigment expression in the chick retina

Visual pigment (VP) expression in the chick embryo retina was investigated in ovo, in dissociated and explant cultures, and in cDNAs from individual cells. While VP mRNA is not detectable by in situ hybridization until embryonic day (ED) 14-16 in ovo, analysis of VP expression by RT-PCR showed that VP messages are present in the retina as many as 7-10 days before they become detectable by in situ hybridization, and are also detected in other regions of the embryonic CNS. On the other hand, red opsin expression is markedly accelerated when cells are isolated from their intraocular microenvironment at ED 6, and placed in pigment epithelium-free dissociated or explant cultures. This acceleration occurs regardless of cell density, birth date, or serum presence in the medium, suggesting that many photoreceptors are already programmed to express red opsin on or before ED 6, and that microenvironmental inhibitory factors prevent implementation of this program until ED 14 in ovo. The selectivity of this phenomenon is suggested by the finding that other VPs are not observed by in situ hybridization in ED 6 cultures, although they are detectable in cultures of older retinas. Taken together, these findings suggest that red opsin expression may be constitutive for many developing photoreceptor cells in the chick.