The expression of Stra6 and Rdh10 in the avian embryo and their contribution to the generation of retinoid signatures.

Two new components of the retinoic acid (RA) synthetic pathway, the cell surface receptor for retinol, Stra6, and the enzyme converting retinol into retinal, Rdh10, have recently been described. To understand how different tissues of the chick embryo generate different retinoid signatures, we describe the expression patterns of these two genes and ask whether they are altered by RA levels. We performed wholemount in situ hybridisation and altered RA levels by applying RA soaked beads and used vitamin A-deficient quail embryos. In some areas of the embryo, these two genes co-localised with a retinaldehyde dehydrogenase (Raldh), as might be expected allowing retinol to be taken into the cell and converted into RA. In other areas of the embryo, the domain of expression of Rdh10 was much smaller than that of the corresponding Raldh, suggesting that retinal is transferred between cells. In yet other areas, only one of the cytochrome P450 enzymes co-localises with Stra6. In the case of co-localisation with Cyp1B1 in the hindbrain mesenchyme, this reveals that retinol is taken up into the cells for conversion to RA by Cyp1B1 and used in establishing ventral progenitor domains in the hindbrain. In the case of co-localisation with a Cyp26, it suggests that other retinol dehydrogenases (Rdhs) have yet to be discovered. We propose that in certain regions of the embryo, there are new Rdhs and Raldhs yet to be discovered and that RA is not a major regulator of its synthetic enzymes.