in the zebrafish [Tallafuss et al., 2006]) at these stages. Collectively, these data show that there is relatively widespread and overlapping expression of Rars and Rxrs in early gastrulating vertebrate embryos. At this stage, retinoic acid (RA) is first synthesized in posterior mesodermal cells by the retinaldehyde dehydrogenase 2 (RALDH2) enzyme [Niederreither et al., 1997], and it has been shown to act in a region-specific manner as demonstrated, for instance, by using a RA-activated lacZ transgene [Ribes et al., 2009; Rossant et al., 1991]. RA starts to have signaling functions during gastrulation in the posterior region of the embryo, which includes the primitive streak, the node and the newly-formed mesoderm [Niederreither et al., 1999; Ribes et al., 2009; Shiotsugu et al., 2004]. On the other hand, anterior regions of the embryo are devoid of RA signaling, due to the action of the metabolizing enzymes CYP26A1 and CYP26C1 [Hernandez et al., 2007; Ribes et al., 2007; Uehara et al., 2007]. Koide et al., [Koide et al., 2001] showed that in the Xenopus gastrula, RAR(s) need to function as transcriptional repressors in the prospective head region, most likely to prevent any inappropriate activation of genes acting as posterior determinants.
