Differential distributions of Rar mRNAs become evident in gastrulating embryos of various species. In the mouse, Rara and Rarg distributions remain diffuse at presomitic stages (E7.5) [Ang and Duester, 1997; Ruberte et al., 1991; Ulven et al., 2000], whereas Rarb expression is low in headfold and posterior midline tissues, and higher in lateral regions of the egg-cylinder [Ruberte et al., 1991]. In zebrafish, differential dorso-ventral distributions of the raraa and rarab genes have been reported, whereas the two rarg genes exhibit diffuse – or poorly detectable – expression [Hale et al., 2006; Waxman and Yelon, 2007]. Studies in Xenopus indicate an accumulation of Rara2, and a downregulation of Rara1, at early gastrulation stages [Koide et al., 2001; Shiotsugu et al., 2004]. Expression of all Rxrs has also been detected (by RT-PCR in the mouse [Ulven et al., 2000] and ISH 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
