1991; Ellinger-Ziegelbauer and Dreyer, 1993; Koide et al., 2001; Pfeffer and De Robertis, 1994; Shiotsugu et al., 2004]. There are no specific reports of Rar/Rxr gene expression at pre-implantation stages in mouse or rat embryos. Rara, Rarg, Rxra and Rxrb transcripts were detected by RT-PCR in pre-implantation bovine embryos at all stages, from oocyte to hatched blastocyst [Mohan et al., 2001; Mohan et al., 2002]. An in situ hybridization study reported ubiquitous expression of Rara in extraembryonic and embryonic tissues, and of Rarg in embryonic tissues, in E6.5 (pregastrula) mouse embryos [Ang and Duester, 1997]. The functional significance of Rar/Rxr genes being expressed at early embryonic stages – or being maternally inherited – is unclear, as current evidence indicates that RA is first produced within the embryo at gastrulation stages (see below).
