Expression of the retinoid receptors during organogenesis has mainly been studied in the mouse [Dolle et al., 1994; Dolle et al., 1990; Ruberte et al., 1990] (see Table 1 for a summary). Although three of the receptors (RARβ, RARγ and RXRγ) exhibit complex, differential expression features, only in some instances do their distributions correlate with specific differentiating cell- or tissue-types throughout the organism. This is the case for Rarg, which is expressed in all precartilaginous cell condensations (see Figure 3B,C and Figure 4D,E), irrespective of their embryological origin [Ruberte et al., 1990]. The same receptor is also expressed in the developing skin epithelium, as well as in all prospective squamous keratinizing epithelia, including the esophagus and left wall of the stomach (Figure 3D) [Ruberte et al., 1990]. Loss of function of mouse RARγ does not lead to any overall defect in chondrogenesis, or histogenesis of the skin and squamous epithelia (although Rarg-/- mutants have squamous metaplasia of the seminal vesicles and prostate [Lohnes et al., 1993]). As observed at early embryonic stages, murine Rara is expressed nearly ubiquitously in differentiating organs [Dolle et al., 1990]. Rarb has more discrete expression features, which are reviewed below.
