The developing retina is particularly rich in RA, and is one of the first structures in which region-specific patterns of RA synthesis were reported ([McCaffery et al., 1992; Wagner et al., 2000] and references therein). At early stages of murine eye development, however, expression of Rar genes is most prominent in extra-retinal tissues: Rarb is strongly expressed in the periocular mesenchyme (including the presumptive choroid and corneal mesenchyme) and Rarg is expressed more homogeneously throughout the head mesenchyme [Dolle et al., 1990]. A detailed immunohistochemistry study has been performed for retinoid receptor expression throughout eye development, which confirmed the presence of the corresponding proteins in these mesenchymal tissues (Figure 4E-H) [Mori et al., 2001]. RARα was the only receptor to be detected within the neural retina (Figure 4F), especially in the inner nuclear and ganglion cell layers. RARα and RARβ were also detected in the retinal pigmented epithelium (Figure 4F,G). RXRα and RXRγ were detected in specific retinal cell layers, whereas RXRβ was ubiquitously expressed [Mori et al., 2001]. Rar and Rxr transcript distributions have also been described in the developing chick retina [Hoover et al., 2001].
