Although RA is actively synthesized within the developing neural retina of various vertebrate species ([Li et al., 2000; McCaffery et al., 1992; Wagner et al., 2000] and references therein), current functional evidence in mouse points to a paracrine mode of action, with RA diffusing and acting within the pigmented epithelium and periocular mesenchyme [Matt et al., 2005; Molotkov et al., 2006]. Possible intrinsic functions of RA within the neural retina, for instance to regulate proper differentiation of retinal cell types (see [Kelley et al., 1994; Prabhudesai et al., 2005] for a description of the effects of exogenous RA on photoreceptor differentiation), remain to be characterized.
