The Rara and Rarb genes remain expressed in the differentiating mouse spinal cord, at least until E14.5 [Colbert et al., 1995; Ruberte et al., 1993]. Rara is expressed throughout the spinal cord, and at higher levels in the ventricular zone neuroepithelium. At early stages (E10.5-11.5), Rarb transcripts are also found in the ventricular neuroepithelium, and by E12.5 they appear in cells of the ventral horns (presumptive motor neurons) within the mantle zone. Eventually, they become restricted to dorsal and intermediate regions of the ventricular neuroepithelium. Along the anterior-posterior (rostro-caudal) axis, they become localized to cervical/brachial and lumbar levels, where limb-innervating neurons are being generated [Colbert et al., 1995]. Although both Rarb isoforms are expressed in the spinal cord, it is the Rarb1 isoform which is most abundant [Mollard et al., 2000]; (see also [Mendelsohn et al., 1994a; Mendelsohn et al., 1991] for lacZ reporter transgene analysis). Rxrg transcripts are restricted to the ventral horns of the spinal cord, where they are coexpressed with Rarb [Dolle et al., 1994].
