REGIONAL PATTERN OF RETINOID-X RECEPTOR-ALPHA GENE-EXPRESSION IN THE CENTRAL-NERVOUS-SYSTEM OF THE CHICKEN-EMBRYO AND ITS UP-REGULATION BY EXPOSURE TO 9-CIS RETINOIC ACID

We have investigated the expression of the retinoid X receptor-alpha ( RXR alpha) gene in the developing chicken embryo by using nonradioacti ve wholemount in situ hybridization. At the earliest stage of developm ent examined (stage 9; Hamburger and Hamilton [1951] J. Morphol. 88:49 -92), we detect RXR alpha transcripts in a stretch of neuroepithelium corresponding roughly to the presumptive caudal hindbrain. Upon format ion of the rhombomeres at stage 12, a strongly RXR alpha-positive regi on extends from a sharp rostral limit at the boundary between rhombome res 6 and 7 caudad to at least the level of somite 9. This pattern of highest expression continues at least until stage 22 but with some var iability in the caudal extent. A lower level of expression extends thr oughout the spinal cord. Transverse sections show that RXR alpha trans cripts are expressed in a gradient, with the highest levels near the r oof plate and decreasing toward the floor plate. At later stages, the level of expression is highest in the proliferative ventricular zone. However, at reduced levels, RXR alpha transcripts are also detectable in the mantle zone as well as outside the developing central nervous s ystem, for example, in the neural crest and the limb buds. Nine-cis-re tinoic acid up-regulates RXR alpha transcripts at stages 19.5-22.0 wit hin a few hours, augmenting but not expanding the expression pattern. Northern blots demonstrate the potential expression of multiple RXR al pha isoforms in the central nervous system at posthatch stages. These results implicate the RXR alpha receptor in both rostrocaudal and tran sverse patterning of the neural tube. J. Comp. Neurol. 398:575-586, 19 98. (C) 1998 Wiley-Liss, Inc.