CHANGES OF THE EXPRESSION AND DISTRIBUTION OF RETINOIC ACID RECEPTORSDURING NEUROGENESIS IN MOUSE EMBRYOS

The expression and distribution of three retinoic acid receptors, alph a, beta and gamma, were investigated in the CNS of mouse embryos durin g development. mRNAs and proteins of RAR-beta that were expressed in t he spinal cord of the 12.5-day mouse embryo decreased during developme nt but they were not decreased in the brain. The RAR-beta-positive cel ls were already present in the ventral region of the spinal cord of 10 .5-day mouse embryos, gradually appeared in the dorsal region during d evelopment and then disappeared from the spinal cord after birth. In t he brain, RAR-beta-positive cells were detected in the mesencephalon a nd rhombencephalon but not in the telencephalon of the 12.5-day mouse embryos. RAR-beta-positive cells were present in the hippocampus and c ingulum but not in the neocortex of 14.5-day mouse embryos. Most neuro ns in the hippocampus of 16.5-day mouse embryos and the cortex of newb orn mice were RAR-beta-positive. In the spinal cord, RAR-alpha mRNAs a nd proteins also decreased during development but more gradually than RAR-beta mRNAs and proteins. During development, the distributions of RAR-alpha and -beta in the spinal cord and brain did not differ substa ntially. The main difference was the appearance of a subtypes of RAR-a lpha, a 52-kDa protein, in the brain of newborn mice. On the other han d, RAR-gamma proteins were only faintly detected in the spinal cord an d the brain of the mice during the embryonal stages but these increase d after birth. The distribution of RAR-alpha- or -beta-positive cells were consistent with the neurogenesis during development in the spinal cord and brain.