Patterning of motor neurons by retinoic acid in the chick embryo hindbrain in vitro.

Motor neurons are found throughout the developing chick hindbrain, while somatic motor (SM) neurons develop only in rhombomeres 5 to 8 (r5-8), and in r1. In r2-8 neuroepithelial explants from stage 7-10 embryos cultured in collagen gels, we found that motor neurons were generated throughout r2-8, while SM neuron differentiation was restricted to r5-8, as in vivo. Exposure of such explants to retinoic acid (RA) resulted in SM neuron differentiation throughout r2-8, while inclusion of the mesoderm and endoderm suppressed this effect. In explants with mesoderm/endoderm, RA-dependent SM neuron differentiation in rostral rhombomeres was restored by the application of an inhibitor of the RA-degrading enzyme CYP26. We found that the mesoderm/endoderm (either with or without RA) induced Cyp26 expression in the neuroepithelium in vitro, suggesting that the modulatory effect of CYP26 on RA-dependent patterning might be dependent on local signals.