DEVELOPMENT OF THE SPATIAL PATTERN OF RETINOIC ACID RECEPTOR-BETA TRANSCRIPTS IN EMBRYONIC CHICK FACIAL PRIMORDIA

Retinoic acid causes a range of embryonic defects, including craniofac ial abnormalities, in both birds and mammals and is believed to have a number of roles in normal development. We have previously shown that the distribution of retinoic acid receptor-beta (RAR-beta transcripts is spatially restricted within the neural-crest-derived upper beak pri mordia of the chick embryo. We have now used in situ hybridisation to trace the distribution of RAR-beta transcripts during the migration of cranial neural crest cells and during formation of these primordia. R AR-beta transcripts were present in a subset of migrating neural-crest -derived cells in the head of the stage 10 embryo. These cells were si tuated in pathways followed by cells that migrate from the neural cres t overlying the posterior prosencephalic/anterior mesencephalic region of the developing brain. Cells containing RAR-beta transcripts accumu lated around the developing eyes and in the regions of the ventral hea d from which the upper beak primordia later develop. We mapped the dis tribution of RAR-beta transcripts as the facial primordia were forming , with particular reference to the development of the maxillary primor dia. We found that these form in a region of the ventral head that inc ludes the boundary between regions of high and low levels of RAR-beta transcripts. The boundary between these two groups of cells persisted as the maxillary primordia developed. The restriction of RAR-beta tran scripts to a subset of migrating neural crest cells which arise from a specific region of the neural crest, and which give rise to precisely distributed populations of cells, provides further evidence that ther e is some form of prepatterning in the neural crest from which the fac ial primordia originate.