OVERLAPPING ORIGINS OF PHARYNGEAL ARCH CREST CELLS ON THE POSTOTIC HIND-BRAIN

The developing hind-brain of vertebrates consists of segmental units c alled rhombomeres. Although crest cells emigrate from the hind-brain, they are subsequently subdivided into several cell populations that ar e attached to restricted regions of the hind-brain. At the preotic lev el, only even-numbered rhombomeres are accompanied by crest cells, whi le the odd-numbered ones are not. At the postotic level, such the birh ombomeric repetition becomes obscure. In order to map the origins and distributions of postotic crest cells, focal injections of Dil were ma de into various axial levels of the postotic neural tube. Cephalic cre st cells at the postotic level first form a single cell population dep osited by cells along the dorsolateral pathway. They are called the ci rcumpharyngeal crest cells (CP cells) and are secondarily subdivided i nto each pharyngeal arch ectomesenchyme. The neural tube extending fro m r5 to the somite 3/4 boundary gave rise to CP cells. The neuraxial o rigins of each pharyngeal ectomesenchyme extended for more than three somite lengths, most of which overlapped with the other. Unlike in the preotic region, there is no segmental registration between neuraxial levels and pharyngeal arches. Caudal portions of the CP cell populatio n show a characteristic distribution pattern that circumscribes the po stotic pharyngeal arches caudally. Heterotopic transplantation of the Dil-labeled neural crest into the somite 3 level resulted in a distrib ution of labeled cells similar to that of CP cells, suggesting that th e pattern of distribution depends upon dynamic modification of the bod y wall associated with pharyngeal arch formation.