RHOMBOMERE FORMATION AND HIND-BRAIN CREST CELL-MIGRATION FROM PRORHOMBOMERIC ORIGINS IN MOUSE EMBRYOS

Prior to rhombomere development, structures called prorhombomeres appe ar in the mammalian hindbrain. This study clarifies the developmental relationship between prorhombomeres and their descendent rhombomeres a nd hindbrain crest cells in mouse embryos by focal dye injections at v arious levels of prorhombomere A (proRhA), proRhB, and proRhC, as well as at their boundaries. ProRhA gives rise to two rhombomeres, rhombom eres 1 and 2 (r1 and r2), as well as to crest cells that migrate into the first pharyngeal arch, including the trigeminal ganglion. ProRhB d evelops into r3 and r4 and produces crest cells populating the second arch and acousticofacial ganglion. The anterior portion of proRhC give s rise to r5 and r6 and to crest cells migrating into the third pharyn geal arch and the IXth ganglion; its posterior portion develops into r 7 and releases crest cells into the fourth pharyngeal arch region as w ell as the Xth ganglion. These results suggest that the boundaries bet ween prorhombomeres serve as lineage restrictions for both hind-brain neuroepithelial cells and for segmental origins of crest cell populati ons in mouse embryos. The Hox code of the mouse head can be schematize d in a much simpler way based on this prorhombomeric organization of t he hind-brain, suggesting that prorhombomeres primarily underlie mamma lian hind-brain segmentation.