THE CONTRIBUTION OF BOTH FOREBRAIN AND MIDBRAIN CREST CELLS TO THE MESENCHYME IN THE FRONTONASAL MASS OF MOUSE EMBRYOS

Migration of cranial neural crest cells is a crucial event in the form ation of facial organs such as the frontonasal mass and branchial arch es. However, the source of the populating crest cells that occupy the frontonasal mass remains unclear in mammalian embryos. To elucidate th is, we performed focal Dil injections at various sites in the prosence phalon (forebrain, including the future telencephalon and diencephalon ), mesencephalon (midbrain), and the anterior part of the rhombencepha lon (hindbrain) separated posteriorly by the preotic sulcus (i.e., rho mbomere A; future rhombomere 1 and 2) of cultured mouse embryos from t he 3- to 10-somite stage. Results directly revealed that during these stages the lateral edge of the prosencephalon produced crest cells whi ch migrated to the frontonasal mass. On the other hand, labeled cells at the anterior neural ridge in the prosencephalon contributed mainly to the head epithelium, including the nasal placode, Rathke's pouch, a nd oral epithelium As for the crest cells of the mesencephalon and rho mbomere A, their destinations were significantly dependent on the inje ction site and somite stage. At the 3- to 4-somite stage, the crest ce lls emigrating from both the mesencephalon and rhombomere A migrated t o the first branchial arch. Moreover, the mesencephalic region, but ne ver rhombomere A, produced another group of crest cells that migrated to the frontonasal mass. In the 5- to 10-somite stage, the destination s of late-emigrating crest cells were restricted depending on their pr emigratory positions, i.e., the region producing crest cells migrating toward the frontonasal mass was restricted to the anterior portion of the mesencephalon, and the crest cells from the posterior portion of the mesencephalon primarily migrated to the first branchial arch, whil e those from the rhombomere A predominantly migrated to the trigeminal ganglion. Migration toward the frontonasal mass from the mesencephalo n ceased at the earliest in the 7-somite stage, followed by terminatio n of mesencephalic and rhombencephalic crest cell migration toward the first branchial arch at the 8-somite stage, whereas the contribution from rhombomere A to the trigeminal ganglion continued even at the 10- somite stage. This behavior suggests that both the prosencephalic and mesencephalic crest contribute to the mesenchymal cells in the fronton asal mass and also that the migration patterns of crest cells released from the prosencephalon, mesencephalon, and rhombencephalon depend on their axial level and developmental stage at initial emigration. (C) 1994 Academic Press, Inc