RETINOIC ACID STAGE-DEPENDENTLY ALTERS THE MIGRATION PATTERN AND IDENTITY OF HINDBRAIN NEURAL CREST CELLS

This study investigates the migration patterns of cranial neural crest cells in retinoic acid (RA)-treated rat embryos using DiI labeling. W istar-Imamichi rat embryos were treated at the early (9.0 days post co itum, d.p.c.) and late (9.5 d.p.c.) neural plate stages with all-trans RA (2x10(-7) M) for 6 hours and further cultured in an RA-free medium . RA exposure stage dependently induced two typical craniofacial abnor malities; that is, at 9.0 d.p.c. it reduced the size and shape of the first branchial arch to those of the second arch, whereas, in contrast , at 9.5 d.p.c. it induced fusion of the first and second branchial ar ches. Early-stage treatment induced an ectopic migration of the anteri or hindbrain (rhombomeres (r) 1 and 2) crest cells; they ectopically d istributed in the second branchial arch and acousticofacial ganglion, as well as in their original destination, i.e., the first arch and tri geminal ganglion. In contrast, late-stage treatment did not disturb th e segmental migration pattern of hindbrain crest cells even though it induced the fused branchial arch (FBA); labeled crest cells from the a nterior hindbrain populated the anterior half of the FBA and those fro m the preotic hindbrain (r3 and r4) occupied its posterior half. In co ntrol embryos, cellular retinoic acid binding protein I (CRABP I) was strongly expressed in the second branchial arch, r4 and r6, while weak ly in the first arch and r1-3. CRABP I was upregulated by the early-st age treatment in the first branchial arch and related rhombomeres, whi le its expression was not correspondingly changed by the late-stage tr eatment, Moreover, whole-mount neurofilament staining showed that, in early-RA-treated embryos, the typical structure of the trigeminal gang lion vanished, whereas the late-stage-treated embryos showed the featu re of the trigeminal ganglion to be conserved, although it fused with the acousticofacial ganglion. Thus, from the standpoints of morphology , cell lineages and molecular markers, it seems likely that RA alters the regional identity of the hindbrain crest cells, which may correspo nd to the transformation of the hindbrain identity in RA-treated mouse embryos (Marshall et al., Nature 360, 737-741, 1992),