ROSE, C.S.*; JOHNSON, A.L.: Thyroid hormone-mediated cell proliferation in frog Meckel’s cartilage is specified before neural crest migration.

The shape and arrangement of neural crest-derived cranial cartilages are known to be specified before the neural crest cells start to migrate from the neural fold. Metamorphic vertebrates pose an additional problem in that their cranial cartilages undergo profound remodeling midway through life in response to TH. In frogs, the cells of the lower jaw (Meckel’s cartilage or MC) proliferate whereas those of the branchial arch cartilages (BA) undergo cell death. This study investigates the stage at which the cell proliferation response to TH is specified in the mandibular neural crest cells of Xenopus laevis. Neural fold segments were transplanted from mandibular crest regions of GFP-labelled embryos to branchial crest regions of unlabelled embryos. Tadpoles that developed GFP-labelled ectopic MCs in their gill baskets were then immersed in a T3 and BrdU solution for 5 days. Ectopic MCs were compared with untransplanted MCs and BAs in both T3-treated and control specimens. Comparisons of BrdU labelling, cell size and cell packing suggest that the medial regions of ectopic MCs responded to T3 in the same manner as untransplanted MCs. This result indicates that the cell proliferation response to TH which generates the adult shape of MC is specified in mandibular neural crest cells prior to their migration from the neural fold. This is consistent with the hypothesis that the embryonic formation and metamorphic remodeling of neural crest-derived tissues in metamorphic vertebrates are specified at the same stage of embryonic development and by the same mechanisms. Additional research is focused on the stage of specification of the cell death response in BA precursor cells.