SKELETAL MORPHOGENESIS IN THE URODELE SKULL .3. EFFECT OF HORMONE DOSAGE IN TH-INDUCED REMODELING

This study examines the dosage dependency of thyroid hormone (TH)-medi ated remodelling in the cranial skeleton of the hemidactyliine plethod ontid urodele, Eurycea bislineata. One set of experiments quantifies m orphogenetic responses in 21 tissues for four size-age classes of larv ae immersed in four different T-4 concentrations. A second set varies both the period and concentration of T-4 treatment to evaluate the eff ect of different TH profiles on adult tissue shape. The tissues survey ed in this study exhibit a 100-fold range in TH sensitivity. Those in regressive morphogenesis have tissue-specific sensitivities which corr elate with the timing of their remodelling in natural development: bon e resorption is more sensitive than cartilage resorption and is initia ted earlier in metamorphosis. In contrast, the TH sensitivities of tis sues in progressive morphogenesis vary within each tissue type and eve n within some tissues, and they do not correlate with timing in natura l development. Some explanation for this discrepancy is offered by the constant spatial and temporal relationships between nasal cartilage a nd dermal bone, which suggest that some TH-mediated ossification may a dditionally require induction by cartilage. Also, the failure of nasol acrimal duct morphogenesis at all but the lowest dosage correlates wit h the induction of integumentary changes that may preclude duct format ion. Variable T-4 treatments produce no effect upon the adult skull, o ther than loss of the nasolacrimal duct and/or foramen. These results have two developmental implications, First, the dosage dependencies of the nasolacrimal duct, ossification sequences, and cranial remodellin g patterns all support a TH profile with exceptionally low levels at l arval stages and at least a 100-fold increase at metamorphosis. Second , a small change in the rate of TH activity has the potential to effec t a large-scale rearrangement and restructuring of TH-dependent remode lling. The lack of such transformations in metamorphic plethodontids s uggests that TH activity is highly conserved in this group. (C) 1995 W iley-Liss, Inc.