Meeting Abstract
Investigators of how thyroid hormones (TH) regulate frog metamorphosis often apply TH to induce metamorphic changes precociously in tadpoles. However, precociously induced remodeling of skeletal tissues might not resemble natural remodeling for many reasons. Remodeling might be induced before larval tissues become competent to fully respond to TH or before they attain the shapes at which natural remodeling starts. Remodeling might also be induced during faster growth than in metamorphosis. Further, induced remodeling means applying exogenous TH at fixed concentrations, which does not simulate the changing T3 and T4 levels during metamorphosis. To test how precocious induction affects remodeling, this study quantifies the stage-dependency of size and shape changes induced in two pharyngeal arch cartilages (Meckels cartilage or MC and ceratohyal or CH) by TH. We treated Xenopus tadpoles at early, mid and late tadpole and early metamorphic stages (NF 46, 53, 57 and 59/60) with 50 nM T4 or T3 or no hormone and measured the changes in body size and size and shape of the MC and CH. Treated and control specimens at NF 53, 57 and 59/60 were pretreated with methimazole to arrest them and methimazole and iopanoic acid were applied during experiments to block endogenous TH production and prevent T4 or T3 being converted to other forms of TH. Animals were photographed before and after treatments, cleared and stained for cartilage and bone, and their MC and CH dissected and photographed. Body sizes and final cartilage sizes and shapes were quantified from photographs; initial sizes and shapes were estimated from allometric relationships for controls. Cartilage dimensions responded similarly to both T3 and T4 at all stages, though the magnitude of change and impact on shape varied with stage and TH type.
