Fig. 3.

Homozygous TRβ mutant tadpoles have drastically reduced responses to exogenous T3. A Representative photos of tadpoles treated with 5 nM T3 for 5 days. Age-matched stage 54 tadpoles were randomly selected and kept in a 4L plastic container with (c and d) or without (a and b) 5 nM T3 treatment for 5 days. They were then genotyped and photographed dorsally. Note that after T3 treatment, the wild type tadpole (c) showed obviously metamorphic changes and was morphologically distinct from TRβ^(−/−) tadpoles (d), which was similar to the untreated tadpoles. Bars: 1 mm. BTRβ knockout tadpoles had reduced intestinal shortening in response to T3. The lengths of the intestine of the tadpoles as treated in A were measured and normalized against the snout-vent length. Note that after T3 treatment, the intestine was significantly shortened in wild type tadpoles but this change was reduced and delayed in the TRβ^(−/−) tadpoles. Different lower-case letters denote statistically significant differences (P < 0.05) when compared in each genotypes and asterisks (** or ***) indicate a significant difference when compared to the wild type group at the indicated days for T3 treatment (P < 0.01 or P < 0.001). ns: no significant difference. C T3 treatment failed to induce epithelial remodeling in the TRβ knockout tadpoles. Age-matched tadpoles at stage 54 were treated with 5 nM T3 for 3 days. Cross-sections of the intestine isolated from wild type (a, c and e) and TRβ knockout (b, d and f) tadpoles were stained with MGPY (methyl green-pyronin Y). Dashed red boxes (c and d) indicate the higher magnification (e and f), respectively. The black-dotted lines depict the epithelium-mesenchyme boundary, drawn based on morphological differences between epithelial cells and mesenchyme cells in the pictures of the stained tissues. The yellow-dotted lines indicate the cluster of proliferating epithelial cell which are well stained in wild type tadpoles after 3 days T3 treatment (e). T typhlosole. Bars: 100 µm