FIG. 1.

TRα knockout inhibits intestinal remodeling during T3-induced metamorphosis. (A) The relative intestinal length shortens less in the TRα^−/− tadpoles than the wild-type ones after two to three days of T3 treatment. Tadpoles at stage 54 were treated with 10 nM T3 for up to three days. Intestinal length was measured from bile duct junction to colon and normalized against body weight. The relative intestinal length was calculated as a % of that in the control animals (0 day) for each genotype, respectively. Each group included more than five tadpoles. Note that the animal weight or intestinal length change little within three days around stage 54 without T3 treatment (53). Each bar represents the mean + S.E. The asterisk (*) indicates a significant difference between TRα^−/− tadpoles and wild-type tadpoles (p < 0.05). (B, C) The morphological changes induced by T3 are inhibited/delayed in the intestine of TRα^−/− tadpoles. Cross sections of the intestine from TRα^−/− and wild-type tadpoles treated with T3 were stained with MGPY, which stains DNA blue and RNA red (B) with the boxed regions shown at higher magnification in (C). The DNA was stained blue while the RNA was stained red. Note that there were more epithelial cells in clusters stained by MGPY after two to three days of T3 treatment of the wild-type animals (black arrowheads). The connective tissue in the wild-type intestine was also thicker (more developed), especially after three days of T3 treatment. Scale bar, 20 μm (B) and 10 μm (C). The experiment was repeated twice with similar results. MGPY, methyl green-pyronin Y; S.E., standard error; T3, triiodothyronine; WT, wild type.