Figure 1.

T3-dependent intestinal remodeling during Xenopus metamorphosis involves larval cell apoptosis and de novo formation of adult epithelial stem cells. Xenopus undergoes a biphasic development. Its embryogenesis, when there is little TR or T3, leads to the formation of a free-living premetamorphic tadpoles by stage 45. During premetamorphosis (stage 45–54), there are high levels of TR but little T3, and the intestine has a simple structure with only a single fold, the typhlosole. During metamorphosis, the T3 level in the plasma rises to peak around stage 62, and most larval epithelial cells in the intestine undergo apoptosis, as indicated by the circles. Concurrently, the proliferating adult progenitor/stem cells are formed de novo from larval epithelial cells through dedifferentiation, as indicated by black dots. By the end of metamorphosis (stage 66), the levels of both TR and T3 drop lower and the newly differentiated adult epithelial cells in the intestine form a multiply folded epithelium.