T3 and postembryonic development. (A) Plasma T3 levels peak during postembryonic development in human and anuran metamorphosis. Postembryonic development in mammals refers to the perinatal period when many organs mature into their adult forms, i.e., about 4 months before to several months after birth in human (Tata, 1993). This period corresponds to metamorphosis in anurans such as Xenopus laevis (Leloup & Buscaglia, 1977; Nieuwkoop & Faber, 1965). (B) Xenopus metamorphosis involves three major types of transformations: de novo development of adult organs such as limbs, complete degeneration of larval organs such as the tail, and remodeling of organs that are present in both tadpoles and frogs such as the intestine. Note that at least some T3-induced apoptosis occurs in these examples of all three types. EP, epithelium; CT, connective tissue; MU, muscle.