Prolactin inhibits both thyroid hormone-induced morphogenesis and cell death in cultured amphibian larval tissues

We describe for the first time the successful organ culture, in a serum-free chemically defined medium, of hind limb buds from stage 54/55 Xenopus laevis tadpoles in which 2 x 10(-9) M triiodothyronine (T3) precociously induces morphogenesis to give rise to morphologically normal limbs within 7 days. It was important to retain the mesenchymal tissue joining the two limb buds in order to obtain limb development in culture. T3 added to tail explants from the same larvae, cultured in parallel with limb buds, induced regression and cell loss at rates comparable to those seen during T3-induced metamorphosis in intact tadpoles. We also demonstrate for the first time that 0.2 units of prolactin (PRL) added at the same time as 2 x 10(-9) M T3 totally blocked both limb development and tail regression over 8 days in culture. When added after T3 had initiated its metamorphic action. PRL arrested further morphogenesis and regression of these two tissues, respectively. Retinoic acid at 10(-7) M had only a marginal effect. Histological examination showed that T3 added to limb buds produced normal chondrogenesis and osteogenesis in vitro as well as skin, muscle, and digit formation, while it produced a rapid and marked histolysis of fin and connective tissue of the tail. The ease of hormonally manipulating both morphogenesis and cell death in culture in opposite directions offers a simple, effective model system for molecular analysis of mechanisms underlying hormone-regulated postembryonic developmental processes.