Analogies between mammalian and amphibian biochemical differentiation; the role of thyroxine

There are important analogies between biochemical differentiation in livers of mammals around the time of birth and in amphibia during metamorphosis. Late fetal rats and early metamorphic tadpoles accumulate glycogen in their livers which is then depleted immediately after birth and during the last phase of metamorphosis, respectively. Both birth and the metamorphic climax are preceded by the development of glucose-6-phosphatase and of urea cycle enzymes in the liver. In fetal rats, as in tadpoles, thyroxine enhances the formation of glucose-6-phosphatase and of arginase; the former enzyme is subject to an additional controlling factor, associated with starvation, in both classes. The results indicate that analogous endocrine processes regulate very similar enzymic differentiations that prepare mammals and amphibia for terrestrial existences.