A change of the hepatocyte population is responsible for the progressive increase of vitellogenin synthetic capacity at and after metamorphosis of Xenopus laevis.

Synthesis of the egg yolk precursor protein, vitellogenin, can be induced in adult, but not in larval, amphibian hepatocytes by estrogen treatment. The transition process for this inducibility of hepatocytes during development of Xenopus laevis was examined, using primary cultures of hepatocytes. This was found to occur at about the metamorphic climax of stage 62, although the level of vitellogenin production was very limited at this stage. This low level seemed due neither to insufficient estradiol-17 beta nor to high estrogen-degrading activity. The level of synthesis gradually increased following metamorphosis. Immunohistochemical analysis showed that fewer than 5% of the hepatocytes at stage 62 could be stained with antivitellogenin antibody and that the stained cell fraction subsequently increased gradually for several months after metamorphosis. These findings indicate that adult-type cells capable of synthesizing vitellogenin appear at metamorphosis and then expand their population in the liver during postmetamorphic maturation.