Differential ontogeny of estrogen responsiveness in the chick embryo liver

The development of estrogen responsiveness in embryonic chicken liver has been studied in terms of specific genomic responses to the hormone. Embryonated eggs were injected with estradiol at various stages of development, and after 48 h the hepatic synthesis of the apoproteins II and B of very low density lipoprotein (VLDL) was determined by incubation of liver slices with [³H]leucine and analysis of the tissue supernatant by specific immunoprecipitation. Significant estrogen induction of synthesis of apoprotein II of VLDL could be seen by the 11th day of embryonic development. The induction of apoprotein B, the other major apolipoprotein of VLDL, paralleled that of apoprotein II. It was reported earlier that the synthesis of vitellogenin did not become estrogen inducible until about the 15th day of embryonic development. The dissociation of the vitellogenin and apoprotein B responses can be confirmed by direct electrophoretic analysis of [³H]serine-labelled liver homogenates. The results are discussed in relation to the ontogeny of the estrogen receptor and the possible involvement of other controlling factors.