Precocious synthesis of oestrogen-induced vitellogenin by embryos pre-treated with thyroid hormones.

Treatment of 9-day-old chicken embryos with oestrogen results in only barely detectable amounts of induced vitellogenin being present in their sera after 13 days of incubation. Readily detectable amounts of vitellogenin are normally first seen in sera at 16 days of incubation (oestrogen having been administered at 12 days of incubation). However, pre-treatment of such embryos (those above) with thyroxine (T4) at 6-8 days of incubation results in substantial amounts of vitellogenin being present in sera of 14-day-old embryos, with slight increases being detected on the 13th day. Pre-treatment of 6 to 8 day-old chicken embryos with T4 also permits induction by oestrogenic substances of substantial amounts of very low density lipoproteins (VLDL) in sera after 13 days of incubation. Without pre-treatment with T4, oestrogen given at 9 days of incubation provokes only a moderately increased increment of VLDL by 13 days of incubation.