Synthesis of steroids in postimplantation mouse embryos cultured in vitro

Steroid and total lipid synthesis have been assessed in postimplantation stage mouse embryos cultured in vitro from the blastocyst to early somite stage. A large increase in acetate incorporation into these compounds is observed during this period. Cholesterol (60–70%), lanosterol (1–15%), and a fraction containing pregnenolone (0–5%) are the major components of the embryo-associated steroid fraction. When embryos are labeled with [³H]pregnenolone, ³H-labeled progesterone, pregnanedione, and a compound identified as acylpregnenolone are produced and secreted into the medium. Production of progesterone and pregnanedione, but not acylpregnenolone, is severely inhibited by the drug cyanoketone (1 μM). Another drug, SU-10603 (10 μM), severely inhibits pregnanedione production, with only a partial repression of progesterone synthesis, and no effect on acylpregnenolone synthesis. Neither drug affects embryonic development. When embryonic tissues were carefully separated and analyzed for their ability to metabolize [³H]pregnenolone it was observed that all tissues (embryo/yolk sac, yolk sac, and trophoblast) can produce progesterone and acylpregnenolone from pregnenolone. Only embryo/yolk sac and yolk sac, but not trophoblast tissue, can produce pregnanedione. The significance of these observations in relation to metabolic communication between the embryo and its mother is discussed.