Summary. Day-6 rabbit blastocysts were incubated with 3H-labelled neutral steroids. Pregnenolone metabolism was insignificant, but progesterone was extensively metabolized to 5β-pregnanedione (5β-pregnane-3,20-dione) and 5β-pregnanolone (5β-pregnan-3β-ol-20-one). Some dehydroepiandrosterone (DHA) was converted to 5-androstene-3β,17β-diol; androstenedione and testosterone were interconverted, and both 4-ene steroids were metabolized to 5α-reduced compounds. There was no evidence for oestrogen synthesis from progesterone or androgens. Exogenous cofactors had no significant effect on the course of steroid metabolism. Oestradiol and oestrone were determined in blastocysts, and high levels of oestrogen were found in blastocysts from superovulating rabbits. Injection of [3H]oestradiol into pregnant rabbits gave rise to radioactivity associated primarily with oestradiol in uterine tissue, uterine flushings and blastocysts.
It is concluded (1) that the capacity of the Day-6 rabbit blastocyst to metabolize progesterone and androgen may be utilized in vivo to protect the blastocyst from excess maternal progesterone, and to involve androgens in an anabolic role, and (2) that blastocyst oestrogen may be partly of maternal origin.