Accumulation of steroids in rabbit preimplantation blastocysts

The concentrations of progesterone, androstenedione, testosterone, oestrone and oestradiol were measured by radioimmunoassay in blastocysts and uterine fluid flushings collected from rabbits 110 to 159 h post coitum (p.c.). None of the blastocysts or uterine flushings contained detectable levels of androstenedione, testosterone or oestrone. All uterine flushings contained large amounts of progesterone and some of the flushings also contained oestradiol. A small amount of progesterone (approximately 7-5 pg/blastocyst) was first detectable in some blastocysts at 135 h p.c.; progesterone levels/blastocyst then increased progressively, reaching levels of about 122-158 pg/blastocyst at 159 h p.c. Micropuncture of blastocysts at 159 h p.c. indicated that greater than or equal to 90% of the progesterone in the embryo was in the blastocoelic fluid. Blastocysts from rabbit uterine horns containing oestradiol also contained oestradiol but those in which oestradiol was detected were never observed in uteri lacking the hormone. It is inferred that rabbit blastocysts accumulate both progesterone and oestradiol from uterine fluid.