In the absence of protein, estradiol suppresses meiosis of porcine oocytes in vitro.

The effect of estradiol on the spontaneous maturation of porcine oocytes was investigated. Cumulus-enclosed (intact) and cumulus-free (denuded) oocytes were cultured in the presence of estradiol-17 beta (0 to 10 microgram/ml) in a chemically defined bicarbonate-buffered medium that contained either dextran or BSA, or in a complex Hepes-buffered medium that was supplemented with serum. After 24 hr, chromatin spreads were prepared and meiotic maturation was scored. The biochemical integrities of the cumulus cells were assessed by determination of the estradiol and progesterone content of spent media after culture of intact oocytes in the presence of 0.5 X 10(-6) M testosterone and 10 microgram/ml follicle-stimulating hormone. Estradiol did not significantly affect the onset of maturation of either intact or denuded oocytes that were cultured in medium containing either BSA or serum. In serum-supplemented medium, however, the progression of maturation beyond metaphase I was significantly affected by the steroid in a dose-dependent manner. The steroid significantly inhibited the release from meiotic arrest of both types of oocyte cultured in medium supplemented with dextran. Supplementation of all media with testosterone and FSH significantly stimulated the synthesis of estradiol by the cumulus cells, compared with that of control groups. The synthesis of progesterone, however, was significantly stimulated by testosterone and FSH only in the BSA and serum-supplemented media. It is concluded that exogenous estradiol has the capacity to arrest meiosis in vitro but that this capacity can only be expressed if no exogenous protein(s) is present. In the absence of exogenous protein, progesterone synthesis by the adherent cumulus cells is minimal.