Naturally occurring steroids in Xenopus oocyte during meiotic maturation. Unexpected presence and role of steroid sulfates

In the ovary, oocytes are surrounded by follicle cells and arrested in prophase of meiosis I. Although steroidogenic activity of follicle cells is involved in oogenesis regulation, clear qualitative and quantitative data about the steroid content of follicles are missing. We measured steroid levels of Xenopus oocytes and follicles by gas chromatography-mass spectrometry. We show that dehydroepiandrosterone sulfate is the main steroid present in oocytes. Lower levels of free steroids are also detected, e.g., androgens, whereas progesterone is almost undetectable. We propose that sulfatation is a protective mechanism against local variations of active steroids that could be deleterious for follicle-enclosed oocytes. Steroid levels were measured after LH stimulation, responsible for the release by follicle cells of a steroid signal triggering oocyte meiosis resumption. Oocyte levels of androgens rise slowly during meiosis re-entry whereas progesterone increases abruptly to micromolar concentration, therefore representing the main physiological mediator of meiosis resumption in Xenopus oocyte.

► The amphibian follicular cells and oocytes contain a broad range of steroids.
► The oocyte contains an unexpected bulk of sulfate-conjugated steroids.
► Sulfatation could be used to prevent oocyte stimulation at inappropriate time.
► Progesterone is the major steroid produced by follicle cells at time of ovulation.
► Testosterone could control oocyte growth, progesterone triggers meiotic divisions.