Background. Among oocytes obtained during aspiration of preovulatory ovarian follicles in hormonally stimulated cycles, we ascertained the percentage of immature oocytes with the nucleus in the metaphase (M I oocytes) or even in the prophase (GV oocytes) of the first meiotic division and their capacity to mature in vitro in a simple medium without hormonal supplements.

Methods. In 818 women, stimulated by gonadotropin releasing hormone agonist (GnRHa) and gonadotropins, aspiration of preovulatory size follicles yielded 4972 oocytes. From these we denuded cells of cumulus oophorus and corona, meiotic maturity was evaluated under a microscope. Cells in the metaphase of the second meiotic division (M II oocytes) and those maturing after 5 hours were used clinically in the intracytoplasmic sperm injection (ICSI) procedure. Immature cells were left in the simple medium. The degree of their nuclear maturity was evaluated after one and after two days of culture. In vitro maturation was clinically used also in 14 cycles with no mature oocytes.

Results. Among 4731 oocytes with denuded corona and cumulus, 4199 (88.8%) were mature M II oocytes, 295 (6.2%) immature M I oocytes and 237 (5%) immature GV oocytes. Under in vitro conditions, 68.7% (90/131) GV oocytes attained maturity. Among M I oocytes, 63.6% (136/214) cells matured already after 5 hours and 26.6% (57/214) until the next day. In all 14 women with only immature oocytes, the embryos for embryotransfer were obtained after in vitro maturation and ICSI procedure. The result was four pregnancies and two deliveries.

Conclusions. Immature oocytes, obtained in hormonally stimulated cycles, may become clinically applicable if left to mature in vitro in a simple medium without supplementation of growth factors and hormones.