Polycystic ovarian syndrome (PCOS) patients are extremely sensitive to gonadotropin stimulation. Despite intensive monitoring and fine adjustment of gonadotropin dosage during ovarian stimulation, they still have significantly higher incidence of ovarian hyperstimulation syndrome (OHSS) compared with women with normal ovaries1. Limited or even no stimulation in IVM cycles help to eliminate this iatrogenic complication. According to this benefit and its reasonable pregnancy rate2, in vitro maturation of oocytes (IVM) has become an alternative treatment of in vitro fertilization (IVF), especially for the women who are infertile due to PCOS.
Successful oocyte maturation includes complete maturation of three compartments, namely meiotic, cytoplasmic and molecular3, which are crucial for good embryo development. Gonadotropins that produced from pituitary are indispensable for oocyte maturation in vivo, however, in vitro it might be different. Studies of hCG got similar conclusions that hCG priming not only promotes nuclear maturation and embryo cleavage rate4, 5, but also increases the number of oocyte recovered6. In contrast, the data regarding the benefits of FSH priming before oocyte recovery are debatable2, 7, 8. During folliculogenesis, FSH is essential for follicle development and oocyte maturation9. In sheep, FSH receptors are present early in the type 2 follicles10, which is equivalent to primary follicles in human. This suggests that FSH plays important roles in follicle development at both gonadotropin responsive and dependent phase11. In another words, cytoplasmic maturation, including the accumulation of protein and RNA during oocyte maturation is dependent on gonadotropin stimulation12. Maturation of this compartment is critical to initiate proper embryonic genome at the right time. Hence, theoretically, FSH priming is beneficial in promoting oocyte maturation in both preantral and antral follicles. On the other hand, some studies working on the FSH priming did not find its profit in terms of oocyte maturation, cleavage or even pregnancy rate 00. Although many live births have been reported, the pregnancy rate and implantation rate are inconsistent, since each study used different protocol and studied in various populations....