Ovarian stimulation is an integral procedure in assisted reproduction treatment. It is achieved by the administration of exogenous gonadotropins to increase follicular recruitment and oocyte yield. At present, two follicle-stimulating hormone ( FSH ) products are commercially available for controlled ovarian stimulation(COS): highly purified human-derived FSH(HP-FSH)and recombinant FSH (rFSH). HP-FSH contains a higher proportion of acidic isoforms whereas rFSH contains a higher proportion of less-acidic isoforms. Less-acidic isoforms exhibit high bioactivity in vitro, but they have a faster clearance,and thus a shorter circulatory half-life than the acidic FSH isoforms. The slow clearance of the acidic isoforms results in more estrogenic follicles, follicular maturation and estradiol secretion. Fostimon,a new generation of HP-FSH product, contains more extensively glycosylated(acidic) FSH. Its improvements in specific activity,purity,degradation,and impurities have resulted in a high pregnancy rate similar to rFSH. The sequential treatment protocol with acidic and less-acidic FSH(Fostimon and rFSH) may mimic the physiologic cycle, whereas normal follicle development and ultimately normal oocyte function depend on an appropriate balance of sequential differentiation, and this balance is strongly influenced by FSH isoform distribution. Recently,it is reported that a sequential combined protocol using both acidic FSH and less-acidic FSH for controlled ovarian stimulation(COS)may improve oocytes maturity and embryo cleavage,and increase pregnancy and implantation rate compared with acidic FSH or less-acidic FSH alone. More attention should be paid to FSH isoform distribution of exogenous gonadotropins applied for COS.
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