ABSTRACT
Following the fi rst birth from a cryopreserved human oocyte over 20 years ago (1), very few births were reported during the next decade. The initial success of Chen’s technique, which was based on a slow freezing method developed using the mouse oocyte as a model, could not be repeated. Furthermore, subsequent laboratory studies revealed the potential negative effects of cooling and exposure to cryoprotectants on oocyte physiology, which raised concerns regarding the safety of such a procedure. Meiotic spindle disruption, chromosome abnormalities, zona hardening, and reduced fertilization were the initial indicators that suggested oocyte cryopreservation protocols were suboptimal and consequently had a negative impact on the oocyte. The unique physiology and membrane composition of the metaphase-II-ovulated oocyte evidently provided greater challenges to scientists than that of the embryo. The report of the fi rst birth from a cryopreserved human embryo (2) was around the same time as that of the oocyte (1). The effi cacy of human embryo cryopreservation, however, has established the process as a routine assisted reproductive technique. Frozen embryos were transferred in around 15% of the total number of in vitro fertilization (IVF) cycles performed in the United States in 2005 (Assisted Reproductive Technology Report by Centers for Disease Control and Prevention). Almost 30% of these frozen embryo transfers resulted in a live birth, which equates to around 6000 births in 2005. This is also the case in Europe, where the number of live births derived from frozen embryo transfers is around 8000/yr (3). In comparison, oocyte cryopreservation is still considered an experimental procedure by the American Society of Reproductive Medicine and has resulted in approximately 500 live births worldwide, in two decades. However, encouraging outcomes for clinical oocyte cryopreservation are now being reported (4,5).
