Oocyte cryopreservation : a technical and clinical update

www.expert-reviews.com ISSN 1747-4108 © 2009 Expert Reviews Ltd 10.1586/EOG.09.24 It has been almost 50 years since the feasibility of oocyte cryopreservation was successfully demonstrated in mice [1]. Since then, many cryopreservation protocols have been introduced and tested in a wide range of mammalian species. The era of oocyte cryopreservation began with the announcement of the possible survival of cryopreserved–thawed oocytes by Sherman et al. [1]. The main milestones along the developmental pathway of oocyte cryo technology began with the first report of a live offspring in a mouse model after successful IVF of frozen– thawed metaphase II (MII) oocytes in 1977 [2]. This was followed by attempts to cryopreserve the germinal vesicle (GV) oocyte [3], which resulted in a live offspring using a slow-freezing protocol [4] and, later on, a vitrification protocol [5]. This success in murine cryobiology was later applied to humans, and the first human pregnancy was reported by Chen et al. [6], using slowly frozen–thawed MII oocytes, and by Kuleshova et al., using vitrified–warmed MII oocytes [7]. Nevertheless, poor survival rates of cryopreserved–thawed mature oocytes have led to attempts to cryopreserve the oocyte at an earlier developmental stage [8,9]. Shortly, these attempts were successful, resulting in the first pregnancy from cryopreserved GV oocytes, reported by Tucker et al. [10]. Following the first pregnancy reports with human frozen oocytes [6,11–13], only a few pregnancies were reported over the following decade [14,15]. Subse�uently, viable pregnancies and live births from cryopreserved oocytes renewed the interest in oocyte cryopreservation technology. Over the last few years, survival rates for slowly frozen [16–29] and vitrified [7,30–39] MII oocytes have improved remarkably.