Successful fertilization in humans, achieved when parental chromosomes intermix at first mitosis, requires centrosome restoration and microtubule-mediated motility. Imaging of inseminated human oocytes reveals that the sperm introduces the centrosome. The centrosome then nucleates the new microtubule assembly to form the sperm aster--a step essential for successful fertilization. Oocytes from some infertile patients failed to complete fertilization because of defects in uniting the sperm and egg nuclei, indicating that failure to properly effect the cytoplasmic motions uniting the nuclei results in human infertility. These discoveries have important implications for infertility diagnosis and managing reproduction.

In a retrospective analysis of 637 cycles of ovarian stimulation and transvaginal follicular aspiration for various assisted reproductive technologies, severe ovarian hyperstimulation syndrome (SOH) occurred in six (0.94%) cycles. The patients at a high risk of developing SOH in cycles of assisted reproduction were those who had excessive serum oestradiol levels on the day of human chorionic gonadotrophin (HCG) administration (oestradiol greater than 6000 pg/ml; 38% SOH) and a high number of oocytes obtained (greater than 30 oocytes; 23% SOH). In those patients with both oestradiol greater than 6000 pg/ml on the day of HCG administration and greater than 30 eggs retrieved, the chance of developing SOH was 80%. The higher the serum oestradiol levels and the more eggs retrieved, the higher the pregnancy rates observed. High oestradiol level did not appear to have a detrimental effect on pregnancy rates and outcome. Furthermore, our results are not consistent with suggestions that the addition of gonadotrophin-releasing hormone agonist to ovarian stimulation protocols, follicular aspiration and/or luteal support with progesterone may reduce the incidence of ovarian hyperstimulation syndrome.

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This study reports the subsequent embryo development of cryopreserved mature human oocytes following insemination or intracytoplasmic sperm injection (ICSI). Metaphase II oocytes were cryopreserved using a slow freezing-rapid thawing procedure employing the cryoprotectant 1,2-propanediol. The study was conducted at two centres. The normal insemination of cryopreserved oocytes was undertaken in one centre, and ICSI of cryopreserved oocytes in the other. Both methods resulted in a 50% normal fertilization rate. A low rate of abnormal fertilization was observed in the inseminated group of oocytes (5%) compared with 21% for the ICSI oocytes; this was not significantly different. Embryo development was assessed daily for 7 days. All normal fertilized cryopreserved oocytes in both groups cleaved on day 2, with a similar appearance to in-vitro fertilization and ICSI embryos. In the normal inseminated oocytes, there was a significant decrease in the number of embryos cleaving on day 3 (33%) compared with the development of ICSI oocytes, with a subsequent gradual reduction over days 4 and 5 (22 and 11% respectively) resulting in one early blastocyst on day 7 (11%). In contrast, all ICSI-generated embryos continued to cleave on day 3, with a gradual reduction over subsequent days (day 4, 86%; day 5, 57%; day 6, 43%; day 7, 29%). By day 7, two of the blastocysts had started to hatch, resulting in a 66% hatching rate of blastocysts formed from ICSI of cryopreserved oocytes. This is the first study to show normal development to the hatching blastocyst stage following ICSI of cryopreserved human oocytes.