Understanding and addressing oocyte maturation failure and failed or abnormal fertilization during oocyte-to-zygote transition.

The transition from an immature oocyte to a normally fertilized zygote is a perilous bridge between gamete and embryo life. Failure can occur during oocyte maturation or in the multiple steps required for fertilization. In assisted reproduction cycles, a proportion of retrieved oocytes are found at the prophase I or metaphase I (MI) stages. These oocytes can be cultured in an attempt to assist the progression to metaphase II (MII) stage, and therefore achievement of meiotic maturation, in vitro. However, the absence of robust culture systems considerably limits the rates of oocyte maturation in vitro and the full acquisition of developmental potential of rescued oocytes. Developmental failure can also occur during the sperm-oocyte interactions that lead to the formation of the zygote. Fertilization can fail entirely; in such cases, failed fertilized oocytes can be treated by intracytoplasmic sperm injection (ICSI) during the first 24 hours from the initial insemination. Fertilization and developmental rates of reinseminated oocytes are suboptimal, but clinically significant outcome rates can be achieved. Fertilization can also occur atypically, generating, not bipronuclear (2PN), but monopronuclear (1PN) and tripronuclear (3PN) zygotes. Especially 1PN zygotes, if capable of developing to the blastocyst stage, have notable chances to be biparental diploid. Therefore, they can be cultured at the blastocyst stage and, ideally, undergo chromosomal testing to assess their suitability for clinical use. This range of possibilities for oocyte and zygote rescue is attracting increasing interest, especially for the treatment of cases in which one extra embryo can have a significant impact on clinical outcome.