Oocyte activation: lessons from human infertility

During fertilization, the spermatozoon penetrates through the cumulus cells and the zona pellucida that surrounds the oocyte, before it binds and fuse s with the oocyte plasma membrane to induce activation. In vitro fertilizat ion (IVF) studies performed in non-human mammals have contributed extensive knowledge regarding the mechanisms by which the spermatozoon activates the meiotic-arrested oocyte to resume meiosis, cleave and develop into an embr yo. Although IVF has been used extensively for treating subfertile couples, not all of them were able to benefit from this procedure. In intracytoplas mic sperm injection (ICSI), one viable spermatozoononly is sufficient for s uccessful fertilization of a single oocyte, Moreover, the injected fertiliz ing spermatozoon bypasses several physiological barriers, compared with IVF , which together could explain the high success rate for this procedure. IC SI has also allowed the identification of sperm components that are require d for successful fertilization.