[In vitro maturation of human oocytes]

Maturation of human oocytes has 3 aspects: nuclear maturation leading to the extrusion of the 1st polar body, membranar maturation essential for the fixation of spermatozoa to the zona pellucida and penetration into the oocyte and cytoplasmic maturation which allows protein synthesis required for normal fertilisation and embryo development. In vitro maturation (IVM) of human oocytes may be appropriate in 5 different situations: for PCOs patients (natural cycle), in normoovulatory patients (natural cycles), for oocytes not exposed to hCG (stimulated cycle), for immature oocytes recovered in the course of an ICSI protocol (stimulated cycle) and after freezing-thawing of immature oocytes. Data from the literature show that in vitro maturation of human oocytes together with ICSI can lead to normal fertilisation, embryo development, pregnancies and the delivery of healthy children. However, the overall efficiency is still very low, indicating that embryo viability is compromised. The incidence of chromosome abnormalities in mature oocytes obtained after IVM is similar to that of oocytes recovered after in vivo maturation and therefore does not seem to be the reason of the failures. Conversely, protein synthesis abnormalities and abnormal calcium signalling might explain the poor viability of the embryos. The key factor seems to be cytoplasmic maturation not yet fully understood in the human.