Oocyte activation after intracytoplasmic injection of mature and immature sperm cells.

The inactivation of metaphase-promoting factor (MPF), leading to reactivation of the oocyte cell cycle after fertilization, is one of the most important results of oocyte activation in the case of intracytoplasmic injection of mature or immature sperm cells. Oocyte activation is a cell signalling event that is likely to imply receptors at the oocyte plasma membrane and a signal transduction pathway involving calcium and protein phosphorylation/dephosphorylation. The typical calcium signal during oocyte activation in mammals takes the form of calcium oscillations. Intracytoplasmic sperm injection (ICSI) is associated with a slightly different pattern of calcium oscillations in comparison with normal fertilization. Available data suggest that sperm cytosolic factor(s) play a role of oscillator, modulating the properties of the oocyte's intracellular calcium stores, whereas the role of trigger, normally realized by spermoocyte interactions at the level of their respective cell surfaces, can be supplemented in the conditions of ICSI by an artificial calcium influx generated by the procedure itself. Delayed onset and an abnormal form of the oocyte activation-promoting calcium signal are the two known molecular abnormalities of human oocyte activation; they can lead to fertilization failure and are also suspected to be at the origin of various embryo abnormalities. The impact of oocyte activation abnormalities on future development increases when oocytes are fertilized with immature sperm cells (spermatids) because the chromatin of these cells is less protected than sperm chromatin against a rapid action of the oocyte's MPF. If oocyte stimulation by the sperm calcium oscillation-promoting activity, which first appears at the round spermatid stage of human spermatogenesis, is insufficient to cause a rapid inactivation of MPF, premature condensation of spermatid chromatids may lead to aneuploidy.