Oocyte activation: The impact of calcium signals on fertilization

Fertilization is triggered by changes in intracellular calcium concentration. In mammals, these transients in ooplasmic calcium concentration take the form of repetitive spikes, so called calcium oscillations (Ca2+-oscillations). These oscillations are important for relieve of meiotic arrest and to induce all the other events of oocyte activation. Although a surface mediated way of oocyte activation has been proposed, there is now substantial evidence to suggest that the sperm cell induces these Ca2+-oscillations by introducing a sperm specific phospholipase C, PLCζ, in the ooplasm. Ca2+-oscillations are also observed after intracytoplasmic sperm injection (ICSI), a successful technique in human assisted reproduction. In the rare cases that no fertilization is observed following ICSI, this may be due to a deficiency in PLCζ. However, artificial activating the oocytes after ICSI by increasing the calcium concentration can restore fertilization rates in these cases and support further development, as evidenced by successful pregnancies. Further evaluation of the current protocols for assisted oocyte activation is appropriate and investigation of the future application of PLCζ is warranted.