Electircal Oocyte Activation
Researcher of Genetic Diagnosis Lab, CAO HUI-MEI
Introduction to assisted fertilization
From 1992 and on, Intracytoplasmic Sperm Injection (ICSI) has become a technique beneficial to many male patients with severe infertility, for example, oligozoospermia, azoospermia, asthenozoospermia, teratozoospermia, etc. However, the fertility rate still remains low in some patients receiving ICSI. To overcome this problem, embryologists have focused on the mechanism of fertilization and developed the“Electrical Oocyte Activation” technology to assist fertilization.
Upon resumption of the first meiotic division, the cytostatic factor will arrest a normally matured oocyte at metaphase II (MII) where it awaits fertilization. Upon fertilization, spermatozoa overcome the second meiosis arrest by inactivate cytostatic factor, allow the cell cycle to continue, and induce a series of cellular events within the oocyte collectively called oocyte activation. These events include an early intracellular rise in calcium concentration to activate cortical reaction, which prevents additional sperm from binding and penetrating the zona pellucida and ensure the diploidy of chromosome. The induced calcium oscillation leads to resumption of meiosis, extrusion of the second polar body, decondensation of sperm nucleus, formation of male and female pronuclei, initiation of DNA synthesis, and cleavage. Elevation of calcium concentration is a key step in fertilization, and electrical oocyte activation is to increase calcium concentration in oocyte for a long time via single electric pulse.
A study in 1997 reported successful pregnancy and healthy childbirth through the combination of ICSI and electrical oocyte activation for the couple who suffered from failure in oocyte activation after ICSI. General fertilization rate for ICSI is approximately 70%, and was not improved by the combination of electric oocyte activation. Therefore, embryologists mainly aimed at the application the electric oocyte activation for the sperms with low fertilization rate by ICSI.
We introduced electric oocyte activation technology for assisted fertilization in 1998. In a total of 20 cases whose fertilization rate after ICSI was below 30%, combination of ICSI and electric oocyte activation increased the fertilization rate to 66% in their next cycle of IVF therapy. Based on current results, we recommend the assisted fertilization by electric oocyte activation for the patients with less than 30% fertilization rate with ICSI. The pregnancy rate is comparable to that after conventional ICSI.
▲Figure: oscillogram of oocyte electric activation