A mature egg cell, also known as a metaphase II egg cell, is a cell that has the polar corpuscle in proper position and is susceptible to fertilization. The polar corpuscle contains 50% of the chromosomes of the egg.
When eggs are obtained by follicular puncture in an assisted reproduction treatment, it is necessary to decumulate the egg to know if its stage is mature. Once decumulated, the oocyte is visualized under the microscope to observe the presence or absence of the polar corpuscle.
Only mature oocytes will be fertilized with sperm from the couple or a donor. Immature eggs are discarded or can be matured in vitro.