[Cryopreservation of female gametes after the second meiotic division]

During the past few years, experimentation in the field of assisted fertilisation has been focused on the freezing of the female gamete. This technique would allow the ethical problems relating to embryo conservation to be resolved. Unfortunately, in spite of the continuous progress made in embryology, conventional freezing methods have proved lethal to ovocytes owing to the profound morphological and functional changes that take place caused by the low temperature and the toxic action of cryoprotective agents. These alterations include: the thickening of the pellucid zone, thus preventing fertilisation; the confluence of cytoplasmatic organules into vesicles and masses; the depolymerisation of the microtubular apparatus of the meiotic spindle and loss of its orientation towards the egg cell, leading to lack of fertilisation or in the event that fertilisation takes place, a high incidence of chromosomal alterations, such as polyploidy, which are incompatible with subsequent embryonal development. This study aimed to evaluate the possibility of eliminating part of these alterations, in particular those affecting the meiotic spindle, an element of central importance in the formation of the zygote. For this purpose, ovocytes matured in vitro were treated with ionomycin, a substance capable of activating the second meiotic division, and then frozen. The mechanism of action of ionomycin can be attributed to the depolarisation of the plasmatic membrane of the ovocyte, with an increased flow of calcium ions and inhibition of a protein that blocks the second meiotic division. The activation of the second meiotic division has enabled the authors to avoid damage to the meiotic spindle resulting from freezing and to obtain encouraging results in terms of the percentage of fertilisation and embryonal development after freezing, and the subsequent insemination of the ovocyte. Embryonal development ceased during the subsequent phases for reasons that are not yet clear. However, this study showed that severe cellular damage induced by freezing is attributable to alterations to the meiotic spindle that condition the later phases of fertilisation and embryonal development.