Recent insights in the ultrastructure of human oocyte

The positive outcome of assisted reproductive technologies (ART) depends on the oocyte quality, in terms of maintenance of full morphofunctional integrity during its complex and long maturation. In fact, the completion of maturation, in the preovulatory period, both in vivo and in vitro, as well as the preservation of cytoplasm, permit the oocyte to become competent for fertilization. The oocyte ultrastructure reflects the developmental competence of this cell that, therefore, has been object of several studies, especially after ART. Since, after completion of its maturation, the human oocyte acquires very peculiar fine structural features, likely influenced by the application of ART, we studied the above changes in human oocytes, from young and aged women, after informed consent, subjected to the most recent ART protocols including, in vitro maturation and or cryopreservation. In this paper, we reviewed by electron microscopy and correlated techniques, the changes of the nucleus, cytoplasm and zona pellucida, occurring in the human oocyte during the final stages of maturation, after ART. We could observe that the human oocyte undergoes a reorganization of its genome, ooplasm and extracellular matrix characterized by neogenesis, modification and redistribution of organelles, membranes and glycoproteins of the ZP. Our analysis showed that the completion of the maturation in the preovulatory period and the absence of cytoplasmic degenerative alterations, make the female gamete competent for fertilization. However, several subcellular aberrations, detectable only by electron microscopy analysis, occur in the oocyte as the consequence of the application of ART protocols (IVM and cryopreservation, especially) and could be responsible for ART failures, or even affecting early embryo development. Electron microscopy observations, associated with other morphological studies, and analyzed together with clinical, epidemiological, biological, molecular/ biochemical data, are useful in the assessment of the cell structural integrity, in order to define the proper “oocyte health” state.