Morphodynamic events of human oocytes during folliculogenesis and in the extraovarian microfollicular unit

Abstract Background: The efficacy of assisted reproductive technology (ART) protocols is often affected by problems related to oocyte behavior, mainly in relation to its maturative stage, storage, and, ultimately, fertilizability. The aim of this article is to review the ultrastructural characteristics of the human oocyte and its vestments (the zona pellucida and the cumulus oophorus) during folliculogenesis, after ovulation or during ART management. Methods: Fetal ovarian tissues (from cases of spontaneous abortion or surgical delivery, hysterectomy or hysterotomy), adult ovarian biopsies, and cumulus-enclosed oocytes (collected from the oviduct or obtained during ART procedures), obtained after the informed consent of the patients, were processed for transmission and scanning electron microscopy. Results: Ultrasound features associated with oocyte-zona pellucida maturative changes are described. Complex relationships between cytoplasmic projections of the innermost follicular cells and oocyte during folliculogenesis are discussed, and the organellar pattern (typical for steroid synthetic cells) shown by many cumulus cells at the time of ovulation and in vitro fertilization is described. Conclusions: Oocyte behavior seems closely related to morphodynamic metabolic activities of neighboring somatic cells. This activity is not restricted to the cells of ovarian follicle but continues inside the extraovarian microfollicular unit (oocyte and its vestments) at ovulation time or at follicular aspiration for ART procedures.