Ultrastructure of Maturing Human Oocytes

The material for study consisted of 41 nonatretic oocytes obtained at follicular aspiration in women in whom ovarian stimulation had been carried out. Collected oocytes were clinically classified as preovulatory and immature by the amount, dispersion, and viscosity of cumulus‐corona cells surrounding the oocytes. Preovulatory oocytes are surrounded by a large, sticky and loose cumulus mass and immature oocytes by, at the most, a few layers of corona cells. Some oocytes were fixed immediately, but most preovulatory oocytes were incubated with spermatozoa and cultured for 2–3 days before fixation. All of these latter oocytes were unfertilized at fixation.