Ultrastructural characteristics of human oocytes fixed at follicular puncture or after culture.

The material consisted of 73 oocytes obtained at follicle aspiration in ovarian-stimulated women. Oocytes in various stages of maturation were either immediately fixed or cultured before fixation. Observations by transmission electron microscopy disclosed that 20% of the immature oocytes, which appeared normal in the light microscope, had commenced atresia, and one immediately fixed oocyte surrounded by a normal-appearing preovulatory cumulus mass was found to be degenerated. Further, a cumulus mass judged as preovulatory contained an oocyte possessing a germinal vesicle. Light microscopy is thus not always adequate for judging the condition of ova surrounded by cumulus cells. Cytoplasmic changes which were regarded as being related to oocyte maturation were a decrease in the number of vacuoles and multivesicular bodies, an increase in the volume of the endoplasmic reticulum, a clustering of mitochondria, and the appearance of aggregates of tubuli of the smooth endoplasmic reticulum surrounded by mitochondria and mitochondria-vesicle complexes. Certain features were found to cause misinterpretations at examination of oocytes in a light microscope. For instance, polar bodies can be mimicked by corona cells in the perivitelline space and pronuclei in oocytes by large mitochondria-vesicle complexes in the ooplasm. Consequently, not all oocytes in which polar body-like or pronuclei-like structures are observed are necessarily fertilized oocytes.