Egg-sperm interactions in humans: ultrastructural aspects

A large cumulus mass usually covers the human ovulated oocyte, and volumino us clusters of cumulus cells are still seen after fertilization around the egg. Cumulus cells surround oocytes and fertilized eggs also during in vitr o fertilization (IVF) procedures. This study describes, by transmission and scanning electron microscopy, the morphology and the microtopography of th e cells forming the human cumulus mass surrounding IVF samples (insemined b ut not fertilized oocytes and fertilized eggs). Particularly emphasized is their morphodynamic role in sperm-egg interactions. A comparison with the b ehavior in vivo of cumulus-enclosed oocyte/fertilized eggs has been also pe rformed. All patients have given their informed consent to participate in t his protocol. An inner layer (corona radiata cells) and an outer layer (proper cumulus ce lls) can be microtopographically recognized in the cumulus mass. Numerous c umulus-corona cells, particularly after fertilization, showed ultrastructur al characteristics typical for steroid synthetic cells, thus undergoing a s ort of "luteinization" parallel to that occurring in the sister granulosa c ells of the postovulatory follicle. This steroid synthetic activity, partic ularly enhanced in vitro but present also in vivo, may be finalized to the release of small amount of steroids (estrogens and progesterone) in the ooc yte/fertilized egg milieu. Various proteins, secreted by other cell subpopu lations - as revealed in other studies by our research group -, may even en rich this milieu. Lymphocytes and macrophages were often found in the cumul us mass. They may modulate the steroid secretion of the neighboring cumulus cells by production of cytokines, mimicking what occurs in the ovarian fol licle and, later, in the corpus luteum. Spermatozoa, both normal (acrosome- intact or - reacted) and abnormal, were frequently seen in the cumulus mass , free in the intercellular spaces or close to the cumulus cells, that can induce sperm capacitation and acrosome reaction. Leukocytes and cumulus-cor ona cells appeared both capable of actively phagocytizing supernumerary and /or abnormal sperms. Such spermiophagic response is present in a lesser ext ent around oocytes and eggs fertilized in viva. In vitro instead, cumulus s permiophagy leads to the elimination of a large part of the excess spermato zoa that have reached the oocyte, thus restoring in an extracorporeal mediu m the spermiophagic activity physiologically exerted by leukocytes and epit helial cells in the female and male genital tracts. In conclusion, the cumulus mass surrounding oocytes and fertilized eggs app ears as a highly dynamic system, in which Various subpopulations of cells c ooperate in order to provide a suitable and healthy microenvironment for fe rtilization and early embryo development.