Ultrastructure of human egg investments during IVF

Abstract The zona pellucida (ZP) and cumulus-corona cells (CC cells) surrounding human oocytes, polypronuclear ova and embroys derived from IVF eggs were studied by SEM/TEM to evaluate whether their possible alteration may influence oocyte fertilizability and early embryonic development. The outer surface of the ZP surrounding preovulatory oocytes showed a spongy structure, with numerous penetrations. In immature and atretic oocytes, the external aspect of the ZP had a compact, smooth architecture. Numerous penetrating sperm were present on the spongy ZP, whereas only a few, flattened spermatozoa were attached to the compact, smooth-surfaced zona. Condensation of the outer aspect of the ZP may cause disruption and/or reduction of its sperm-binding sites, thereby impeding sperm attachment. This, in turn, may reduce oocyte fertilizability. The CC cells were packed loosely and projected many microvilli and blebs into the wide intercellular spaces. Most CC cells possessed large cytoplasma containing mitochondria with tubular cristae; well-developed smooth endoplasmic reticulum membranes, and lipid droplets. This arrangement typical of steroidogenic cells, is comparable to that of parietal granulosa cells, just prior to and following ovulation, and subsequent luteinization. The presence of numerous enlarged, luteinized CC cells about oocytes, polypronuclear ova and embroys did not seem to exert a negative effect on fertilization. Indeed, these elements may promote fertilization by modifying, selecting and helping to orient the sperm present around the oocyte and cotribute toward early embryonic development by secreting small amounts of progesterone.