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Microscopic analyses of mammalian gamete ultrastructure have provided among the most seminal insights into fertilization. Here we present a summary of ultrastructural studies of mammalian fertilization, together with a review of the effects of post-ovulatory aging in eggs, and our own results using scanning electron microscopy to examine the effects of post-ovulatory aging on the egg membrane topography. Our previous work detected two abnormalities in egg membrane function in aged eggs: aged eggs appeared to be less able to support sperm-egg membrane interaction, thus rendering the eggs less fertilizable, and aged eggs appear to have a reduced ability to prevent polyspermy at the level of the egg membrane, i.e., to establish a membrane block to polyspermy. In the work presented here, we tested the suitability of both environmental (ESEM) and low-vacuum (LVSEM) modes of the FEI Quanta 200 Environmental Scanning Electron Microscope to study mammalian gametes. While ESEM mode was not sufficient under the conditions we used to observe fine cell surface details, the LVSEM mode proved to be an excellent way to view egg membrane topography. Unfertilized aged eggs have an abnormally distended amicrovillar region over the meiotic spindle, and fertilized aged eggs had three different abnormalities detected, from reduced sperm-induced membrane remodeling to abnormal sperm-induced membrane remodeling and membrane blebbing. Combining these insights of egg membrane function and topography with others in the field of post-ovulatory aging, we have expanded insights into why fertilization at increased times after ovulation is associated with poor reproductive outcomes. [ABSTRACT FROM AUTHOR]