How to Handle Gametes in the Laboratory-Kinetic Studies

Gametes from several animal groups lend themselves to laboratory studies, both due to their large numbers and the ease of collection. But when it comes to some aspects of fertilization studies, care must be taken to adhere to natural conditions and not to create artifactual conditions. One such requisite is the study of the kinetics of sperm-oocyte interactions. For over 100 years scientists have arbitrarily chosen sperm-oocyte ratios in the laboratory, without considering natural conditions and in the process have created perhaps one of the greatest dogma in fertilization studies, the existence of polyspermy preventing mechanisms. The first consideration to be made when contemplating whether oocytes have evolved mechanisms to allow one spermatozoon to enter while repelling all others is to study sperm- oocyte ratios at the site of natural fertilization. High ratios may have favoured an evolutionary process by which oocytes established polyspermy preventing mechanisms, low ratios would suggest polyspermy blocks are not necessary. That oocytes change morphologically and physiologically at activation is the subject of innumerable publications, but, whether these activation events have evolved to block the entry of supernumerary spermatozoa, or are merely part and parcel of changing the inert oocyte into the dynamic zygote, is pure conjecture. Two animal groups that are amenable to laboratory experimentation, the mammals and the sea urchins, have been the focal point for the study of fertilization for several decades. Mishandling the gametes from these animals in the laboratory have led to the idea that oocytes possess mechanisms to block polyspermy and this idea was then promulgated across the animal kingdom. In this review, I will first look at the sperm-oocyte ratios at the natural site of fertilization in mammals and sea urchins and then describe and criticise the laboratory experiments that led to idea of polyspermy blocks. Finally, I will prevent an overview of sperm-oocyte interaction across the phyla identifying strategies that assure monospermy.