Sperm Bind but Do not Unbind from the Fixed Sea Urchin Egg: (sperm binding kinetics/fixed eggs)

In S. purpuratus sperm remained bound to aldehyde-fixed eggs and did not exhibit a detachment phase. Sperm bound in high numbers and the binding curve was similar to that for unfixed homologous gametes. Binding kinetics were analyzed using sequential still photographs of slightly flattened, fixed eggs under supported coverslips. In this way, a single egg could be observed. Alternatively, sperm remaining in the supernatant over settled eggs were counted at successive time points postmixing using spectrophotometry (340 nm) and hemocytometry. Additionally, aliquots of mixed gametes were fixed at successive time points and the numbers of bound sperm determined microscopically on egg perimeters. The maximum number of sperm bound by 2 min; this number remained at or near maximum for as long as 10 min when the experiments were terminated. When sperm were jelly-reacted before addition to eggs, fewer sperm bound, although binding curves were similar to the above experiments. It appears that the binding efficiency of sperm decreases with time after initiation of the acrosome reaction: (1) fewer prereacted sperm bound; and, (2) sperm did not continue to bind after 2 min even though the egg surface was not saturated. Whether these effects are related to motility or other factors is unclear. Furthermore, the above results indicate the importance of the cortical reaction to sperm unbinding. Such studies enable one to observe sperm behavior while precluding the effects of egg secretion during the initial stages of fertilization.