Oocyte activation in mammals involves the action of a soluble sperm factor (SSF) that enables oocytes to develop a characteristic series of Ca2+ spikes (Ca2+ oscillations). SSF is also likely to be responsible for the Ca2+ oscillations driving oocyte activation after intracytoplasmic sperm injection [ICSI]. With an appropriate injection technique, Ca2+ oscillations do not develop spontaneously after ICSI but can be triggered by subsequent treatment of sperm-injected oocytes with Ca2+ ionophore. Here we show that Ca2+ oscillations, quite similar to those developing after ICSI, can be triggered by the ionophore treatment in human oocytes previously injected with human round spermatids. In contrast, oocytes injected with earlier spermatogenic cell (primary and secondary spermatocytes) and with non-germ cells (polymorphonuclear leukocytes) did not develop Ca2+ oscillations after the ionophore challenge although the subsequent injection of SSF did induce typical Ca2+ oscillations in these oocytes. Disintegration of the plasma membrane of the injected cells was detected in all cases by transmission electron microscopy. Thus, the absence of the typical oscillatory Ca2+ response in spermatocyte-injected oocytes was due to the actual deficiency of SSF in the spermatocytes rather than to a defective responsiveness of the injected oocytes or to the failure of SSF release into the oocyte cytoplasm. The ability of human round spermatids to induce a response to calcium in oocytes that is similar to that induced by mature spermatozoa may be important for normal embryonic development after spermatid conception.
Calcium Cytoplasm