SPERM-ASSOCIATED OOCYTE-ACTIVATING FACTOR IS RELEASED FROM THE SPERMATOZOON WITHIN 30 MINUTES AFTER INJECTION AS A RESULT OF THE SPERM-OOCYTE INTERACTION

We have previously shown that sperm plasma membrane damage makes the s perm plasma membrane permeable and the sperm nucleus accessible for lo w molecular weight molecules such as eosin and dithiothreitol. In the present study, we investigated whether this damage is associated with a passive release of the sperm-associated oocyte activating factor (SA OAF) from the spermatozoon and, if so, its time sequence, In a first s tudy, human oocytes remaining unfertilized after conventional in-vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) and fr eshly ovulated mouse oocytes were injected with a whole spermatozoon o r a sperm head respectively, They were randomly allocated to one of th ree groups: oocytes in group 1 were injected with a spermatozoon immob ilized or sperm head detached immediately prior to the injection; oocy tes in group 2 were injected with a spermatozoon immobilized or sperm head detached 2-4 h before injection; oocytes in group 3 were injected with a spermatozoon or sperm head that had been subjected to heat tre atment, The activation rate of oocytes injected with a spermatozoon or sperm head was the same for groups 1 and 2, and significantly higher than in group 3 (P < 0.001), In a second series of experiments, human oocytes remaining unfertilized after IVF or ICSI were injected with a sperm head that was subsequently removed from the ooplasm 20-30 min af ter injection, The activation rates were compared to that of oocytes i njected with heat-treated spermatozoa which subsequently were removed from the ooplasm, We found that the removal of the spermatozoon 30 min after injection did not prevent oocyte activation, Our data indicate that the initial damage to the sperm plasma membrane induced at immobi lization, although essential for the onset of sperm nuclear swelling a fter ICSI, does not by itself lead to the release-of SAOAF from the sp ermatozoon. We postulate, however, that SAOAF is released during the s perm nuclear swelling phase, which is induced by the so-called sperm n ucleus decondensing factor (SNDF) of the oocyte.