Sperm-induced oocyte activation in the rhesus monkey: nuclear and cytoplasmic changes following intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) has increased the potential of the assisted reproductive technologies to propagate mammalian species and has provided an opportunity for research into cell cycle control and the mechanisms involved in sperm-induced oocyte activation. We have investigated the efficacy of ICSI in the rhesus monkey, the mechanism of fertilization following sperm injection and the cytoskeletal rearrangement that occurs upon oocyte activation. These studies were conducted on mature, and to a lesser extent, immature oocytes. Ejaculated spermatozoa, washed, capacitated and activated before immobilization, were injected into oocytes using conventional ICSI methodology. Sperm injection into mature oocytes induced oocyte activation (19/22; 86%) and pronuclear formation. In contrast, sham-injected oocytes did not activate readily (2/16; 13%). To localize oocyte activation factor(s), spermatozoa were separated mechanically into heads and tails which were then injected individually into mature oocytes. Activation occurred in 87% (20/23) of oocytes receiving heads. After tail injection, a single microtubule aster was nucleated and one pronucleus (PN) was seen in four of 21 oocytes. Intracytoplasmic injection of sperm extract (SE) resulted in oocyte activation at a significantly higher rate than occurred following sham injection (76 versus 13%). Sperm-induced oocyte activation was also evaluated in immature metaphase (MI) oocytes; activation occurred in 46% (12/26) of cases; however, only 8% of the activated oocytes exhibited 2 PN. Finally, beta-tubulin localization in untreated and taxol-treated oocytes was established as a marker for cytoplasmic changes associated with oocyte activation. These results are consistent with the hypothesis that spermatozoa contain an oocyte activating factor(s) which is primarily localized in the sperm head. Moreover, an activation response is limited to mature oocytes and is accompanied by cytoskeletal changes analogous to those seen following conventional fertilization.