Comparison of Ca2+ responses in human oocytes fertilized by subzonal insemination and by intracytoplasmic sperm injection.

OBJECTIVE To evaluate the consequences of bypassing the normal interaction between the sperm and oocyte surfaces for the form of Ca2+ responses developing in oocytes at fertilization. DESIGN Oocytes were fertilized by subzonal insemination (SUZI) (maintaining the normal interaction between cell surfaces of both gametes) or by direct intracytoplasmic sperm injection, and changes in intracellular free Ca2+ concentration were evaluated by confocal laser scanning microscopy after loading oocytes with a fluorescent Ca2+ indicator. SETTING Private hospital, public research center, and university-based laboratory. PATIENTS, PARTICIPANTS Patients participating in an assisted reproduction program. INTERVENTIONS In vitro fertilization, SUZI, intracytoplasmic sperm injection. MAIN OUTCOME MEASURES Changes in intracellular free Ca2+ concentration. RESULTS All oocytes fertilized after SUZI showed an oscillatory Ca2+ response introduced by a short initial phase with faster Ca2+ oscillations. In contrast, oocytes fertilized after intracytoplasmic sperm injection did not show a similar change in the oscillation rhythm. In both cases, Ca2+ increases were propagated throughout the ooplasm in a wave-like manner. CONCLUSIONS The results show that there is a relationship between gamete surface contact and the form of Ca2+ fluxes accompanying fertilization. When the contact between gamete surfaces is skipped by direct sperm injection to the ooplasm, a delayed, truncated Ca2+ response is produced which, however, can maintain the form of Ca2+ waves and Ca2+ oscillations typical of normal fertilization.