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.