Is fine morphology of the human sperm nuclei affected by in vitro incubation at 37°C?

Objective: Incubation of ejaculated spermatozoa at 37°C is recommended for IVF-intracytoplasmic sperm injection. Preselection of sperm cells with morphologically normal nuclei before microinjection, adapted in our laboratory, is usually a time-consuming procedure. Therefore, we aimed to verify whether incubation at 37°C could affect the morphologic integrity of sperm nuclei. Design: Time-kinetics studies testing fine morphology of the sperm nuclei upon prolonged in vitro incubation. Setting: Male Fertility Laboratory at Bar-Ilan University, Ramat Gan, Israel. Patient(s): Forty-two males selected at random, who were referred for sperm preselection before ICSI. Main Outcome Measure(s): Morphologic integrity of the sperm nuclei, obtained by motile sperm organelle morphology examination. Result(s): After 2 hours of incubation at 37°C a significant decrease occurred in the morphologic integrity of the sperm nuclei, compared with the initial state (4.7% ± 2.8% vs. 6.8% ± 3.5%, t = 3.2, P≤.01). Correspondingly, a significant increase occurred in the frequency of vacuolated nuclei (80.8% ± 7.2% vs. 75.0% ± 7.6%, t = -2.8, P≤.01). No significant morphologic changes in sperm nuclei were observed upon prolonged incubation at 21°C. After 2 hours of incubation, the incidence of spermatozoa with vacuolated nuclei was significantly higher at 37°C compared with 21°C (56.5% ± 10.8% vs. 45.5% ± 10%, t = 6.6, P≤.01). Conclusion(s): Prolonged (≥2 hours) sperm manipulations for assisted reproduction therapy should be performed at 21°C rather than 37°C.