Atypical Decondensation of the Sperm Nucleus, Delayed Replication of the Male Genome, and Sex Chromosome Positioning Following Intracytoplasmic Human Sperm Injection (ICSI) into Golden Hamster Eggs: Does ICSI Itself Introduce Chromosomal Anomalies?

Objective: To examine nuclear decondensation, positioning of sex chromosomes, and the S-phase in human sperm nuclei following intracytoplasmic sperm injection (ICSI) into hamster eggs.

Design: Prospective analysis of hamster eggs and human sperm following ICSI.

Setting: Division of Reproductive Sciences, Oregon Health Sciences University and Oregon Regional Primate Research Center.

Patient(s): Fertile donor sperm from a commercial source.

Intervention(s): Human sperm were examined by immunofluorescence stain, bromodioxyuridine (BrdU) uptake assay and fluorescence in situ hybridization following ICSI into hamster eggs.

Main Outcome Measure(s): Immunostaining and fluorescence in situ hybridization.

Result(s): Decondensation of human sperm nuclei occurred initially in the basal region, and perinuclear theca of sperm persisted around the condensed apical region. In some sperm nuclei, following ICSI the sex chromosomes were in the apical region, remaining condensed for longer than in the basal region. S-phase entry of human sperm nuclei following ICSI was delayed compared to the zona-free hamster egg penetration assay.

Conclusion(s): These results force questions about the mechanism of male pronuclear formation after ICSI and suggest new strategies for understanding the basis of chromosomal anomalies leading to birth defects as well as continuing improvements in the safety and efficacy of infertility therapies.