Ultraviolet-irradiated spermatozoa activate oocytes but arrest preimplantation development after fertilization and nuclear transplantation in cattle

Artificial means of parthenogenetically activating mammalian oocytes are be lieved to lack an essential sperm epigenetic component required for normal development. The main goal of this study was to examine the potential of ul traviolet (UV)-irradiated sperm as a means of functionally eliminating the chromatin component of spermatozoa without affecting the ability to induce activation and support parthenogenetic development in cattle. Spermatozoa w ere stained with a DNA dye, exposed to various UV irradiation doses, and us ed to fertilize secondary oocytes, Although the percentage of pronuclei at 18 h postinsemination was similar using treated and control sperm, most ooc ytes fertilized by UV-irradiated sperm failed to develop beyond the 2-cell stage, suggesting that UV irradiation can functionally destroy the genomic component of spermatozoa with limited effects on the ability to induce oocy te activation. However, when oocytes activated with UV-irradiated sperm wer e used as hosts for nuclear transfer, developmental rates to cleavage and t o blastocyst improved only marginally and remained lower than in the contro ls, indicating that UV-treated spermatozoa blocked development even in the presence of a diploid donor nucleus. Although DNA replication was not inhib ited by UV irradiation treatment, abnormal chromatin morphology after cleav age suggests improper segregation of chromatin to daughter blastomeres duri ng the first mitotic division. Together, these results indicate that althou gh sperm exposed to UV can activate oocytes, a developmental block occurs a t or soon after the first mitosis in parthenotes and oocytes reconstructed by nuclear transfer.