The effects of spermatozoal irradiation with X-rays on chromosome abnormalities and on development of mouse zygotes after delayed fertilization

The chromosome complements of zygotes derived from oocytes aged post ovulation and fertilized in vivo with X-ray-irradiated sperm were studied. Ovulation was induced by an injection of luteinizing hormone-releasing hormone (LHRH) at pro-estrus and fertilization was achieved by artificial insemination at 13 h and 24 h after LHRH in order to obtain embryos from unaged and aged (12 h post-ovulation) oocytes respectively. Post-ovulatory aging prior to fertilization did not significantly affect the percentage of zygotes with irradiation-induced chromosome abnormalities. However, post-ovulatory aging had a negative effect on the morphology of male as well as female pronuclear chromosomes of the first cleavage metaphase. When fertilized with control spermatozoa this effect was apparent in both the male and the female pronucleus. When unaged oocytes were fertilized with X-irradiated spermatozoa chromosome morphology was also adversely affected in both pronuclei. In zygotes from aged oocytes, there was an extra negative effect of X-rays on the male pronuclear chromosomes only. After fertilization with X-irradiated sperm 27% of zygotes from aged oocytes were arrested at interphase compared to 7% from unaged oocytes. We suggest that post-ovulatory aging and X-rays affect the male and female pronuclear chromatin structure after fertilization. These chromatin alterations could interact with DNA lesions induced in the spermatozoa prior to fertilization, such that development to first cleavage can be blocked.