THE STAGES AT WHICH HUMAN FERTILIZATION ARRESTS - MICROTUBULE AND CHROMOSOME CONFIGURATIONS IN INSEMINATED OOCYTES WHICH FAILED TO COMPLETEFERTILIZATION AND DEVELOPMENT IN HUMANS

The goal of fertilization is the union of one, and only one, sperm nuc leus with the female pronucleus within the activated oocyte. For this to occur successfully, several events must transpire, including the in corporation of the entire spermatozoon into the oocyte, the completion of meiotic maturation with the extrusion of the second polar body, th e metabolic activation of the previously quiescent oocyte, the deconde nsation of the sperm nucleus and the maternal chromosomes into the mal e and female pronuclei respectively, and the cytoplasmic migrations of the pronuclei, which bring them into apposition. Defects in any of th ese events are lethal to the zygote and might prove to be causes of in fertility. In this study, the microtubules and DNA were imaged in inse minated human oocytes that had been discarded as unfertilized. The pre sence and number of incorporated sperm tails were also documented usin g a monoclonal antibody specific for the post-translationally modified acetylated-alpha-tubulin found in the tail, but not the oocyte, micro tubules. An analysis of 211 oocytes from failed in-vitro fertilization s from 58 patient couples resulted in the determination of several pre viously undetectable phases at which fertilization arrests: (i) metaph ase II arrest; (ii) arrest after the successful incorporation of the s permatozoon; (iii) arrest after the formation of the sperm aster; (iv) arrest during mitotic cell cycle progression; and (v) arrest during m eiotic cell cycle progression. Data on polyspermy and arrested embryon ic development are also presented. These results have implications for the diagnosis and treatment of female, as well as male, infertility. They also provide a rationale for the reasonable use of intracytoplasm ic sperm injection (ICSI) therapy, although they suggest that cases in tractable to this approach will be found. Concerns are raised about th e use of seemingly 'unfertilized' but inseminated oocytes for subseque nt re-inseminations, ICSI or even research, since the fertilization pr ocess might have arrested after sperm penetration. These results demon strate that the proper union of the parental genomes requires a series of cytoskeletal-mediated events on the oocyte surface and within the oocyte proper, and that failure at any phase results in the arrest of human fertilization.