THE IMPLICATIONS OF A PATERNALLY DERIVED CENTROSOME DURING HUMAN FERTILIZATION - CONSEQUENCES FOR REPRODUCTION AND THE TREATMENT OF MALE FACTOR INFERTILITY

PROBLEM: Successful fertilization in humans follows a complex series o f events, including the completion of meiotic maturation of the oocyte with the extrusion of the second polar body, the decondensation of th e sperm nucleus and the maternal chromosomes into male and female pron uclei, the restoration of the sperm centrosome, and the nucleation of microtubule-mediated motility necessary to bring the male and female p ronuclei into close apposition. These events occur after both fertiliz ation in vitro and after intracytoplasmic sperm injection (ICSI), a ne w technique which is currently being applied in many clinics to overco me severe male infertility. Defects in any of the events leading to fe rtilization can be lethal to the zygote and may prove to be causes of infertility. METHODS: Imaging of inseminated human and rhesus oocytes using immunohistochemical techniques reveals several phases at which f ertilization arrests. RESULTS: Oocytes from some infertile patients fa iled to complete fertilization due to failure of the sperm aster micro tubules in uniting the sperm and egg nuclei. The rate of sperm aster f ormation, size, and organization during fertilization has been used as a measurement of bovine sperm quality. The development of an assay us ing Xenopus laevis oocyte extract can also be used to test sperm from various species for their ability to form asters and perform other cen trosomal functions in vitro, as well as another indicator of sperm qua lity. Semen from men with questionable fertility was found to contain sperm which are generally incapable of producing sperm asters. In addi tion, the activity of centrosomal proteins such as gamma-tubulin and c entrin have been detected in mammalian eggs and sperm. The levels of g amma-tubulin increase markedly after exposure to X. laevis egg extract . CONCLUSION: Defects in either male or female nucleus decondensation also resulted in the arrest of fertilization and was found to occur in both inseminated human oocytes and in rhesus oocytes fertilized by IC SI. These discoveries on the molecular basis of infertility in humans have important implications for infertility diagnosis and managing rep roduction.