CONDITIONS OF OOCYTE STORAGE AND USE OF NONINSEMINATED AS COMPARED WITH INSEMINATED, NONFERTILIZED OOCYTES FOR THE HEMIZONA ASSAY

Objectives: To examine differences in sperm binding to the zona and re covery of oocytes from the storage vessel after oocyte preservation fo r the hemizona assay (HZA) by the method currently in predominant use, salt storage at 4-degrees-C, as compared with a new method that shoul d allow for indefinite preservation of zona receptors, dimethylsulphox ide (DMSO)/sucrose in liquid nitrogen (-196-degrees-C). A second objec tive was to compare sperm binding to noninseminated zona as opposed to zona from inseminated, nonfertilized oocytes and to examine whether d ifferences in binding potential were related to the patient's fertiliz ation rate from the cycle in which the oocytes for the HZA originated. Design: Binding and recovery were evaluated after 1, 2, 3, 6, 9, 12, and 17 to 25 months of storage. Setting: In vitro fertilization and an drology laboratories at the University of Iowa Hospitals and Clinics; academic tertiary care center. Results: Binding of sperm was significa ntly lower for nonfertilized oocytes stored > 12 months in salt at 4-d egrees-C than for those stored in liquid nitrogen. Binding was similar after storage for 1, 2, 3, 6, 9, and 12 months. Oocyte recovery was s ignificantly lower after storage in salt for > 12 months as compared w ith storage in liquid nitrogen. Greater variability in sperm binding w as observed between matching zona halves of nonfertilized as compared with noninseminated oocytes. Nonfertilized oocytes also bound fewer to tal sperm than noninseminated oocytes. The number of sperm bound to no ninseminated oocytes was not related to the patient's fertilization ra te from the cycle in which the oocytes originated. However, significan tly fewer sperm bound to the zona of nonfertilized oocytes when the oo cyte originated from a cycle in which the patient's fertilization rate was > 50%. Conclusions: These results indicate that storage of oocyte s in DMSO/sucrose in liquid nitrogen results in superior long-term (> 12 months) preservation of zona receptors for sperm binding and improv es oocyte recovery as compared with salt storage at 4-degrees-C. Altho ugh noninseminated oocytes appear to be optimal for use in the HZA, no nfertilized oocytes can be used successfully if the oocytes originate from an IVF cycle in which the fertilization rate is less-than-or-equa l-to 50%.