Conditions of oocyte storage and use of noninseminated as compared with inseminated, nonfertilized oocytes for the hemizona assay

Abstract

To examine differences in sperm binding to the zona and recovery of oocytes from the storage vessel after oocyte preservation for the hemizona assay (HZA) by the method currently in predominant use, salt storage at 4 degrees C, as compared with a new method that should allow for indefinite preservation of zona receptors, dimethylsulphoxide (DMSO)/sucrose in liquid nitrogen (-196 degrees C). A second objective was to compare sperm binding to noninseminated zona as opposed to zona from inseminated, nonfertilized oocytes and to examine whether differences in binding potential were related to the patient's fertilization rate from the cycle in which the oocytes for the HZA originated. Binding and recovery were evaluated after 1, 2, 3, 6, 9, 12, and 17 to 25 months of storage. In vitro fertilization and andrology laboratories at the University of Iowa Hospitals and Clinics; academic tertiary care center. Binding of sperm was significantly lower for nonfertilized oocytes stored > 12 months in salt at 4 degrees 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 significantly lower after storage in salt for > 12 months as compared with storage in liquid nitrogen. Greater variability in sperm binding was observed between matching zona halves of nonfertilized as compared with noninseminated oocytes. Nonfertilized oocytes also bound fewer total sperm than noninseminated oocytes. The number of sperm bound to noninseminated oocytes was not related to the patient's fertilization rate from the cycle in which the oocytes originated. However, significantly fewer sperm bound to the zona of nonfertilized oocytes when the oocyte originated from a cycle in which the patient's fertilization rate was > 50%. These results indicate that storage of oocytes in DMSO/sucrose in liquid nitrogen results in superior long-term (> 12 months) preservation of zona receptors for sperm binding and improves oocyte recovery as compared with salt storage at 4 degrees C. Although noninseminated oocytes appear to be optimal for use in the HZA, nonfertilized oocytes can be used successfully if the oocytes originate from an IVF cycle in which the fertilization rate is < or = 50%.