Production of normal mice from oocytes fertilized and developed without zonae pellucidae

Mouse oocytes, freed from zonae pellucidae, were inseminated and cultured in vitro. When inseminated with a modest concentration of spermatozoa (approximately 5/microliter), 79% of 151 oocytes were fertilized normally, of which the majority developed into blastocysts. When 177 blastocysts were transferred to 12 pseudopregnant foster mothers, 10 (83%) of the mothers became pregnant and 37 live pups were obtained. Although the overall rate of development of zona-free embryos was slightly inferior to that of zona-intact (control) embryos, these results nevertheless indicate that the oocyte and embryo are potentially capable of normal fertilization and development in the total absence of the native zona pellucida. Since the acrosome reaction is essential for successful sperm-oocyte fusion, zona-free oocytes must have been fertilized by spermatozoa which had undergone the reaction either in the medium or on the surface of the oocyte plasma membrane. Such acrosome reactions, which are not mediated by the native zona pellucida, can be called 'unphysiological' or 'spurious' but the spermatozoa with such reactions were certainly functional, as they produced healthy offspring.