Fertilization in vitro of hamster and mouse eggs in a chemically defined medium

Hamster and mouse eggs with follicular cells were penetrated by epididymal spermatozoa in a modified Krebs-Ringer-bicarbonate solution known to be suitable for fertilization of rat eggs in vitro. The highest proportion (84-88%) of hamster eggs penetrated was observed when 6-30 eggs with follicular cells were introduced into 10 microliter sperm suspension. Greater volumes of sperm suspension reduced the proportions of eggs fertilized, while increased numbers of eggs in the same volume gave greater penetration rates. No fertilization was observed when 21-30 denuded hamster eggs were introduced into 10-100 microliter sperm suspension, indicating that capacitation and/or acrosome reaction of hamster epididymal spermatozoa was induced only by the post-ovulatory oviduct contents in this medium. However, when small numbers (10-20) of mouse eggs with or without follicular cells were incubated in a large volume (400 microliter) of sperm suspension 84-96% and 91% were penetrated, respectively, suggesting a lack of importance of the products of ovulation on mouse sperm capacitation. The incidence of polyspermy was very low for the hamster (0-13%) and mouse eggs (0-10%).