Fertilization in the Mouse

To the murine spermatozoon, its conspecific egg in vivo must seem as remote and screened by obstacles as was the Holy Grail to the Knights of Chivalry. In order that the egg be fertilized, the sperm must overcome obstacles found in the female reproductive tract and surrounding the egg itself as well as prepare itself for the membrane fusion events that lead to its entry into the egg’s cytoplasm. In the laboratory, the technique of in vitro fertilization removes most of the naturally occurring impediments posed by the female reproductive tract and provides almost unobstructed access for the sperm to the egg and an unobstructed view for the investigator. The process by which the sperm and egg interact from time of initial contact to formation of male and female pronuclei may, as a result, be studied in detail. In the mouse, successful in vitro fertilization of isolated eggs was accomplished over 20 years ago (Whittingham, 1968). In the intervening period, clarification of the sequence of the reactions involved in mouse sperm—egg interaction leading to fertilization has begun, and the number and complexity of those reactions are now more fully appreciated. However, too much emphasis on in vitro experiments may serve to obscure the biology of fertilization. Section 2 of this chapter is, therefore, concerned with the natural route taken by the sperm to reach the egg and the ensuing events as fertilization occurs in vivo. Section 3 then describes in vitro studies aimed at understanding the reactions involved in direct sperm—egg interaction that culminates in fertilization of the egg. It is hoped that information gained from both in vivo and in vitro studies will permit the construction of a unifying description of the process of fertilization in the mammal.