Fertilization of mammalian eggs by sperm injection

Abstract

Mammalian sperm normally fertilize eggs in the ampulla of the oviduct after a long trip through the female reproductive tract. During this trip, the sperm become capacitated to fertilize but must overcome substantial barriers in the form of egg investments before reaching the plasma membrane of the egg. Once inside the egg the head of the sperm responds to cytoplasmic factors to decondense and form a pronucleus. All the problems of sperm penetration of the egg and its investments can be circumvented by injecting the sperm into the egg with a micropipette. Such injected sperm can participate normally in the subsequent events, typical of fertilized eggs, that lead to the formation of embryos. Sperm can also be injected into normally fertilized eggs to produce a third pronucleus and a triploid embryo. To avoid the mechanical problems of capturing a swimming sperm with a micropipette, the sperm suspension can be sonicated to break off the sperm tails. The resulting sperm heads can easily be injected into unfertilized eggs to induce the normal events of fertilization. With these injection procedures it is possible to test the fertilizing capacity of foreign sperm, of defective sperm, and even of “dead” sperm. We have found that the phenotype of the sperm does not reflect the genotype in terms of fertilizing ability after microinjection. Immotile and grossly defective sperm of the mouse when injected into the egg produce the same reactions that are produced by fertilization with healthy robust sperm. Thus we conclude that the complex set of interactions normally required for sperm to penetrate the egg investments and to fertilize the egg are biologically unnecessary and can be circumvented by direct injection of the sperm into the egg.