Gamete manipulation for development: new methods for conception.

Mammalian oocytes microsurgically injected with spermatozoa can develop into normal offspring. Apparently the oocyte has the ability to decompose or eliminate such sperm components as the plasma membrane and acrosomal contents, which normally do not enter its cytoplasm. Species in which normal offspring were obtained by direct sperm injection include: human, mouse, rabbit, horse, sheep, cattle, pig, and monkey. In the mouse, normal offspring can also be obtained routinely by the injection of round spermatid nuclei into oocytes. This suggests that all post-meiotic modifications of spermatozoa (spermiogenesis, sperm maturation, capacitation and the acrosome reaction) evolved as processes solely dedicated to delivering male genomes into female gametes. Birth of normal offspring after injection of spermatocytes into maturing or mature oocytes suggests that the mechanisms controlling meiosis of male and female germ cells are similar, if not the same. Spermatozoa do not need to be morphologically normal or alive in the conventional sense to participate in embryo development; as long as they have intact genomes, they are able to produce normal offspring. Chromosomes within the first and second polar bodies can be used as substitutes for female pronuclei for the production of normal offspring. The nuclei of adult somatic cells can be used for production of animals. This procedure, involving introduction of cell nuclei into enucleated oocytes (genomic cloning), is rather inefficient at present. Many obstacles must be overcome before it is accepted as a safe, novel method to reproduce scientifically, medically or economically valuable animals. For human cloning the prime consideration must be the welfare of the child.