Fertilization and developmental initiation of oocytes by injection of spermatozoa and pre-spermatozoal cells

The essence of fertilization is the union and mingling of male and female genomes. Therefore it is not surprising that microsurgical deposition of a single spermatozoon in an oocyte (ICSI) results in the development of normal offspring. Poorly motile or structurally aberrant spermatozoa, which are unable to fertilize under ordinary conditions, are not necessarily genomically abnormal. This is the reason why normal offspring are obtained after ICSI using such spermatozoa. At present, ICSI is most successful in humans and mice, but there is no reason to believe that ICSI does not work in other animal species as well. Injection of round spermatids into oocytes (ROSI) works routinely in the mouse, but it is controversial in humans. While some investigators have claimed successes, many others have reported complete failure. There must be several reasons for this, including the difficulty of distinguishing true spermatids from other small cells. Insufficient oocyte activation following ROSI and the functional immaturity of the centrosome could also be responsible for this. In mice, it is possible to obtain normal offspring by injection of primary or secondary spermatocytes into oocytes. The nucleus of a spermatocyte undergoes meiotic division(s) within the oocyte's cytoplasm before a haploid sperm pronucleus unites with an oocyte's haploid pronucleus. However, only a few of the produced zygotes have developed into fertile offspring. There are many hurdles to clear before ROSI and spermatocyte injection becomes efficient and medically safe methods for assisted fertilization.