Intracytoplasmic Sperm Injection (ICSI): Applications and Insights

When the oocyte (egg) and sperm combine in fertilization, one remarkable process is set in motion and another completed. The first is to initiate the establishment of totipotency, the faculty of a cell to give rise to an entire individual. The second is to produce a new and unique embryo genome comprising one meiotically rearranged nuclear contribution from each parent. Manipulating fertilization may therefore allow us to influence the genome of an entire individual at the stage of the one-cell embryo, and to learn how the totipotent state is established. In the last 50 years or so, such manipulation has been facilitated by injecting sperm directly into oocytes through a fine needle—a process termed intracytoplasmic sperm injection (ICSI). The application of ICSI to mammals now includes widespread use in research, particularly in the mouse, and in human assisted reproduction. Because ICSI bypasses facets of fertilization that are upstream of sperm–oocyte union, this technique has been adapted in the mouse for genome manipulation, the delivery of non-sperm nuclei into oocytes (e.g., somatic cell nuclear transfer) and to dissect totipotency. With this dissection in mind, we here briefly describe ICSI and discuss a recent extension to it in which sperm are injected into one-cell parthenogenetic haploid embryos rather than oocytes. The parthenogenote injection method, termed phICSI, reveals that totipotency can be established by a non-oocyte, mitotic cell-type, taking a different epigenetic path to the one taken in ICSI and natural fertilization. The implications of these findings are also discussed.