Intracytoplasmic Sperm Injection (ICSI) is one method of human egg fertilization, involving the injection of one sperm cell into an egg cell. Sperm must be collected and prepared by washing off the seminal plasma and running the sperm through a special filter to identify the best quality sperm. One sperm is chosen, and it is concentrated in a tiny volume of culture fluid. Ova from the female must also be collected, in the same process used with in vitro fertilization. The cumulous cells that surround the ovum are removed, and the selected sperm cell is immobilized so the beating tail will not damage the ovum. Next, the single sperm cell is injected into the ovum using a 5 µm needle, one-fourteenth the size of a human hair. The newly fertilized eggs are usually cultured for an additional one or two days, during which the embryos are monitored to make sure they are developing properly. After this culture period, the embryos are either transferred to the uterus or cytopreserved, depending on the patient's condition.
Usually several embryos are implanted, due to a pregnancy rate of only about 10-40% with current ICSI techniques. Leftover embryos that are properly developed are usually frozen and stored, while those that were not successfully fertilized are usually thrown out.
Gamete manipulation began in the late seventies, culminating with the first successful birth from in vitro fertilization in 1978. However, for couples with severe male factor infertility, in vitro fertilization had poor results. As a result, new techniques, such as zona drilling and partial zona dissection (PZD) and Subzonal insertion of sperm (SUZI), were developed for these patients. The high rate of polyspermy, which was fatal for the embryo, led to the development of ICSI, in which only one sperm was injected directly into the egg cell.
ICSI was first applied to human gametes in 1985, although the first reported pregnancies resulting from ICSI came only in 1992.