Extremely low sperm counts, impaired motility, and abnormal morphology represent the main causes of failed fertilization in conventional IVF. Today, ICSI is the ultimate option to treat these cases of severe male-factor infertility. One single viable spermatozoon, preferably of good morphology, is selected by the embryologist and injected in each oocyte available.
ICSI is based on micromanipulation of oocytes and spermatozoa. Initially, partial zona dissection (PZD) was established to facilitate sperm penetration (4-7). The barrier to fertilization represented by the zona pellu-cida was disrupted mechanically so that the inseminated sperm cells obtained direct access to the perivitelline space of the oocyte. Subzonal insemination (SUZI) represented the next step in micromanipulation techniques (8-11). SUZI enabled the immediate delivery of several motile sperm cells into the perivitelline space by means of an injection pipette. ICSI is even more invasive because a single spermatozoon is directly injected into the ooplasma, thereby crossing not only the zona pellucida but also the oolemma. ICSI had been first used successfully to obtain live offspring in rabbits and cattle (12), and a preclinical evaluation was reported by the Norfolk group (13). The first human pregnancies and births resulting from this novel assisted-fertilization procedure were reported in 1992 (2). Thereafter, ICSI was revealed to be superior to SUZI in terms of oocyte fertilization rate (14-17), number of embryos produced, and embryo implantation rate (14-17). As a result, ICSI has been used successfully worldwide to treat infertility due to severe oligo-astheno-teratozoospermia, or azoospermia caused by impaired testicular function or obstructed excretory ducts (18,19).
Since the first publication describing the ICSI procedure, minor modifications contributed to reduced rates of oocyte degeneration, oocyte activation (one-pronuclear), and abnormal fertilization (three-pronuclear). Hyaluronidase may be responsible for oocyte activation; therefore, the concentration used during oocyte denudation and the exposure time of oocytes to the enzyme have been reduced (20). The moment of denudation relative to oocyte pick-up (immediately or four hours later) does not influence the ICSI results (21). The orientation of the polar body during injection does, however, influence embryo quality (22). Motile sperm cells are selected and immobilized prior to injection (23). Cytoplasm aspiration to ensure oolemma rupture is critical to the success of the ICSI procedure because the method of rupture has been correlated with oocyte degeneration (22). Furthermore, the morphology of the injected spermatozoon is related to the fertilization outcome of the procedure as well as to the pregnancy outcome (24).
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