Through the controlled application of ovarian hyperstimulation, it is current practice to time the retrieval of mature oocytes (eggs) from a woman's ovary. The yield may vary anywhere from one to 30 or more eggs that may be retrieved depending on the responsiveness of the ovaries to the gonadotropins used to stimulate them. These eggs are gathered by the embryologist into an appropriately balanced salt solution and maintained at body temperature (37°C) until such time as they are ready to be inseminated. Meanwhile, a sample of semen containing the sperm destined to be used for each specific set of eggs is collected and processed by cell separation techniques to provide as clean and active a sample of sperm as possible. A major emphasis of the IVF laboratory is directed toward guaranteeing that the correct sperm go with the right eggs through good labeling and check systems. Ultimately, following several hours in culture, eggs and sperm can be mixed and allowed to bind and fertilize in a relatively natural fashion. Depending on the quality and maturity of both eggs and sperm, it is common for fertilization rates to vary considerably relative to the original number of eggs collected. Twenty eggs retrieved in no way guarantees 20 embryos. Likewise, 20 fertilized eggs in no way guarantees that there will be 20 embryos of sufficient quality for both cryopreservation and fresh transfer to the woman's body.
Central to the question of how many embryos are actually utilized in any IVF treatment cycle is the period during which the embryos are cultured in vitro. This can be as little as one day, or up to five in the case of blastocyst growth and transfer. Assuming that culture conditions are relatively optimal, there is less and less reason not to culture embryos throughout their pre-implantation stages to allow the embryos to "select" themselves for transfer or cryopreservation. The blastocyst is the term given to the very last stage of an embryo prior to it implanting into the endometrial lining of the uterus. The poorer the rates of blastocyst growth are, the more restricted the choice of embryo is at this stage of development. In any event, growth of any embryos to the blastocyst stage improves the level of discrimination of embryo viability available to the embryologist, and is key to reducing the numbers of embryos used for uterine transfer. The more confidence a clinic has in the viability of the embryos it transfers, the less need there is for multiple transfers of three or more embryos. Thus with the transfer of three or less embryos, the risk of multiple pregnancies is significantly reduced, in turn minimizing risks of pregnancy loss or fetal abnormalities common in multi-fetal pregnancies.
Micromanipulation is the technique whereby sperm, eggs and embryos can be handled on an inverted microscope stage, performing minute procedures at the microscopic level via joysticks that hydraulically operate glass microtools. Micro-manipulation first saw clinical use in IVF for purposes of assisted fertilization in the treatment of male factor infertility, where fertilization potential was low in cases of poor sperm quality. The ultimate evolution of this approach has been the development of the single sperm injection procedure referred to as Intracytoplasmic Sperm Injection, or ICSI. Sperm of virtually any quality and from any level of the male reproductive tract may be used with the only criterion for use being that the sperm is alive even if it is not moving (motile). Dead sperm may be able to achieve fertilization; however, the DNA or genetic material from such sperm is too degenerate to form a viable embryo. Immature sperm from the testicle or the epididymis can be retrieved for use with ICSI for men who possess no sperm in their ejaculated semen (azoospermia). This azoospermia is either due to an obstruction in the tract (obstructive), or to extremely low production of sperm in the testicle itself (non-obstructive). In certain cases, men may produce sufficient sperm, but they do not survive to the point of ejaculation (necrozoospermia). Consequently, instead of using non-viable sperm from the ejaculate, testicular biopsy will provide a ready source of freshly produced viable sperm.
With the almost unlimited potential to achieve some level of fertilization with ICSI regardless of sperm quality, it would seem that male factor infertility would no longer be of concern. It must be noted, however, that sub-fertility in men can be related to certain numerical and structural defects of the chromosomes and, therefore, there is a strong recommendation for all couples that achieve pregnancies from ICSI to undergo prenatal screening. In certain cases of obstructive azoospermia, there is a higher incidence of cystic fibrosis in the male. Hence, before embarking upon treatment of the more extreme forms of male factor infertility, it is advisable to have some cytogenetic screening performed. Incidentally, very subtle compromise in sperm quality may well be responsible for a marginally lower embryonic viability rate and a slightly higher early miscarriage rate even if such embryos implant. Such observations have led to the suggestion that the technique ICSI itself is at fault; but this misses the point that ICSI per se is not causing the problem, merely facilitating the use of sperm, which under other circumstances would never have even achieved fertilization.
The use of ICSI is now routinely applied to a range of clinical situations wherever there is a possibility that conventional in vitro fertilization may be suppressed or not occur. Such situations include the following: idiopathic or unexplained fertility; hyper-responsive ovarian stimulation cases where egg quality may be reduced; post-thaw sperm samples that survive poorly; post-thaw egg insemination; generation of embryos for pre-implantation genetic screening where embryos "clean" from any extraneous contaminating sperm is needed; or, indeed, any case where there is an extreme need to maximize normal fertilization, for example, when a woman has only a few eggs retrieved. It is possible to "rescue" cases following complete failed conventional fertilization with ICSI. The viability potential of these "late-fertilized" embryos is approximately half of timely fertilized embryos; nevertheless, they do generate successful live births. ICSI has become such a common feature of IVF therapy that it is fast becoming the insemination technique of choice.
