The use of assisted fertilization techniques have evolved over the past decades from (a) making small nicks or incisions in the shell of the egg (zona pellucida) called zona drilling or partial zona dissection, to (b) inserting a small number of sperm under the shell of the egg but not within the plasma membrane (oolemma) of the egg called subzonal insertion (SUZI), to (c) inserting individual sperm under the plasma membrane of the egg directly into the contents (cytoplasm) of the egg called intracytoplasmic sperm injection (ICSI). ICSI has far greater success than the earlier techniques in terms of pregnancy. The only sperm requirement of ICSI is having as many living (generally identified due to their motility) sperm as there are eggs for injection.
The first pregnancy from any of the assisted fertilization techniques was reported in 1988, and resulted from partial zona dissection (PZD) in which a small incision or tear was made in the shell (zona pellucida) of the egg to aid access for sperm. These defects in the zona pellucida of the eggs were associated with a high number of polyspermic embryos (eggs fertilized with more than one sperm, therefore abnormal and not viable). Also in 1988 the first pregnancy from subzonal insertion of sperm (SUZI) was reported. In the SUZI procedure several sperm are placed into the perivitteline space between the outer shell of the egg (zona pellucida) and the plasma membrane of the egg. It was not until 1992 that the initial pregnancies with ICSI were reported; yet since that time this technique has overwhelmed the field of assisted fertilization.
The history of ICSI is interesting and shows how long it takes for medical advances to become clinically available even in the most successful of situations.
In 1976, researchers reported that the injection of whole spermatozoa into hamster eggs resulted in the transformation of the sperm’s nucleus into the male pronucleus. This illustrated that whole mature sperm can be microsurgically injected into the cytoplasm of eggs and the genetic material within the chromosomes (DNA) will organize into the normal structure (the male pronucleus). This important discovery allowed for the development of ICSI in humans.
In 1988, researchers first reported the finding that microinjection of human spermatozoa into human oocytes resulted in pronuclear formation. This confirmation that a similar procedure with human material resulted in a similar outcome occurred some 13 years after the initial hamster finding.
In 1992 the first pregnancies following intracytoplasmic injection of single spermatozoon into an oocyte were reported.
In 1997, ICSI has about a 60-80% fertilization rate per egg with an overall “take home baby” rate that compare nicely to rates of standard IVF performed for couples without a male factor.
This success for ICSI is incredible since the only criterion required for the sperm is that they are alive. This is because the sperm do not have to accomplish fertilization on their own; rather, they are placed directly into the egg’s cytoplasm. Live sperm can be most easily detected by their motility but even nonmotile live sperm or immature sperm from testicular biopsy has been used successfully with ICSI.
The primary disadvantage of ICSI is a practical one. Oocyte (egg) retrieval is required so that the embryologist has the egg to work with in order to perform the assisted fertilization (ICSI) procedure. Therefore, ICSI is performed in the context of In Vitro Fertilization. The practical concern is that these procedures (IVF with ICSI) tend to use a tremendous amount of resources, usually requiring the woman to undergo controlled ovarian hyperstimulation with egg retrieval and IVF following ICSI. These resources may be very expensive and are often (in USA) not covered by insurance.