Terminology

It is worthwhile becoming comfortable with the technical terms for several of the words that will be used interchangeably in this chapter, and in case you wish to make more comprehensive searches of the literature, such explanation of terms will improve
your level of understanding. Firstly, the rather loose term egg is usually referred to as an oocyte by biologists, which is the unfertilized female gamete (sex cell). The sperm(atozoon) is the male counterpart gamete. Cryopreservation is the very
specific term for all stages involved in the cryostorage of the oocyte, and refers to the freezing, storage and the thawing processes. Cryoprotectant is the term used to refer to the fluid that is used to place the eggs in prior to freezing, and is usually a mixture of sugars and organic chemical liquids that are designed to buffer the egg cell during the stresses of freezing and thawing.To put egg freezing into context, it is interesting to consider that human sperm (spermatozoa) have been successfully frozen for decades, and that the first successful report of human embryo freezing that generated a pregnancy was in 1983. Subsequently both human sperm and embryo cryopreservation have become considered routine and consistent technologies.
Frustratingly human egg freezing has not yet reached such apparent levels of acceptance or consistency.The technology so far applied clinically has been based directly on traditional human embryo cryopreservation protocols, and has produced relatively few offspring when compared with human embryo cryopreservation. ortunately to date, no abnormalities have been reported from these pregnancies, regardless of the persistent concerns that freezing and thawing of mature oocytes may disrupt the chromosomal apparatus in these cells (meiotic spindle), and so increase the potential for chromosome abnormalities (aneuploidy) in the embryos that arise from such eggs. With respect to cryostorage of donated oocytes, for use eventually as eggs to donate to recipient women, there have been several reports that have shown some success with
this approach. In fact, it has been reported that there have been 10 babies born from frozen-thawed donor oocytes. In another unusual case, frozen donor eggs after thawing have been used, not for whole egg donation, but actually for ooplasmic transfer which
gave rise to the successful delivery of a twin following thawed ooplasmic donation. In this procedure the cytoplasm of the egg was injected into the eggs of another woman. The cytoplasm is the part of the egg outside of the nucleus that does not
contain the genetic elements (chromosomes), but contains elements of cellular unctioning. Transfer of this is thought to bolster the quality of the recipient egg making it healthier, and possibly more likely to give rise to a healthy embryo.
The first successful cryostorage of women’s own oocytes occurred with the reporting of three births over a decade ago by two centers in Australia and
Germany. However, at that time reports of egg freezing studies in mice suggested that although eggs could survive freezing and thawig, they might possess higher levels of chromosomal anomalies following this procedure when compared to fresh eggs. Of note, is the fact that these studies were not performed on human tissue, and the procedures used were totally different from that utilized with the successful human egg freezing cases.Nevertheless, the suspicion of this problem was enough to prompt a sort of voluntary worldwide moratorium on clinical oocyte cryopreservation, until studies prove the fear of chromosomal abnormalities to be unsubstantiated. Unfortunately for oocyte freezing research, human embryo cryopreservation was just starting to be undertaken much more routinely and successfully in the mid-eighties, due to the
growing ethical concerns with the fate of surplus embryos following in vitro fertilization (IVF). This presented a pressing clinical problem that drove embryo freezing research, while egg freezing languished without any clear clinical need, worsened still by the worries over its safety.