The Cryobiology of Assisted Reproduction: Gametes and Gonads

[*]The science of low-temperature biology, or cryobiology, is a cornerstone of 21st-century reproductive medicine. Its advent can be dated fairly accurately to the late 1940s and geographically located to London, England. A group of researchers led by (later Sir) Alan Parkes was attempting to preserve spermatozoa but had little success until a serendipitous discovery of the cryoprotective properties of glycerol. Their breakthrough led to the freeze banking of bull semen, with obvious applications for agriculture, and a few years later to clinical applications. Encouraged by their progress, the researchers applied themselves to cryopreserving a wide range of objects, from corneas to whole organs and even intact hamsters! But in the 1950s reproductive medicine had hardly been born and cryotechnology had little impact, apart from semen banking. After the advent of in vitro fertilization (IVF) technology at the end of the 1970s, fresh needs and demands arose, and these are the topics addressed in this volume.

The first chapter by Pegg provides the theoretical background to low-temperature biology and technology. The physics of cooling cellular constituents is complex but an understanding is crucial for developing optimal protocols for preserving living cells. Most of the techniques employed today are based on equilibrium cooling protocols, though ultrarapid or vitrification methods have recently attracted interest. If this volume is updated in a few years, we can expect to find vitrification will deserve an entire chapter to itself.

The subsequent six chapters consider different cell types in reproduction. Critser discusses the current technology of sperm banking, which has undergone great advances since Parkes' day, although still requires improvements. Küpker et al consider methods for cryopreservation of immature male germ cells, which are now needed because of the advent of epididymal or testicular microaspiration for intracytoplasmic sperm injection. In the future, we may see the clinical preservation of spermatogonia for patients prior to undergoing sterilizing chemotherapy, which could even enable the fertile potential of children to be preserved.

Since 1983, spare human embryos have been banked so that IVF success rates can be augmented by transferring the thawed embryos without needing to repeat a cycle of controlled ovarian stimulation. Byrd discusses this now routine but crucial technology. After years of disappointing results, oocyte cryopreservation is also being applied with more success, albeit on a small scale. Wininger and Kort discuss this progress, which offers an alternative to embryo banking in some circumstances. Storage of ovarian follicles is a logical extension that has applications for fertility conservation in female cancer patients. Shaw and Trounson consider these options, which are currently restricted to cortical strips of tissue but may one day enable whole-organ banking to fully restore natural fertility. Utilization of banked ovarian tissue will require either an effective culture system to produce mature gametes or, more realistically at present, autotransplantation to restore fertility and estrogen status. Ovarian transplantation is not new but the applications and techniques are novel and are reviewed by Oktay and Yih.

Cryopreservation of reproductive cells and tissues is controversial because it has opened up formerly undreamed of scenarios, such as fertility after iatrogenic sterilization and natural menopause and even postmortem. Treatment of this range of topics would be incomplete without a chapter considering the ethical implications, and this topic has been contributed by Bahadur.

We are honored to be editors of chapters that are of great importance to physicians and scientists and that have been authored by distinguished contributors in reproductive medicine. At a time when technology is advancing faster than ever before in history, we hope that these reviews will bring specialists up-to-date with reproductive cryobiology and will provide knowledge that can be applied in medical practice.