ABSTRACT

The established principles of cell injury or survival during the freezing process have been set out in detail in previous chapters (see Chapters 1 and 2). However, there are additional complicating factors resulting from the specialized nature of human reproductive cells that affect survival after cryopreservation. These relate, in the main, to the fact that both male and female cells are designed to deliver the haploid set of chromosomes at the point of fertilization, in the correct sequence of events, to achieve normal embryo production and eventual normal birth. In both male and female, the mature gametes develop from immature precursor cells in their respective sites (testicular tissue or ovary). It is beyond the scope of the current chapter to describe in detail these developmental processes, and they can be found elsewhere (Edwards and Brody, 1995; Jones, 1997). However, it is worth pointing out some overriding principles that affect cryopreservation of the various stages.

On achieving maturation, both male and female gametes have acquired highly specialized structural components essential to normal fertilization (e.g., the sperm-tail contractile machinery, or the oocyte protective coat of the zona pellucida) that may respond to the freezing process in ways different from that of basic cell structures such as the plasma membrane. Also, the haploid state exists with chromosomes devoid of a surrounding nuclear membrane. Earlier developmental stages of the gametes exist as a less complex cell population, which superficially may be more attractive to consider for cryopreservation. However, in the human patient, the question then becomes how to access these primary-stage cells, and how to achieve their essential maturation after cryopreservation. In many ways, in the development of clinical reproductive technologies, progress in cryopreservation has gone hand in hand with that for in vitro maturation by cell culture methods. The comparative complexities of in vivo and in vitro maturation (of both male and female gametes) have been described in an excellent review by Leibfried-Rutledge et al. (1997).