ABSTRACT
The cryopreservation of human oocytes, zygotes, cleavage-stage embryos, and blastocysts has become an integral part of a human in vitro fertilizationembryo transfer (IVF-ET) program. Oocyte freezing has remained one of the most elusive tasks in the field of assisted reproductive techniques (ART). Although several pregnancies using oocyte cryopreservation in conjunction with IVF have been reported, adverse effects of cryopreservation on the integrity of several unique features of the oocyte involved in normal fertilization and embryonic development have been observed. These include premature cortical granule exocytosis leading to zona hardening,1 increased parthenogenetic activation,2,3 damage of cytoskeletal elements,4-6 and disruption of the meiotic spindle.7,8 The difficulties in overcoming these adverse effects have stalled further progress in research.9-12
In 1992, the introduction of intracytoplasmic sperm injection (ICSI) into the human ART procedure overcame fertilization failure associated with these problems, and oocyte cryopreservation has now become an alternative choice in some IVF programs. More than 30 healthy babies have been born around the world13,14 using the slow freezing-rapid thawing cryopreservation protocol combined with ICSI. However, the efficiency of the technique is not yet satisfactory,15 as rates of survival and development in vitro and in vivo are still low.16,17
The main biophysical factor affecting human oocyte survival and subsequent embryonic development during and after cryopreservation is the intracellular ice crystal formation that generally pierces the membrane. This causes lysis, and breaks the meiotic spindle resulting in chromosome aneuploidy. Because the human oocyte has a large quantity of water in the cytoplasm, it is difficult to avoid
ice crystal formation with the slow cooling and thawing protocol.
