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Steven Mullen Discusses History of Oocyte Cryopreservation

Last week, Mary Zelinski reported to us about the annual meeting of the Society for Cryobiology held in Corvallis, Oregon, July 24-27. She continues her review of the meeting and the keynote presentation by Dr. Steven Mullen, below. Read the first blog post on Cryo2011.


By Mary Zelinski, PhD-Our Oncofertility colleague, Dr. Steven Mullen, Head of Reproductive Cryobiology, 21st Century Medicine, started the keynote session with his excellent presentation on “The Evolution of Methods to Cryopreserve Human Oocytes”.  He discussed how the initial attempts in the 1980’s to cryopreserve human oocytes using dimethyl sulfoxide (DMSO) as the permeating cryoprotectant followed by the slow-cooling method yielded poor outcomes.  This raised clinical and social concern for the health of potential children derived from this technique, thereby essentially halting research until the late 1990’s when it was discovered that inclusion of propylene glycol as a cryoprotectant could improve outcomes.  Follow-up reports using slow-cooling methods confirmed that reasonable survival and embryonic development can be achieved when propylene glycol and sucrose (at the appropriate concentrations) were used.

In the late 1990’s, the first case report on successful vitrification of human oocytes was followed by studies indicating successful cow oocyte vitrification using extremely fast cooling and warming rates.  Remarkable success based on developmental potential was achieved when similar vitrification techniques were applied to human oocytes in the mid-2000’s. Since then, numerous clinics applied vitrification, using various methods to achieve “ultra-rapid” cooling and warming, with good outcomes.

Direct comparisons between slow-cooling and vitrification reveal, in most instances, that vitrification can provide a better outcome.  Dr. Mullen noted that vitrification has its drawbacks including the use of very small volumes of solution applied to the surface of a thin carrier device that is directly exposed to liquid nitrogen for the fastest cooling rates poses a potential cross-contamination between patients.  Development of closed systems lags behind the open systems in terms of success.  Furthermore, a steep learning curve is required to obtain technical expertise, thus repeatability among clinics remains variable.  Nonetheless, some studies using large numbers of young patients have reported comparable pregnancy rates between vitrified and fresh oocytes.  Studies with similarly large cohorts of older patients as well as cancer survivors remain to be demonstrated.  Furthermore, long-term follow-up of the health of offspring health is also unknown at the present time.


Stay tuned to hear reviews of the other keynote speakers at this year’s meeting of the Society for Cryobiology or read the first Oncofertility blog post.

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