Embryo freezing, or cryopreservation, was one of the most significant revolutions in the field of assisted reproduction. From the start in the 80s, this technique has improved substantially, allowing for an increased success rate of fertility treatments and offering higher flexibility and security for the patients.
The first human embryos were frozen using a slow-freezing method, which involved progressively cooling the cells to prevent the formation of damaging ice crystals. Despite that, the slowly frozen embryos had limited survival chances after defrosting.
In the first decade of the 21st century, vitrification was introduced. This technique prevents the formation of crystals by fast freezing and converts the intracellular liquid into a vitreous (glassy) state. The embryonic survival rate today exceeds 90% thanks to vitrification, and the clinical results (pregnancies and births) are comparable to fresh embryo transfers.
More recently, a new advance was made: the ultra-fast vitrification – an improved version of conventional vitrification. This method involves even faster cooling (exceeding 20,000ºC per minute) and extremely reduced volumes of cryoprotectant medium. This minimises cellular toxicity and maximises embryonic viability, especially in the day 5 embryos (blastocysts) that are more sensitive to cryogenic stress. Recent studies suggest that ultra-fast vitrification can provide higher efficiency in high-quality embryo cryopreservation and reduce the risk of cellular structure alterations after thawing.
In addition to technical progress, cryopreservation has paved a new road for reproductive medicine as it permits deferred embryo transfers (this improves the endometrial preparation), preserves fertility in oncologic patients or delayed childbearing, and stores embryos for scientific investigations or future treatments.
At present, cryopreservation – and more recently, ultra-fast vitrification – are essential tools in in vitro fertilisation (IVF) treatments and continue to evolve through biotechnology and cryobiology breakthroughs, by consolidating as fundamental pillars in the modern assisted reproduction.
Daniel García
Embryologist
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