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Embryo Vitrification and Cryopreservation
by Geoffrey Sher, M.D.
Cryopreservation of human embryos has been a routine procedure since the early 1980s. Conventional techniques (which still being widely used by most IVF centers involve slow freezing which unfortunately results in ice crystal formation inside the cell(s),damaging them and reducing embryo “competency”, i.e. the subsequent ability upon thawing ,to propagate a viable pregnancy. The relatively recent introduction of ultra-rapid freezing or vitrification changed all this. This innovative technology, rapidly freezes the cells (literally within the blink of an eyelid), thereby avoiding intracellular ice from forming and preventing cell damage.
Vitrification involves rapid freezing of embryos and eggs in a tiny amount (less than 0.1 microliters) of a special vitrification solution, before storing them in liquid nitrogen. When compared to the older conventional slow embryo freezing method survival rate for human embryos of about 75-80%, vitrification improves post-thaw (warming) embryo survival to >90%.
It is my personal preference to only cryopreserve blastocysts. This is based upon our own recent research, very recently reported in “Fertility and Sterility” which clearly demonstrates that embryos failing to develop to the blastocyst stage are virtually always chromosomally abnormal (aneuploid) and thus not worthy of keeping, let alone transferring. While many IVF programs still freeze day 2 or 3 embryos, many SIRM physicians agree with this position of exclusively freezing only “good quality (Grade 1 or 2) blastocysts ( 120-144 hours post egg retrieval).
1. Staggered IVF (St-IVF) Staggered IVF (St-IVF) involves performance of a blastomere biopsy on the 3rd day post ICSI. Blastomer-derived DNA is subjected to Embryo/egg competency testing (ECT) using a genetic technique known as Comparative Genomic Hybridization (CGH). CGH allows identification of ALL 23 pairs of the embryo’s chromosomes the full presence of which (euploidy) is highly likely to result in a viable pregnancy provided that the embryo(s) is transferred to an ideal (receptive) uterine environment. Since it takes at least 5 days to complete CGH testing , it is not possible to have the test result available in the time to transfer a fresh blastocyst in the same cycle ( 2-3 days following the PGD biopsy). Accordingly it is necessary to cryopreserve and store the blastocyst(s) for subsequent FET in an upcoming menstrual cycle. This is where vitrification comes in..The advent of this freezing method now makes it possible to transfer unimpaired warmed blastocysts by FET with virtual impunity. The process of splitting the IVF cycle in to two phases (separate cycles) in order to make it possible to select only “competent” embryos for subsequent FET is referered to as St-IVF .It coprises the following: