In BioNews 484, we published a story reporting on new evidence to suggest that in IVF 'vitrified' embryos may be better than 'fresh' embryos. It has been brought to our attention that the technique used to store embryos in these studies was in fact 'slow-freezing', or 'controlled-rate freezing', and not vitrification as reported.
Controlled-rate freezing avoids the formation of potentially damaging ice crystals by allowing time for the concentration of solutes in the embryo to reach equilibrium in a cryoprotectant (a form of anti-freeze), before cooling in a predetermined, controlled way. Vitrification also aims to avoid the formation of ice crystals in the embryo, but does so by boosting the levels of cryoprotectant in order to avoid freezing and produce a glassy or 'vitreous' state at very low temperature.
While controlled rate freezing has been used in embryo storage since the 1980's, resulting in hundreds of thousands of healthy births, vitrification is, by comparison, a relatively new technology, which has so far only been attempted in a relatively small number of births. It would therefore have been impossible to achieve the large sample sizes recorded in these studies if not using the significantly more established slow-freezing technique.