Slow Freezing

Slow freezing of embryos had been the gold standard of embryo freezing for almost three decades prior to the advent of the vitrification procedure. However, one of the major drawbacks of this was the formation of intracellular ice crystals, these proved to be rather dangerous to the cells as the sharp crystals could kill cells by shredding the cell membranes. As water turns to ice, it can rupture the cell walls by expanding its volume beyond the membrane’s capacity.
To overcome this situation, the embryos were run through various solutions to dehydrate the cells of water and replace them with cryoprotectants such as ethylene glycol and glycerol. Individually labelled cryopreservation straws were put into special freezers and the embryos cooled to -35C in liquid nitrogen. Over time, it was observed that slow freezing worked well with embryos and sperm but was not as successful for eggs. With a much larger volume of water, the possibilities of ice crystallization were much higher leading to cell degeneration.

Vitrification

The difficulties in freezing eggs via the slow freezing mode required a new cryopreservation method which is fast freezing or vitrification. This process allows for water molecules to be removed and replaced with a higher concentration of cryoprotectant by reducing the possibility of cell degeneration.

The embryos are directly put into liquid nitrogen and this reduces the temperature at a drastic rate of -12,000° Celsius per minute. Such drastic freezing changes the egg or embryo into a chemical glass in milliseconds and leaves no possibility for any significant ice crystals to form.