Spermatozoa were the first mammalian cells to be cryopreserved successfully (Polge et al.,
1949). This success was due to the serendipitous discovery by Polge and co-workers of the
cryoprotective effect of glycerol. Since then, many methods have been developed for various
types of cells, tissues and organs. Much progress in the field has come from empirical work
as well as from fundamental cryobiology. Increased understanding of the causes of cryo-injury
has continually helped to improve cryopreservation methods. Research into fundamental
cryobiology has provided the basis for new cryopreservation methods such as vitrification.
The two most commonly used cryopreservation methods for animal germplasm are
slow-freezing and vitrification. These are quite different methods, but relate to the same
physico-chemical relationships. The differences between the two can be explained by first
describing what happens during slow freezing