Vitrification of mammalian spermatozoa in the absence of cryoprotectants: from past practical difficulties to present success.

The use of cryoprotective agents for the conventional cryopreservation of human spermatozoa, oocytes, zygotes, early cleavage stage embryos and blastocysts is an integral part of almost every human IVF programme. Moreover, the cryopreservation of these types of cells by direct plunging into liquid nitrogen usually requires high cryoprotectant concentrations with consequent cytotoxic effects. This review covers the history of this problem, and in this light offers an explanation, through physico-chemical concepts, for one of the most recent developments in this area: the recovery of motile and potent spermatozoa after cryoprotectant-free vitrification.