ABSTRACT
During the last 30 years, various innovative technologies have been devised and are now used with increasing frequency to produce animals of laboratory, domestic, and nondomestic species. These technologies include in vitro maturation and fertilization of oocytes; in vitro production of embryos; micromanipulation of embryos to produce genetically identical twins, triplets, or quadruplets; and genetic engineering by various means to alter the genome of animals, including nuclear transfer of somatic cells into enucleated oocytes, or cloning. This latter method, first reported by Wilmut
et al. (1997), has revolutionized the field of animal reproduction. A recent review of somatic cell cloning lists the production of live offspring of sheep, cattle, goats, pigs, and mice by this method (Brem and Kühholzer, 2002). The potential power of this method is illustrated by the fact that clones of 10 heifer calves (Wells et al., 1999), of six bull calves (Kubota et al., 2000), and of four rabbits (Chesné et al., 2002), all produced by nuclear transfer of adult somatic cells, have been reported. Such methods are being applied not only for the routine production of animals and for alteration of animal genomes but also as research tools to determine molecular mechanisms of embryonic development, and even as a means to protect endangered species. Efficient implementation of all of these methods has required cryopreservation of gametes, embryos, and other reproductive cells and tissues.
