A Method for Producing Cloned Pigs by Using Somatic Cells as Donors

Nuclear transfer (NT) is a procedure by which genetically identical individuals can be created. The applications of these NT techniques will be in agriculture, biomedicine, and basic research. Based on the source of donor cells, NT can be classified into embryonic cell NT and somatic cell NT. Somatic cell NT was first reported in 1996 (1 ) and includes more practical applications. Most importantly, it provides a promising method for producing transgenic animals. This concept is exemplified by the generation of transgenic sheep (2 ), pigs (3 ,4 ) and calves (5 ), along with gene-targeted sheep (6 ) and pigs (7 ), derived from NT approaches by using transfected somatic cells. For pigs, somatic cell NT has another specific significance, as it allows the use of genetic modification procedures to produce tissues and organs from cloned pigs with reduced immunogenicity for use in xenotransplantation (7 ). However, when measured as development to term as a proportion of oocytes used, the efficiency of somatic cell NT, has been very low (1–2%). Several variables influence the ability to reproduce a specific genotype by cloning. These include species, source of recipient ova, cell type of nuclei donor, treatment of donor cells prior to NT, the method of artificial oocyte activation, embryo culture, possible loss of somatic imprinting in the nuclei of reconstructed embryos, failure of adequate reprogramming of the transplanted nucleus, and the techniques employed for NT. In some species (e.g., pigs) there is an additional difficulty in that at least four good embryos are required to induce and maintain pregnancy.