Nuclear Transfer in Cattle

For decades, a dream of animal breeders has been to clone (make exact genetic copies of) outstanding animals. This was first achieved by splitting embryos (1 ); however, with this technique, only a few animals of the same genetic origin could be produced (2 ). The second approach, which seemed to be more promising, was the embryo reconstruction using a single nucleus from an embryo to an enucleated one-cell embryo. This technique was based on an idea proposed by Spemann in 1938 (3 ). With only few exceptions, experiments on frogs and fish provided scientific evidence that all the cells in the body of an animal appear to contain the same genetic information, being contained in DNA, a molecule located in the nucleus of cells. Thus, within an animal, the DNA sequence in the mammary cells is identical to skin cells. These cells differ in their appearance and function because they utilize different parts of the genetic information, not because the total amount of information differs. Furthermore, all these cells have genetic information present in the one-cell embryo that develop into the animal. Therefore, if the nucleus of any of these cells were used to replace the genetic information in any one-cell embryo, an exact genetic copy of the animal whose cells donated the nucleus would develop. This theory was confirmed by successful experiments in frogs and fish (4 ,5 ). Using similar procedures and inner cell mass cells (ICM) as nuclear donors, Illmensee and Hoppe (6 ) reported the birth of live offspring (mice) in 1981. However, these results were never confirmed.