The word clone can mean different things to different people.
In molecular biology, it refers to the process of making identical copies of DNA.
Using this procedure, Willadsen obtained live lambs (19) and calves (20) after transfer of the reconstructed embryos into surrogates.
Subsequently, several other research groups with ties to the agricultural industry began exploring the possibilities of embryo and embryonic cell nuclear transfer, achieving success with progressively later stages of embryos (Table 2).
Although the fact that an adult nucleus could indeed direct normal development (resulting in a live offspring) was revolutionary for developmental biology, it followed a series of discoveries that suggested such a possibility (Fig. The initial attempts to artificially clone domestic animals involved embryo splitting.
Steen Willadsen demonstrated that twins could be produced in sheep (17) and cattle (18) after splitting of cleavage-staged embryos and transfer of the demi-embryos into recipients.
These studies demonstrated that triplets and even quadruplets could be obtained, albeit at lower frequencies due to the loss of cellular mass.
To overcome this limitation, Willadsen used a modified version of the nuclear transfer technique that had been used in the earlier amphibian cloning studies described above (Fig. In brief, the oocyte’s DNA was removed by aspirating the portion of ooplasm containing the chromosomes, thus forming a cytoplast; the donor cell was placed abutting the ooplasmic membrane; and the cytoplast and donor cell were fused together.
The fact that such a complex procedure works at all is amazing and is the result of decades of pioneering research.
In this review, the historical work in domestic species leading up to the development of somatic cell nuclear transfer (SCNT), along with the practical applications of this technology, will be discussed.