Figure 5: Schematic illustration of the process of somatic cell nuclear transfer. 

For the technique known as somatic cell nuclear transfer (SCNT) a cytoblast and an oocyte without its own nucleus are required. In the process of cell nuclear transfer, oocytes are extracted from the donor's ovaries by means of a minimally invasive operation following an often physically strenuous hormonal treatment (see IVF). The oocyte is then enucleated by aspirating the cytoblast with a micropipette. In a second step, another cytoblast is aspirated from a body cell and inserted into the enucleated oocyte. In this context, the cytoblast can be isolated from virtually any adult somatic cell of the donor using aspiration through a micropipette.

This launches the so-called “reprogramming” - a process which is as yet not entirely understood and during which the DNA of the original blastocyst loses its differentiation capacity. This is caused by so-called epigenetic modifications: Gene sequences on the DNA that have been “packaged” in the course of cell specialization so that they can be no longer translated are being reactivated by so-called transcription factors. This is the precondition for the further development of the new composed cell. After activating the cell by inducing electric pulses and various medium admixtures, the oocyte containing the somatic cytoblast starts to divide and can then develop into a blastocyst or even further (Figure 5) in an appropriate culture medium. Embryonic stem cells can be obtained from this blastocyst (Figure 3).

This technique is generally limited because the reprogramming is often not completed and thus the cell does not lose its specification completely. 

The procedure for producing mammal embryos through transfer of the cytoblast of an adult somatic cell to an enucleated oocyte was first described in 1997 by Wilmut et al., who used it to create Dolly the cloned sheep. This method has since then been successfully performed for other species of mammals. For example, in 1998 Wakayama et al. reported on the successful use of a similar method in mice. However, the method of reproductive cloning is associated with a high rate of mortality and physical deformities.