However, there is a debate as to whether inducing quiescence is required for successful nuclear transfer. Most laboratories that have succeeded with nuclear transfer have utilized a serum starvation treatment. Serum starvation induces quiescence of cultured cells, and arrests them at the cell cycle stage of G0. Serum starvation was used in the creation of Dolly and was believed essential to the success of nuclear transfer. In the following section, we will discuss several strategies used to improve nuclear transfer efficiencies. These observations suggest that further studies on nuclear reprogramming are needed in order to understand the underlying mechanisms of reprogramming and significantly improve the ability of the differentiated somatic nuclei to be reprogrammed. Although the efficiency of nuclear transfer has been dramatically improved from the initial success rate of one live clone born from 277 embryo transfers, none of the aforementioned efforts abolished the common problems associated with nuclear transfer. These include: a) synchrony of the cell cycle stage of donor cells, as well as synchrony between donor cells and recipient oocytes b) using somatic cells from donors of various ages, tissue origins, passages and culture conditions c) transfer of stem cells with low levels of epigenetic marks and d) modifying epigenetic marks of donor cells with drugs. Most of these efforts are focused on donor cells. Various strategies have been employed to modify donor cells and the nuclear transfer procedure in attempts to improve the efficiency of nuclear transfer. It has been proposed that low cloning efficiency may be largely attributed to the incomplete reprogramming of epigenetic signals. Developmental defects, including abnormalities in cloned fetuses and placentas, in addition to high rates of pregnancy loss and neonatal death have been encountered by every research team studying somatic cloning.
Currently, the efficiency for nuclear transfer is between 0–10%, i.e., 0–10 live births after transfer of 100 cloned embryos. One of the most difficult challenges faced, however, is cloning's low efficiency and high incidence of developmental abnormalities. The blastocyst can then be transferred to a recipient (h) and cloned animals are born after completion of gestation (i).
The somatic cell and the oocyte is then fused (f) and the embryos is allowed to develop to a blastocyst in vitro (g). A matured oocyte (c) is then enucleated (d) and a donor cell is transferred into the enucleated oocyte (e). Cells are collected from donor (a) and cultured in vitro (b).
Schematic diagram of the somatic cloning process. In addition to its practical applications, cloning has become an essential tool for studying gene function, genomic imprinting, genomic re-programming, regulation of development, genetic diseases, and gene therapy, as well as many other topics. With optimization, it also promises enormous biomedical potential for therapeutic cloning and allo-transplantation. Somatic cloning may be used to generate multiple copies of genetically elite farm animals, to produce transgenic animals for pharmaceutical protein production or xeno-transplantation, or to preserve endangered species. It demonstrated that genes inactivated during tissue differentiation can be completely re-activated by a process called nuclear reprogramming: the reversion of a differentiated nucleus back to a totipotent status. The success of cloning an entire animal, Dolly, from a differentiated adult mammary epithelial cell has created a revolution in science. Somatic cell cloning (cloning or nuclear transfer) is a technique in which the nucleus (DNA) of a somatic cell is transferred into an enucleated metaphase-II oocyte for the generation of a new individual, genetically identical to the somatic cell donor (Figure (Figure1).