ABSTRACT
The technique of nuclear transfer (NT) involves the transfer of a nucleus from a donor cell (karyoplast) into an enucleated oocyte or zygote from which the nuclear genetic material has been removed (cytoplast). The term ‘nuclear reprogramming’ has been used to describe the changes in gene expression which are induced in the donor nucleus following NT, or more generally the changes that are induced experimentally by introducing nuclei into a new cytoplasmic environment.1 NT was proposed by Spemann2 as a technique to study cell differentiation and commitment; by transferring nuclei from progressively later stages of development, the point at which the nucleus became fixed (committed), or lost the ability to control all development, could be determined. We now know that nuclear instructions are in the form of DNA and that the majority of nuclei within an adult contain two complete copies of the genome derived from the maternal and paternal germ cells at the point of fertilization. In 1996 the birth of ‘Dolly’, a sheep produced by nuclear transfer from an adult-derived somatic cell3 demonstrated that the oocyte was able to reprogram nuclear DNA from a differentiated cell and recapitulate the temporal and spatial pattern of gene expression associated with development to produce offspring which developed to adulthood and were reproductively normal. Thus, NT provides a route to the dedifferentiation and redifferentiation of somatic nuclei, thereby producing the entire range of cell types associated with normal development and, in particular, providing a method
for the derivation of autologous multipotential embryonic stem cells for human therapeutics. It is now accepted that nuclear reprogramming following NT is controlled by epigenetic mechanisms and not by rearrangement of DNA sequences. However, it should be remembered that nuclear reprogramming is not restricted to NT embryos and that nuclear programming by epigenetic mechanisms is an ongoing event throughout development and differentiation and has also been implicated in the disease process.
