Factors affecting nuclear reprogramming
Various strategies have been employed to modify donor cells and the nuclear transfer procedure in attempts to improve the efficiency of nuclear transfer. Most of these efforts are focused on donor cells. These include: a) synchrony of the cell cycle stage of donor cells [24-26], as well as synchrony between donor cells and recipient oocytes [27,28]; b) using somatic cells from donors of various ages [29-33], tissue origins [26,34-39], passages [16,40,41] and culture conditions [42]; c) transfer of stem cells with low levels of epigenetic marks [43-48]; and d) modifying epigenetic marks of donor cells with drugs [49-51]. Although the efficiency of nuclear transfer has been dramatically improved from the initial success rate of one live clone born from 277 embryo transfers [1], none of the aforementioned efforts abolished the common problems associated with nuclear transfer. 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. In the following section, we will discuss several strategies used to improve nuclear transfer efficiencies.