Nuclear reprogramming in zygotes.

Nuclear reprogramming, the conversion of the epigenome of a differentiated cell to one that is similar to the undifferentiated embryonic state, can be facilitated by several methods, such as nuclear transfer, cell fusion, use of embryonic stem cell extracts, and more recently, by the introduction of exogenous transcription factors. Amongst these various strategies, somatic cell nuclear transfer (SCNT) is, by far, the most effective method of nuclear reprogramming. The majority of SCNT studies have been carried out using enucleated mature oocytes, as reprogramming is efficient and can be completed within hours following the introduction of the somatic cell nuclei into the recipient oocyte. Fertilized eggs, on the other hand, were regarded as poor recipients for nuclear transfer, as previous studies showed that embryonic blastomeres transferred into enucleated zygotes were unable to develop to blastocysts. However, more recent studies have demonstrated that the method of enucleation and the cell cycle phase of the embryos can impact the success of somatic cell reprogramming when zygotes were used as nuclear recipients. It is, therefore, timely to revisit and further explore the nuclear reprogramming capacity of zygotes as recipients for SCNT. Assessment of the various factors that influence the reprogramming capacity of zygotes in SCNT also provide hints of the mechanistic nature of nuclear reprogramming.