Nucleus transfer in mammals: How the oocyte cytoplasm modifies the transferred nucleus

Successful development of clones depends on the reprogramming of transferre d nuclei in enucleated oocytes. Thus far, oocytes are the only cells that c an convert nuclei, which are already differentiated, into undifferentiated stages resembling pronuclei in freshly fertilized zygotes and that can then complete development of the reconstructed embryos. However, we still don't know exactly how the enucleated oocyte (cytoplast) secures this reprogramm ing. Oocytes exhibit a number of cytoplasmic activities that may be involve d reprogramming. We discuss how these activities may be involved in reprogr amming of transferred nuclei. (C) 2001 by Elsevier Science Inc.