Chromatin remodelling and nuclear reprogramming at the onset of embryonic development in mammals.

Two main strategies are used to produce cloned mammals. The first involves the condensation of donor chromatin into chromosomes directly exposed to the recipient cytoplasm, whereas the second leaves the donor nucleus in interphase until the time of the first mitosis. Both strategies, which induce marked changes in chromatin organization, allow full reprogrammation of somatic-differentiated fetal and adult cells. This paper reviews some of the recent data that contribute to our understanding of chromatin remodelling at the onset of normal development, as well as after the introduction of a foreign nucleus into a recipient enucleated oocyte. These data indicate that the coordinated changes in chromatin organization that take place up until the first cellular differentiations at the blastocyst stage are determinants for successful cloning. Although some degree of synchronization between the cell cycle stages of donor and recipient cells is necessary for correct remodelling of a transferred nucleus, the kinetics of remodelling events occurring during the one-cell stage appears to be the determining factor for the normal onset of gene expression.