[Cloning and reprogramming of the nucleus in the preimplantation embryo]

In mammals, nuclear transfer (or cloning) consists of introducing the nucleus which is generally transcriptionally active into a new cytoplasmic environment, usually that of a transcriptionally inactive metaphase II oocyte. Following the transfer, intense nucleocytoplasmic exchange takes place, and is responsible for remodeling of the nuclear structure and gene reprogramming. The original development program of the donor nucleus is effaced, and the reconstructed embryo adopts that of the recipient oocyte. Both remodeling and reprogramming are regulated by maternal cytoplasmic factors. Overall transcriptional activity, splicing and translational functions as well as the specific expression of certain genes are fairly similar to corresponding activity in normal embryos. However, the development program seems to be read some hours in advance by the reconstructed embryos, and some basic information is lacking. It is particularly important to study these reprogramming abnormalities during the early stages of development in the context of specific abnormalities in fetal and neonatal development observed after transfer of somatic cells.