Résumé
Early embryonic development is characterized by dramatic modifications in chromatin organization. These contribute to the formation of the totipotent cells that will differentiate into the embryonic and extra-embryonic lineages. However, some groups of genes are not affected by these global changes. Amongst these, are the genes controlled by genomic imprinting, an epigenetic mechanism that makes the maternal and the paternal genomes functionally non-equivalent in development. Somatic cell nuclear transfer (SCNT), used to clone animals, mimics the chromatin reprogramming that occurs in the post-fertilization embryo. However, SNCT leads to aberrant development, particularly of the extra-embryonic tissues. As a consequence, live-born animals are obtained at very low frequency. Here, we review how different steps in the SCNT procedure may give rise to aberrant development. Imprinted genes are amongst the frequently affected genes, and their perturbed expression explains in part the abnormal developmental phenotypes. The future challenge will be to modify currently used cloning procedures, in order to increase the efficiency of nuclear reprogramming, without affecting the gene loci at which chromatin is normally not altered during development.