[Therapeutic use of stem cells]

Stem cells display important capacities of self renewing, proliferation and differentiation. Because those present in the embryo have the more remarkable properties, their potential use in the therapy of until now incurable degenerative diseases have been envisioned. Embryonic stem (ES) cells are located in the inner mass of the balstocyst at early stages of the development. Even in long-term cultures they still retain their undifferentiated features. Under specific culture conditions, ES cells can be committed into a variety of differentiation pathways, giving rise to large amounts of cells corresponding to different tissues (neurones, cardiomyocytes, skeletal muscle, etc.). However, producing these tissues from already established ES cell lines would lead to immune rejection when transplanted to patients. To prevent this pitfall and using the expertise accumulated by animal cloning by nucleus transfer, it has been proposed to adapt this technique to human ES cells. The therapeutic cloning consists in transferring the nucleus of somatic stem cells isolated from the patient into an enucleated oocyte, to allow blastocyst development from which ES cells will be derived. From these stem cells, compatible tissues will be then produced. The problem is that it is in theoretically possible to reimplant the cloned blastocyst into a surrogate mother for obtaining a baby genetically identical to the donor. This is called reproductive cloning. This worrying risk raises important ethic and legal questions.