CLONING - BLESSED OR DAMNED

The lamb derived recently (WILMUT et al.: Nature, February 27, 1997) f rom an embryo which had been cloned using adult somatic cell as nuclea r donor, can revolutionize our knowledge in developmental biology. Dol ly, the first mammal cloned from the tissue of a grown-up animal, prov es that even the genome of highly differentiated somatic cells does no t undergo irreversible modifications and retain the capability for nuc lear reprogramming. Obtaining identical copies from adult animals woul d facilitate the exploitation of genetic potential achieved by convent ional breeding. In addition to that, nuclear transfer can provide nove l approaches for the genetic manipulation of mammals. However, serious ethical concern has been raised over the possibility and desirability of cloning human beings. This review article summarizes the history, current status and perspectives of cloning mammals, indicating the adv antages and limitations of the different methods available today. The simplest way to clone a mammal used routinely for about 15 years is th e splitting of embryos in morula stage. This method is easy to perform under practical conditions with acceptable efficacy but - due early d ifferentiation of embryonic cells - the number of identical twins obta ined by splitting is very limited. Nuclear transfer (Fig. 1) is based on the finding that nuclei from totipotent cells taken from early embr yos and transferred into enucleated oocytes are able to be reprogramme d and start their development from the beginning. So far, viable offsp ring has been born only if the transferred nucleus derived from early stage embryos the cells of which were not yet differentiated. Although this cloning procedure resulted in several pregnancies in the last de cade, its success rate is rather low and the newborn animals have an i ncreased chance for developmental abnormalities. Now, the successful e xperiment of WILMUT et al. (Fig. 2) using quiescent cells from a grown -up animal confirms that - at least in sheep and probably in cattle - obtaining identical copies From adult mammals is possible. At this mom ent it is not clear, whether or not this method can be used in species other than the sheep.