Previous attempts by scientists to produce animal clones directly from fully differentiated B cells, T cells and neurons had failed beyond the blastocyst stage. Only with a second step that involved combining the blastocyst with a fertilized embryo, which produces what biologists call a chimera, or by performing another nuclear transfer using the embryonic stem cells derived from these blastocysts, could "cloned" pups be produced. Even so, other researchers have countered these are not bona fide clones because they possess chromosomes that are not identical to those of the original donor nucleus.
Since Dolly, animal cloning using adult cells has been accomplished in more than a dozen mammalian species, but the process is highly inefficient. Even if the reconstructed eggs survive to the blastocyst stage, only a handful, at most, of these result in live young when implanted into a female.
Many have attributed cloning's limited success to a theory that clones must be derived from adult stem cells, which reside in a specific area of each tissue and remain quiescent until they are activated by the presence of disease or tissue injury. Yet, if this were true, Drs. Yang and Cheng point out, the results of their studies would have found the adult stem cells to be more efficient than the other, more differentiated cells.
"Of the 1,828 nuclear transfers we performed with stem cells, very few could develop to the blastocyst stage and not one clone was produced. With such odds, it's hard to believe that Dolly and other cloned animals could have possibly been derived from adult stem cells. Much more likely is that these animals were derived from fully differentiated tissue cells," Dr. Yang argues.
Source:University of Pittsburgh Medical Center