Calvin Simerly of the University of Pittsburgh School of Medicine and his colleagues studied 716 monkey embryos that were created by nuclear transfer, in which the genetic material from one egg is removed and implanted into a donor egg. Of these, 33 were implanted into surrogates after initial cell division had occurred. But despite the fact that they appeared to be progressing normally on the surface, none of the embryos resulted in a successful pregnancy. On closer inspection, the researchers determined that the cloned monkey embryos were far from normal. Says Simerly: "When cells divide, there are very basic things that are supposed to happen, and they just didn't happen."
Using antibody tags, the team tracked proteins and DNA within the cells as they progressed (see image). The researchers found that the cells' mitotic spindles, which help separate and align the chromosomes, were not functioning properly: some cells had too many chromosomes, whereas others had too few. Moreover, the spindles lacked two proteins crucial to proper functioning. In successfully cloned animals such as mice and cows, in contrast, the cells have extra copies of these proteins, which allow successful cell division. "Current techniques such as those used to create Dolly the sheep, mice and other domestic animals do not work in nonhuman primates," says study co-author Gerald P. Schatten of the University of Pittsburgh. "I don't want to say that this will never happen. Given enough time and materials, we may discover how to make it work. It just doesn't work now."