Human embryo cloning starts with a standard in vitro fertilization procedure. Sperm and an egg cell are mixed together on a glass dish. After conception, the zygote (fertilized egg) is allowed to develop into a blastula (a hollow mass of cells). The zygote divides first into two cells, then four, then eight... A chemical is added to the dish to remove the "zona pellucida" covering. This material provides nutrients to the cells to promote cell division. With the covering removed, the blastula is divided into individual cells which are deposited on individual dishes. They are then coated with an artificial zona pellucida and allowed to divide and develop. The experiment by Sillman et al. showed that the best results could be obtained by interrupting the zygote at the two cell stage. Many of these pairs of zygotes were each able to develop to the 32 cell stage, but stalled at that point. They might well have had the potential to develop further and even mature into a viable fetus, except that the original ovum was defective and would have died anyway. For ethical reasons, the researchers had selected embryos which had no possibility of ever maturing into fetuses, and thus becoming newborn babies.

Labels: Human embryo cloning