Therapeutic Cloning

Human "Therapeutic Cloning": How It's Done [text from Scientific American, Jan 2002]

"Eggs are coaxed to mature in a culture dish. Each has a remnant egg cell called the polar body and cumulus cells from the ovary clinging to it. While an egg is held still with a pipette, a needle is used to drill through the zona pellucida, removing a plug. "

"After ejecting the zona plug, the needle is inserted back in the egg through the hole to withdraw and discard the polar body and the egg's genetic material. A cumulus cell from another egg is taken up into the needle. Cells called fibroblasts (or their nuclei) can also be used in this step."

"The cumulus cell is injected deep into the egg that has been stripped of its genetic material. The injected egg is exposed to a mixture of chemicals and growth factors designed to activate it to divide."

"After roughly 24 hours, the activated egg begins dividing. The cells contain genetic material only from the injected cumulus cell."

Commentary: Injection of nuclei from fibroblast cells was unsuccessful. Cumulus cells were used because their small size permit injection of the entire cell into the ovum. Among eight ova injected with cumulus cells, two completed two cell divisions and one began a third division before halting. The developers of the procedure have dubbed it "therapeutic cloning" to differentiate it from "reproductive cloning", in which a nucleus from an adult is transplanted into an ovum with the intention of producing a child that is genetically identical to the donor. The purpose of "therapeutic cloning" is to produce stem cells that can be used for treatment of disease.