Therapeutic Cloning: How It's Done

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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.

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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.

By the fourth or fifth day, a hollow ball of roughly 100 cells has formed. It holds a clump of cells called the inner cell mass that contains stem cells.

The blastocyst is broken open, and the inner cell mass is grown in a culture dish to yield stem cells.

The stem cells, in turn, can be coaxed to grow into a variety of cells that might one day be injected into patients.

Images: JANA BRENNING

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