D3 embryonic stem cells cultured in Life Technologies' KNOCKOUT D-MEM
Just last year Science hailed stem cell isolation and culture as the "breakthrough of the year."1 Much of the excitement over stem cells derives from their potential to differentiate into any cell type in the body. A fertilized egg, for example, is a single cell that is capable of eventually creating all the different cells that make up the mature organism. As such, a fertilized egg and the daughter cells of the first few divisions are often referred to as "totipotent."

After a few days of division, a human fertilized egg reaches the blastocyst stage. The inner cell mass of the blastocyst eventually forms the mature adult body. This inner cell mass cannot form an organism because it cannot form a placenta, which is derived from the outer cells of the blastula. But cells from the inner cell mass...

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