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Human embryonic stem cells are isolated from early-stage embryos, four or five days after fertilization, called blastocysts. The blastocyst is a hollow sphere made of approximately 150 cells and contains three distinct areas: the trophoblast (a surrounding outer layer of cells), the blastocoel (a fluid-filled cavity), and the inner cell mass (a cluster of cells that can become the fetus). hESCs are isolated from the inner cell mass.

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