Hatched blastocyst. Light micrograph of a hatched human blastocyst, 7 days after fertilisation. The blastocyst is a hollow ball of cells with a fluid centre. The inner cell mass (bright white) contains the cells that would form the embryo proper, the embryonic stem cells (ESCs). These cells will be harvested and grown in culture. Human ESCs are a potential source of cells to repair damaged tissue in diseases such as Parkinson's and insulin-dependent diabetes. They are pluripotent, able to differentiate into any of the 200 cell types in the body. Research using human ESCs is controversial as it requires the destruction of an embryo.
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