Hatched blastocyst. Coloured light micrograph of a hatched human blastocyst (right), 7 days after fertilisation. To the left is the zona pellucida, a glycoprotein shell that originally surrounded the unfertilised egg. The blastocyst is a hollow ball of cells with a fluid centre. The inner cell mass (dark grey) 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. However, this research is controversial as it requires the destruction of an embryo.