Getting from a fertilized egg to a fully formed human, it’s all in the epigenome.

So far we have described some specific cases of epigenetic regulation, but we now know that epigenetics in its broad sense, (how genes are expressed and used, rather than the DNA sequence of the genes themselves) is central to how a fertilised egg can eventually give rise to a whole organism and how cells of, let’s say your skin, remain skin cells and are different from your brain cells, despite containing exactly the same genes. Shortly after fertilisation, a developing human embryo consists of a ball of cells called embryonic stem cells. Each of these cells has the capacity to give rise to any of the types of cells in the body as the embryo grows (for example, brain cells, skin cells or blood cells). By contrast, 9 months later when a baby is born, most of the cells making up his or her body are committed to be a specific type of cell with specific functions. So as the cells divide, the ball of embryonic stem cells gradually develops into all the cell types and structures of the baby at term. For this to happen, thousands of genes must be switched on or off at just the right times and in the right cells as an embryo grows. For example, genes that make the fibrous keratin protein that gives our skin its strength, are only switched on in skin cells and not in the developing brain and genes required for brain cells to develop and make their interconnections are on in the brain but not in the skin.