Embryonic stem cells are derived from very early embryos called blastocysts; the diameter of a human blastocyst is roughly four times that of a human hair

A human blastocyst next to a human hair

A mouse blastocyst aged 3.5 days; the inner cell mass is coloured green and the trophectoderm is coloured red

Embryonic stem cells genetically modified to glow green under a fluorescent lamp

A chimeric mouse and his offspring; the offspring have a gene for black hair from the ES cells used to make their father

Neurons (nerve cells) made in the lab from human embryonic stem cells

Embryonic stem cells are grown from cells found in the embryo when it is just a few days old. In humans, mice and other mammals, the embryo is a ball of approximately 100 cells at this stage. It is known as a blastocyst and has two parts:

Some of the cellsin the inner cell mass are pluripotent: they can make every type of cell in the body.

If an inner cell mass is taken from a mouse blastocyst and given the right nutrients, the pluripotent cells cangrow in the laboratory. The process of cell maturation and specialization that would normally take place in the embryo stops. Instead, the cells multiply to make more undifferentiated cells that resemble the cells of the inner cell mass. These laboratory-grown cells are called embryonic stem (ES) cells.

Embryonic stem cells can make copies of themselves and make other types of more specialized cells

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Embryonic stem cells: where do they come from and what can ...