What stem cells are

A zygote divides into 2, 4, 8 cells and so on. Most of the daughter cells mature into specialised cells: muscle, skin, liver and the rest. But some stay 'immature', keeping the ability to become other cells. These are stem cells.

Stem cells have two special properties:

• Self-renewal: they can divide by mitosis to make more stem cells, keeping their own supply going.
• Potency: they can differentiate, becoming many kinds of specialised cells.

Stem cells are found in most multicellular organisms. In humans, they come from the umbilical cord, the placenta, the inner cell mass of the early embryo, some fetal tissues, and some adult organs. The first stem cells to be isolated came from blood.

Why are they so important?

• To grow new cells that replace damaged tissue.
• To study genetic defects and how diseases develop.
• To test new drugs on human cells.

The field of regenerative medicine aims to regrow damaged tissues and organs.

Some milestones: human embryonic stem cells were first grown in 1998; then came culture methods free of animal products (xeno-free), clinical trials, and induced pluripotent stem cells (iPSCs), made by reprogramming ordinary adult cells. Shinya Yamanaka and John Gurdon shared the 2012 Nobel Prize for showing that mature cells can be reprogrammed. More recent advances include CRISPR editing in stem cells, and organoids: miniature organs grown from stem cells.