HOW WERE THEY GROWN?

Although our bodies naturally produce blood, the red blood cells in the experiment never existed inside anyone.

The blood cells were created in a lab where experts from Bristol in Cambridge used stem cells from a blood sample to grow billions of brand-new red blood cells.

Stem cells are special human cells that can develop into different cell types such as muscle cells, brain cells and blood cells.

They can also fix damaged tissue.

Under the right conditions in the body or a laboratory, stem cells divide to form more cells called daughter cells.