Why are scientists growing tiny hearts, each the size of a rice grain?

Answer: to test which drugs increase the risk of congenital heart defects. (As can maternal conditions like diabetes.)

The heart is the first organ to become fully functional in the growing embryo. Indeed the human heart starts to beat when it’s no more than a tube. So it’s the organ that’s been around for the longest time during pregnancy, to get damaged by toxins from the mother.

So it’s no surprise that congenital heart disease is the commonest type of birth defect in people, occurring in over 1 in 100 births worldwide.

Experiments on living foetuses are unethical to many people – and illegal anyway in western countries. But foetal mini-hearts can be grown from (pluripotent) stem cells obtained from an adult. So no foetuses are harmed in their making.

Other human organs can be grown this way too – including mini-brains. Could such an embryonic organ be dropped into a mouse foetus? Perhaps Flowers for Algernon isn’t so far-fetched after all?