One of the biggest challenges facing research around how the human heart develops is access to a developing heart. That may no longer be a problem thanks to a team of scientists who have created the first-ever functional miniature human heart in the lab.

Grown from stem cells, the human heart organoids (hHOs) are made up of all primary heart cell types and have a functioning structure of chambers and vascular tissue. Because of their close structural similarity to the real thing, they could help researchers understand how hearts develop and build better models for treating disease.

To create the mini-organs, scientists first take samples of skin or blood cells from adults, then reprogram them into stem cells, known as induced pluripotent stem cells (IPSCs), that can differentiate into whatever other cell types are needed.

“This process allows the stem cells to develop, basically as they would in an embryo, into the various cell types and structures present in the heart,” says Aitor Aguirre, senior author of the study. “We give the cells the instructions, and they know what they have to do when all the appropriate conditions are met.”

So what’s the point of growing tiny hearts in the lab? The researchers say that they could allow for new treatments of conditions like congenital heart disease and provide an ethical platform for testing drugs and treatments on the heart.