Researchers at MIT have taken a step toward replicating the complexity of an organ by growing different types of cells in precisely the right arrangement.

The scientists put embryonic stem cells into “building blocks” containing gel that encourages the cells to turn into certain types of cell. These building blocks can then be put together to make more complex structures. The gel degrades and disappears as the tissue grows.

Eventually, the group hopes to make cardiac tissue by stacking blocks containing cells that have turned into muscle next to blocks containing blood vessels, and so forth.