The ability to create living tissues and organs in the lab holds great promise for transplant medicine. But the traditional approach to tissue engineering--seeding the outside of a biodegradable scaffold with cells, without regard to their organization--hasn't gotten cells to behave the way they would in the body.

Ali Khademhosseini, assistant professor in medicine and health sciences and technology at Harvard Medical School, hopes to improve engineered tissues with an approach he likens to building with living Legos. As a first step toward creating a heart, he aligns cardiac muscle cells to form small, beating strings. He then embeds these strings in a supportive, gelatinous polymer to make building blocks that can be assembled into bundles resembling the sheets of muscle that make up the heart. He can also add other types of cells to the building blocks to provide support for the muscle. This aspect of the system is crucial, since natural tissues comprise cells of multiple types in structurally complex arrangements. By giving cells the same interconnections they have in the body, Khademhosseini hopes to create tissues that can be used to test new drugs and, eventually, to rebuild organs.

1) Khademhosseini begins by seeding a patterned slide with heart muscle cells

2) Guided by the pattern, the cells elongate until they resemble the cells in a living heart

3) After six days, the cells have formed "organoids" that beat on their own and may be removed from the slide

4) The organoids are embedded into blocks of gel that can be molded into any shape needed--and combined like tissue­-engineering "Legos"

 --Katherine Bourzac