It could eventually help patients who had suffered heart attacks, they said.

A mixture of "progenitor" cells, taken from blood and bone marrow, made cells lining the vessels, and also those surrounding the lining.

A UK expert said that the Harvard research was "promising", and could eventually help lab-grown organs to be implanted successfully.

The ability to develop swiftly a new network of tiny blood vessels - known as capillaries - would be a prize for scientists.

There are dozens of potential applications in medicine, particularly in the treatment of conditions which involve damage to a tissue's blood supply, such as that to the heart muscle following a heart attack.

However, the complex structure of these vessels has slowed progress.