Scientists have used human cells to grow new blood vessels in a mouse for the first time, a US journal reports.
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.
"What's really significant about our study is that we are using human cells that can be obtained from blood or bone marrow rather than removing and using fully developed blood vessels"
The latest study, published in the journal Circulation Research, uses two types of "progenitor" cells, which have the ability, like stem cells, to form different cell types.
In this case, "endothelial" progenitor cells have the ability to form the cells which line blood vessels, while "mesenchymal" progenitor cells can form the cells adjacent to this lining, which help to support it.
Unlike more controversial stem cell therapies, which might require cells taken from an embryo, these progenitor cells can be harvested from the blood or bone marrow of an adult, or from the umbilical cord.
They were mixed together in growth-promoting chemicals in the laboratory, then implanted into mice whose immune systems had been weakened to avoid rejection.
Pig progress
Within seven days, a "vigorous network" of new vessels formed, joined up with the host animal's blood vessels and started transporting blood.
Dr Joyce Bischoff, who led the research team, said: "What's really significant about our study is that we are using human cells that can be obtained from blood or bone marrow rather than removing and using fully developed blood vessels."
Dr Nick Rhodes, from the UK Centre for Tissue Engineering at the University of Liverpool, said that the results were "interesting and promising".
He said: "It could certainly assist in the connection of other engineered organs to the body's blood supply.
"Although this approach is not yet suitable for clinical use, it is interesting that they have demonstrated you have all the elements you need to create a functional network of capillaries from a small amount of blood."
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