Scientists are a step nearer the creation of tiny artificial blood vessels after growing minuscule tubes out of stem cells in the laboratory.
The Massachusetts Institute of Technology team got the cells to stretch and form tubes along a grooved template, Advanced Materials reports.
Now they plan to produce capillaries which could be tested in animals.
One UK researcher said that lab-grown blood vessels was one of the "holy grails" of regenerative medicine.
Researchers have already managed to make wider blood vessels from scratch, but the formation of the tiny diameter capillaries needed to create a blood supply within other tissues and organs is far more challenging.
The US scientists claim to have made progress towards this, using a "nanoscale" template into which stem cells called endothelial progenitor cells are placed.
The cells detected the grooves and elongated themselves along them, aligning themselves in the same direction.
Adding a gel made of growth factors allowed the cells to grow outwards, forming a series of tiny tubes running parallel to each other.
While these tubes are not yet ready to be put inside a human body, the researchers say they are "very excited" by their potential.
Professor Robert Langer, who led the research, said: "It provides a new way to create nano-based systems which we hope will provide a novel way to some day engineer tissues in the human body."
The team now plan to develop capillary tubes which can be inserted into animals to see if they work properly.
Professor Julian Chaudhuri, a researcher in tissue engineering at Bath University, said that the inability to create working small blood vessels in the laboratory was one of the major obstacles to efforts to create replacement organs outside the body.
He said: "This is an exciting paper. The creation of blood vessels is one of the holy grails of regenerative medicine.
"A lot of people are working on this. It's not going to happen overnight, but this is another step towards it."
He added that, aside from the hope of creating replacement tissues and organs for patients, the work could help scientists understand how to tackle unwanted blood vessel growth, such as that found in tumours.