Growing better vessels could keep tissues alive

ONE of the major obstacles to growing new organs—replacement hearts, lungs and kidneys—is the difficulty researchers face in building blood vessels that keep the tissues alive, but new findings from the University of Michigan could help overcome this roadblock.

"It's not just enough to make a piece of tissue that functions like your desired target," said Andrew Putnam, U-M associate professor of biomedical engineering. "If you don't nourish it with blood by vascularizing it, it's only going to be as big as the head of a pen.

"But we need a heart that's this big," he added, holding up his fist.

More immediately, doctors and researchers believe figuring out how to grow working blood vessels might offer treatments for diseases that affect the circulatory system such as diabetes. Perhaps the right drug or injection could save patients' feet from amputation.

Putnam and his colleagues have revealed why one of the leading approaches to building blood vessels isn't consistently working: It's making leaky tubes. They also demonstrated how adult stem cells could solve this problem. A paper on the findings is published online in Tissue Engineering Part A, and will appear in a forthcoming print edition.

Engineered blood vessels built with lung fibroblasts as supporting structure cells were leaky, as this image depicts. Red represents the tracer dye the researchers injected into the bloodstream of mice. The tracer dye is not contained in vessels. The researchers captured this image 14 days after the experiment began (Stephanie Grainger)

Today, biomedical researchers are taking two main approaches to growing new capillaries, the smallest blood vessels and those responsible for exchanging oxygen, carbon dioxide and nutrients between blood and muscles or organs.

One group of researchers is developing drug compounds that would signal existing vessels to branch into new tributaries. These compounds—generally protein growth factors—mimic how cancerous tumor cells recruit blood vessels.

The other group, which includes the U-M team, is using a cell-based method. This technique involves injecting cells within a scaffolding carrier near the spot where you want new capillaries to materialize. In Putnam's approach, they deliver endothelial cells, which make up the vessel lining and supporting cells. Their scaffolding carrier is fibrin, a protein in the human bodySNEAKERS