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See Inside December 2006

Growing Replacement Parts

Bioengineers can now cultivate blood vessels and other tissues from scratch

With the goal of mimicking the mechanical properties of soft tissue, bioengineers William R. Wagner and Michael S. Sacks of the University of Pittsburgh have fashioned an inexpensive polymer, polyester urethane urea, into a biodegradable scaffold. This cylindrical scaffold's strength resembles that of a pulmonary valve because it responds to stress differently depending on the direction in which the stress is applied. A patch of this biomaterial infused with smooth muscle cells (right) functions as vascular tissue, promoting healing and reducing formation of scar tissue in the hearts of rats recovering from cardiac arrest.

Already having reached the phase of clinical trials, the California bioengineering company Cytograft has patented a method for growing blood vessels from a human patient's own cells. In a feasibility trial undertaken in Argentina, Cytograft implanted its engineered vessels into two dialysis patients. Neither patient encountered problems with the implants for at least nine months.

One barrier to progress in tissue engineering results from the inability of thick tissue such as muscle, once implanted in a patient, to receive sufficient penetration of new blood vessels from the body's own network to keep the tissue alive. To address that problem, a multi-institution team spearheaded by Shulamit Levenberg of the Technion-Israel Institute of Technology in Haifa has created small pieces of muscle capable of generating its own blood vessels.

The researchers combined on a plastic biodegradable scaffold three types of cells: myoblasts that become muscle fibers, endothelial cells that form into vessel tubes, and fibroblasts that are the precursors to the smooth muscle cells that stabilize the cell walls. The endothelial cells became vessels, recruited fibroblasts and caused them to differentiate into smooth muscle cells. Once implanted in a rat, less than half the vessels became perfused with blood. But twice as many cells survived when implanted with the three cell types than implants made up of myoblasts and fibroblasts unaccompanied by the vessel-producing endothelial cells. The technique might eventually help address the persistent challenge of supplying engineered cells with oxygen and nutrients and allowing them to remove wastes.

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