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In the first successful attempt to make engineered tissue without synthetic scaffolding, a team of researchers created a blood vessel made entirely from human cells. Such replacement vessels could be used for patients who need coronary bypass surgery or bypass surgery to replace clogged arteries in the legs.

"The blood vessel displays the function, look and feel of real tissue," said Nicolas L'Heureux, a bioengineering post-doctoral fellow at the University of California, San Diego (UC San Diego) School of Engineering and first author of the study.

The research is being published in the January issue of The Federation of American Societies for Experimental Biology (FASEB) Journal. In the article, the researchers report that the engineered tissue is strong enough to handle nearly 20 times the normal blood pressure in the human body.

The research was conducted through the Laboratoire d' Organogénèse Expérimentale (LOEX) at the Laval University School of Medicine in Quebec by L'Heureux, Lucie Germain, Raymond Labbe, and Francois Auger.

L'Heureux, Auger, and Germain hold a patent on the invention, which has been issued to Laval University. The research was funded over the past three years by the Heart and Stroke Foundation of Quebec (Canada).

"We believe this is a fundamental breakthrough that could be used to engineer a variety of tissues," said John Frangos, UC San Diego professor of bioengineering who is now working with L'Heureux to improve the system and apply it to bone tissue. The LOEX will also continue research on the system and UC San Diego and LOEX are exploring collaborative projects.

Currently, most engineered tissues for skin, blood vessels and cartilage have been built on scaffolding made from synthetic materials or animal products. This extracellular matrix gives the tissue shape and form. Human cells such as fibroblasts and muscle cells are seeded into the artificial formwork. Eventually, the human cells grow into the biodegradable artificial material and cover it with extracellular matrix proteins.

A tissue engineered completely from the patient's own cells may offer several advantages.

"We avoid the possibility of an immune response that causes complete rejection and we diminish the chance that the body will react to the graft with a chronic inflammatory response that builds scar tissue," said L'Heureux. "As a result, we might be able to make smaller functional vascular grafts, which allow for more perfect matches to the natural blood vessel diameter and thus improve blood flow patterns in the vessel."

Each year, more than 430,000 Americans with heart disease need coronary artery bypass surgery. The clogged section of the artery is replaced with a graft from another artery or a vein from the patient's leg. However, this technique has drawbacks. A vein graft has to work hard to do the job of an artery, thus building extra muscles that eventually clog the graft. Patients often need repeat surgery, and in many cases, the patient does not have enough healthy tissue available for more grafts. In addition, atherosclerosis can clog peripheral blood vessels such as the blood vessels in the major arteries of the legs. Engineered vascular grafts could be used in all of these cases.

Bioreactor Design is Key to Engineering Completely Biological Tissue

The LOEX technique works by harvesting fibroblasts from the recipient's skin and smooth muscle cells from a superficial vein. These cellular building blocks produce collagen and other proteins, which interweave to form a strong and flexible sheet-like matrix. The sheet is then wrapped around a tube to mold it into the shape of a blood vessel. The tube is placed in a bioreactor, where a nutritious liquid made of amino acids and vitamins flows through and around the vessel. Temperature, pH balance, atmosphere, and nutrient mix are all carefully controlled. Finally, endothelial cells that make up the blood vessel wall are seeded inside the protein matrix and the tube is removed.

"The biochemical and mechanical environment can influence gene expression, and basically change the behavior of the cell," said L'Heureux. "Through the bioreactor, we mimic the body's environment, and transform the skin cells into blood vessel cells."

Although researchers have attempted to create completely biological blood vessels in the past, the work failed because the engineered blood vessels were too weak to withstand the blood pressure in the body and would burst after implantation. As reported in the FASEB journal, the Canadian researchers tested the new engineered blood vessel in both laboratory and animal models. The researchers created a blood vessel made from human cells and implanted it for one week in a dog model. The graft held strong and allowed blood flow. The studies showed that the all-natural tissue was strong enough to handle nearly 20 times the normal human blood pressure.

The UC San Diego team is now working to improve the bioreactor design to more closely simulate fluid flow and pressure inside the body. They also plan to do long-term immunological and biomechanical studies on the whole system. In proposed pig studies, blood vessels would be engineered from the pig's own cells and implanted for a two-year period.

Frangos and L'Heureux are also using the technique in preliminary efforts to engineer bone tissue.

"Blood vessels and bones are very similar with the exception that the protein matrix in bone mineralizes," said Frangos. "To date, we have successfully created a matrix that has the biochemical properties of bones and we are now attempting the harder challenge of achieving mineralization."

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