Heart bypass gets new source for replacement blood vessels

The search for a stable, renewable source of blood vessels, especially for potential use in heart bypass surgery, has reached a milestone at the State University of New York at Buffalo.

A multi-disciplinary team at SUNY Buffalo designed tissue engineered blood vessels (TEVs) using a matrix of vascular smooth muscle embedded in fibrin gels. After only two weeks in culture, the TEVs showed the strength and resiliency necessary for implantation. Even more exciting, 15 weeks after implantation, the fibrin-based TEVs “exhibited remarkable remodeling with considerable production of collagen and elastin, and significantly increased mechanical strength (and) physiological levels of blood flow and vasoreactivity,” according to a paper published online in the American Journal of Physiology-Heart and Circulatory Physiology.

Currently, blood vessels are usually “harvested” from the patient’s own leg, often causing pain and discomfort, as well as extra surgical steps. So the need for a source of strong, yet elastic -- and physiologically responsive – replacement blood vessels has been the subject of laboratory searches and experimentation for decades.

The study, “Fibrin-based functional and implantable small diameter blood vessels,” was written by Daniel D. Swartz and James A. Russell from the SUNY Buffalo Department of Physiology and Biophysics, and Stelios T. Andreadis of SUNY Buffalo’s Department of Chemical and Biological Engineering, Buffalo, New York.

Fibrin-based TEVs develop strength and reactivity after two weeks in culture

The researchers concluded that “fibrin-based TEVs hold significant promise for treatment of vascular disease and as a model system to address interesting questions with regards to blood vessel development and pathophysiology.”

Replacement of large (6-millimeter and larger) blood vessels has been successful using several synthetic materials, but smaller-diameter grafts usually failed due to thrombus or plaque formation. Various tissue-engineering approaches were developed using natural or synthetic biomaterials as scaffolds for cell growth. Biodegradable scaffolds using polyglycolic acid (PGA) have shown promise and collagen gels also worked, though 7mm collagen-based TEVs needed Dacron mesh reinforcement.

Prior to this study, the SUNY Buffalo researchers thought fibrin could be substituted for collagen as a scaffold for TEV because it shares high seeding efficiency (with smooth muscle cells, or SMCs) and uniform cell distribution. Indeed, in “contrast to collagen, fibrin stimulates synthesis of collagen and elastin and yields TEV constructs with improved mechanical properties, suggesting that fibrin may be a more appropriate scaffold for cardiovascular tissue engineering,” they said.

In the current study, the SUNY Buffalo researchers took lamb vascular smooth muscle and endothelial cells to engineer small diameter (4mm) blood vessels, “which attained considerable mechanical strength and vasoractivity after only two weeks in culture.” When the thrombin/fibrinogen solution was poured into the fibrin mold to start the process, it “gelled within 5-10 seconds.”

Tests using vasoactive receptor and nonreceptor substances showed that the fibrin-based TEVs exhibited an ability to expand and contract over time, similar to native vessels. This is a very important property that allows blood vessels to adapt to changes in blood flow rate.

Transplanted TEVs produce new collagen, elastin

Furthermore, after “a short time in culture, SMCs remodeled the extracellular matrix by substituting the fibrin gel with collagen.” They found that after only about two weeks, the structure was ready for transplantation. “TEVs containing SMC and endothelial cells were implanted as interpositional grafts into the external jugular veins of 12-week-old lambs,” Swartz et al. reported. “After implantation TEVs integrated very well with the cephalic and caudal ends of the jugular vein and remodeled successfully producing new collagen and elastin,” they said. When the implants were removed after 15 weeks, there was no fibrin left; it had been completely replaced by collagen.

“Most importantly,” the researchers wrote, fibrin-based “TEVs remained patent and demonstrated blood flow comparable with that of the control (natural, non-operated) jugular vein, suggesting that fibrin based blood vessels may provide a promising therapeutic modality and a good model system to study vascular development.”

Next steps: According to Stelios Andreadis, the team is preparing to submit a grant application to the National Institutes of Health “to improve vessels’ mechanical strength.” They will also seek to make the matrix stronger using recombinant DNA techniques and “engineer vessels from bone marrow-derived stem cells to provide a source of autologous cells for transplantation, and therefore avoid the use of native vessels as a cell source,” Andreadis noted.

Source and funding: The study, “Fibrin-based functional and implantable small diameter blood vessels,” by Daniel D. Swartz, James A. Russell and Stelios T. Andreadis from SUNY Buffalo appears on the online version of the American Journal of Physiology-Heart and Circulatory Physiology, published by the American Physiological Society.