Scientists are striving to construct complex tissue from scratch, for use in transplants for damaged or age-worn organs: "The research demonstrates the potential for eventually growing tissue-engineered vessels out of stem cells harvested from the patients who need them, providing a desirable alternative to the venous grafts now routinely done in patients undergoing coronary bypass operations. ... Although not yet strong enough for coronary applications, the UB group's tissue-engineered vessels (TEVs) performed similarly to native tissue in critical ways, including their morphology, their expression of several smooth muscle cell proteins, the ability to proliferate and the ability to contract in response to vasoconstrictors, one of the most important properties of blood vessels. The TEVs also produced both collagen and elastin, which give connective tissue their strength and elasticity and are critical to the functioning of artificial blood vessels. ... The TEVs were implanted into sheep and functioned normally for five weeks."