Regenerative medicine moves forward, organ by organ: "The trachea and other respiratory tubes, like most tubes in the body, have an intricate, three-layer architecture. The inner layer, or epithelium, interacts with whatever is flowing through the tube; in the case of the trachea, air. The middle layer is composed of muscle that constricts or relaxes the tube, and the outer layer consists of connective tissue that supports microvessels and small nerves. ... researchers found that it is not necessary to recapture the ordered layering to heal injuries. Instead, they concentrated on restoring cellular health. When cells are intact and have regained their biological function, they need only reside near the injured tissue to enhance overall repair. [Scientists] achieved this repair state by delivering a mixture of new healthy cells derived from the epithelial lining and the nourishing blood vessels. The combination of epithelial and endothelial cells take over the biochemical role lost with cell damage. The healthy cells release growth factors and other molecules necessary for healing tissue, and can modulate their delivery in response to physiological feedback control signals."