To flush the animal heart of cellular material, “we start with a nasty detergent that literally bursts the cells,” Taylor says. In the case of larger animal hearts, like those of pigs, a member of the center, Stefan Kren, fills a large receptacle with the detergent and lets it flow into the heart through a long rubber tube. To “decellularize” smaller organs, like rat hearts, Kren uses a piece of enclosed glassware. After cells are removed, the heart appears white and rubbery. Taylor points to a glass jar containing a decellularized pig heart in formaldehyde, noting that the location of the coronary blood vessels is visible. Valves are intact, she says, as are heart chambers.
The bigger challenge is adding new cells to the heart scaffolding and culturing it within the bioreactor. To prepare for this process, Kren places a decellularized rat heart in saline solution and attaches tiny catheters to the left ventricle and aorta. These are for inflow and outflow of nutrient solution, respectively. Then he hangs the organ by the aorta within the bioreactor’s central glass chamber, which is surrounded by a system of tubing. He also attaches two electrodes to the heart, one near its bottom and another near the aorta. These will help pace the heart and encourage the new cells to contract in a coördinated fashion.
The next step is particularly tricky: it involves injecting new cells, isolated from newborn rats, through the walls of the left ventricle. (For human hearts, of course, a different cell source, such as cardiac stem cells, would be needed.) “If we put too few in, they won’t interact with each other properly,” says Taylor. “If we put too many in, they’re not going to get enough oxygen and nutrients and are going to die.”
So far, her team has used a primary culture that includes four types of heart cells from neonatal rats: cardiac myocytes, endothelial cells, smooth muscle cells, and fibroblasts. The researchers have repopulated the left ventricle but have yet to complete the process with other heart chambers. Once the cells are in place, Kren begins the electrical pacing. “We usually wait a day while the cells are settling down,” he says.
At this stage, the bioreactor mimics several features of a heart-lung system. A gas mixture that is 95 percent oxygen bubbles from a steel tank into a cylinder of nutrient solution. A small pump sends that solution over to the heart, down through the catheter, and into the left ventricle. Not only does the solution provide nutrients, says Taylor, but the pumping action stretches the heart mechanically, the way a heartbeat would. “We want to train the cells so they beat in a way that gets the blood out and don’t just sit there and throb,” she says.