To the scientists involved, that failure seems shortsighted. “I think [in vitro meat] will be the only choice left,” says Mark J. Post, head of the physiology department at Maastricht University. “I’m very bold about this. I don’t see any way you could still rely on old-fashioned livestock in the coming decades.”
In theory, an in vitro meat factory would work something like this: First, technicians would isolate embryonic or adult stem cells from a pig, cow, chicken or other animal. Then they would grow those cells in bioreactors, using a culture derived from plants. The stem cells would divide and redivide for months on end. Technicians would next instruct the cells to differentiate into muscle (rather than, say, bone or brain cells). Finally, the muscle cells would need to be “bulked up” in a fashion similar to the way in which animals build their strength by exercising.
For now there are challenges at every stage of this process. One difficulty is developing stem cell lines that can proliferate for long periods without suddenly deciding they want to differentiate on their own. Another challenge is to be sure that when stem cells are prompted to differentiate, the overwhelming majority of them turn into muscle as instructed. “If 10 cells differentiate, you want at least seven or eight to turn into muscle cells, not three or four,” Roelen says. “We can achieve 50 percent now.”
The Utrecht scientists tried to extract and develop embryonic stem cell lines from pigs. Such cells would, in normal conditions, be able to duplicate every day for long periods, meaning 10 cells could grow into a staggering amount of potential meat in just two months—more than 50,000 metric tons. “Culturing embryonic stem cells would be ideal for this purpose since these cells have an (almost) infinite self-renewal capacity,” according to a 2009 report by the Utrecht team. “In theory, one such cell line would be sufficient to literally feed the world.”
Until now, however, such cell lines have been developed only from mice, rats, rhesus monkeys and humans. Embryonic cells from farm animals have had a tendency to differentiate quickly—and of their own accord—into specialized cells. In the report, Utrecht team’s porcine cells often veered toward “a neural lineage”—brains, not bacon.
The Utrecht group also worked with adult stem cells, which have the advantage of being largely preprogrammed. These cells exist within skeletal muscle (as well as other parts of the body) with a specific mission: to do repair work when tissue is injured or dies off. So if you are making in vitro meat and want stem cells that will almost surely turn into muscle tissue, adult stem cells from skeletal muscle tissue should work very well. Until now, however, scientists have not been able to get these cells to proliferate as readily as they can embryonic cells.
Cost is another barrier. The culture used to grow stem cells of any kind is very expensive. With currently available media, it might cost $50,000 to produce a pound of meat, according to Roelen, and the most efficient nutrient bath is derived from fetal calf or horse serum taken from slaughtered animals. In recent years scientists have developed their own recipes for “chemically defined media” that include no animal products. By using recombinant-DNA technology, they have also been able to get plant cells to produce animal proteins that could be used to grow the meat. But both these types of media are, for now, prohibitively expensive. An algae-based medium may eventually work best because algae can produce the proteins and amino acids necessary to sustain cell life, but that, too, is costly—at least for now.
Once the researchers get a big supply of muscle cells, they will need to keep them alive and bulk them up. It is possible now to engineer a thin strip of tissue, but if it gets thicker than a few cell layers, parts of it start to die off. The cells need a constant flow of fresh nutrients to stay alive. In the body, these nutrients are delivered by the bloodstream, which also removes waste. Post is working on how to develop a three-dimensional system that delivers such nutrients.