by George Touloumes

figures by Brad Wierbowski

Would you ever consider eating meat that was grown in a lab instead of raised on a farm? What if it were both healthier and more sustainable than conventional meat? Silicon Valley venture capital firms and major meat companies like Tyson Foods are now investing tens of millions of dollars in bioengineering research to produce exactly that kind of product. Several companies have already received extensive coverage in scientific journals and the mainstream media for successfully making edible and affordable lab-grown burgers. These burgers are the result of many significant bioengineering achievements over the past several decades. However, burgers and other minced meats are far easier to design and manufacture than complex tissues such as steaks, bacon, and chicken breasts. To produce realistic lab-grown meats like these, food bioengineers could take advantage of some results from regenerative medicine research that involved, of all things, spinach.

Why is lab-grown meat desirable?

While lab-grown meat might initially sound like a strange (or even unnecessary) idea, it actually has major advantages over its traditional, farm-raised counterparts in terms of environmental sustainability and human health. Raising livestock to produce meat by traditional practices requires lots of land, water, and energy, accounting for up to 18% of global greenhouse gas emissions. By contrast, lab-grown meat is estimated to use less than half of the land and energy required for agricultural meat production, and it also frees water-based resources and vegetable crops grown for animal feed to be used as human food.

Lab-grown meats could also provide societal and health advantages. With predictions that the global population will approach 10 billion people by 2050, lab-grown meat could be needed to satisfy increasing global demand. It could also be a healthier option than traditional meat; growing cells in a lab allows scientists to precisely test and control variables like fat, protein, and vitamin content while reducing pathogens, suspected carcinogens, saturated fats, and other unwanted hitchhikers.

How is meat currently grown in the lab?

Lab-based meat production got its start by borrowing a page out of the regenerative medicine playbook (Figure 1). In regenerative medicine, researchers hope to grow transplant organs like livers and kidneys for sick or injured patients using the patient’s own cells. In contrast, food scientists practicing bioengineering aim to grow meats from an animal’s muscle cells. This second process is based on 3 main steps:

Acquire a small sample of the animal’s muscle tissue. (Figure 1, Step 1) Break down the muscle mechanically and chemically to separate a type of stem cell called satellite cells from the muscle. (Figure 1, Step 2) Grow the satellite cells in an ideal environment with proper nutrients so they multiply into trillions of specialized muscle cells called myoblasts. (Figure 1, Step 3)

Currently, myoblasts can multiply enough times under ideal laboratory conditions that a single animal muscle sample could theoretically produce several hundred pounds of lab-grown muscle meat. But to turn the myoblasts into muscle tissue, they must first be aligned such that they fuse in a particular way to form muscle fibers. For the muscle fibers to develop, they need to experience tension forces that can trigger protein synthesis. Bioengineers typically achieve this growth by coaxing the cells to contract around small cylinders and beads for about 3 weeks. Thousands of the resulting inch-long, millimeter-thick mini muscle fibers are then packed together with other natural flavor additives to make a burger-sized patty. To grow more complex lab-made meats such as steaks, however, muscle fibers are only the first piece of the puzzle.

Where is lab-grown meat headed?

While the lab-grown meat industry has certainly made progress, no lab or company has yet managed to grow anything with traditional meat’s taste, texture, or nutritional value. Meat is primarily composed of muscle, but it also contains fat, blood vessels, nerves, and connective tissue and tendons (Figure 2). The unique tastes and textures of meats are therefore determined by how much of each of these components are in the tissue, how they are organized, and how the body’s diet and exercise developed the proteins and nutrients within them.

To start resembling a realistic steak with full flavor, texture, and nutrient content, lab-grown muscle needs to incorporate fat and connective tissue. Fat is easier to grow than skeletal muscle because the same muscle-derived satellite cells become fat cells in the presence of naturally occurring chemicals called fatty acids, and fat cells do not need to be as well-aligned or contracted as muscle cells. Fibrous tissue, however, is grown from a different group of cells called fibroblasts, which can also be found in muscle tissue samples. Interestingly, new research has shown that fibroblasts can be reprogrammed into satellite cells, thereby functioning as another source of myoblasts and fat cells. Although these muscle, fat, and connective tissues can all be grown in the lab, each needs its own highly specialized growth environment, making it difficult to grow them together into a complete meat tissue. Scientists in a wide range of biological fields are attempting to find the best ways to grow multiple cell types together effectively.

But even if cell compatibility issues are overcome, scientists must still add blood vessels to (or vascularize) meat tissues. In addition to its importance in lab-grown meat, vascularization is one of the primary goals of researchers hoping to grow replacement organs for medical transplants. Without a blood vessel network to supply nutrients and oxygen and to remove waste, the inner part of any tissue that’s more than ~1 millimeter thick would quickly suffocate. Recently, however, a cardiac (heart) tissue engineering team took a highly vascularized spinach leaf and replaced its plant cells with blood vessels and cardiac cells (Figure 3). Amazingly, the blood vessel cells grew in the old spinach veins throughout the leaf, feeding the heart cells and allowing the repurposed leaf to “beat” on its own. Although a spinach leaf is much different from a fully-grown muscle cell in both size and structure, plant-based vascularization is a promising strategy for getting blood vessels into lab-grown meat.

Outlook

For lab-grown ribeyes and sirloins to appear on the dinner table, regenerative medicine researchers will need to master the science of vascularization and cell compatibility. In the meantime, the price of lab-grown ground beef has rapidly fallen from $325,000 for a 2013 burger to an estimated $2,500/lb today. It’s still expensive compared to the $10 half-pound patty from your local diner, but the dramatic price drop shows that bioengineering is getting closer to contributing to the commercial meat industry in several key areas. These advances include decreasing the number of animals raised for meat consumption, saving valuable environmental resources like land and water, and reducing the amount of carcinogens, pathogens, and other harmful substances in our meat supply. With enough sustained financial support in engineered muscle tissue, there are plenty of reasons to believe that lab-grown meat might one day become a viable food source.

George Touloumes is a student in the Bioengineering Ph.D. program at the Harvard University John A. Paulson School of Engineering and Applied Sciences.

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