Inside the Innovative Lab Growing Mammal Tissue Using Plants as Scaffolds
Researchers at the University of Ottawa have used apple flesh to create human tissue in the shape of an ear and asparagus stalks to regenerate spinal cords
When chemistry major Jessica Holmes joined biophysicist Andrew Pelling’s Lab for Augmented Biology at the University of Ottawa, she was charged with an unusual task: coax mammalian cells to multiply and thrive on pasta noodles. In the name of regenerative medicine, Pelling runs his lab like an experimental kitchen. There, researchers like Holmes are probing common grocery items to determine which contain microscopic structures that could shape nascent cells into functional tissues.
The flat petri dishes that scientists have been using since the 1800s do not mimic the body’s complex environment, so the Pelling lab and others have been hard at work developing more realistic 3D “scaffolds” to support cell growth. The Pelling lab’s approach, though, is rather unconventional. They’ve identified everyday foods containing naturally-occurring scaffolds that, with a little tweaking, could provide the physical foundation for mammalian cells to divide, come together, communicate and assume specialized roles.
As the Covid-19 pandemic escalated during Holmes’ junior year, the university temporarily closed its research facilities to undergraduates. Instead, Holmes’ kitchen became her laboratory. She abandoned her pasta project after exhausting the list of potentially-porous noodles (from Ramen to pea-based pastas) that might make for good scaffolds. Like many during the spring lockdown, she began experimenting with bread recipes. In doing so, she made a surprising discovery: The porous structure of Irish soda bread provided an excellent scaffold. Holmes and her Pelling lab colleagues sterilized the crumbs, soaked them in nutrients, and allowed young cells to adhere to the crumbs and infiltrate the pores. In a study published in November in Biomaterials, Holmes and her labmates show that this fast, simple recipe containing little more than pantry ingredients can foster precursor cells for mouse muscle, connective tissue and bone in a dish for up to four weeks. Although it may seem like a bizarre undertaking, with additional work Holmes’ carbacious cell nursery has the potential to help researchers repair damaged tissue or regenerate organs.
While other research groups have toyed with cellular scaffolds made from wheat-derived proteins such as gluten, these materials are often labor- and resource-intensive to create. One existing technique, for instance, takes over a week and requires specialized equipment to spin wheat proteins into ultrafine fibers, creating a film on which cells can grow. To Pelling’s knowledge, his group is the first to employ entire bread crumbs to grow muscle and bone pre-cursor cells.
Bread is just one of many materials that could fulfill his mission to formulate simple, inexpensive biomaterials that support mammalian cells. In the thirteen years since he started his lab, Pelling has pushed mammalian cells to their limits by challenging them to grow in peculiar environments. Pelling began with Legos, and since then has moved on to celery, apples, asparagus and other plant-derived scaffolds. (Bread contains wheat, so Pelling considers it to be plant-based as well.) “I've convinced myself that cells will grow on pretty much anything,” he says.
The Pelling lab is at the forefront of a practice that dates back to 3000 B.C., when ancient Egyptians used wood to replace teeth and coconut shells to mend skulls. Plants are well-suited to such applications because they have cellulose, a carbohydrate built into their cell walls that provides strength and flexibility. Cellulose not only gives plant cells a structure on which to grow, but it also forms a porous network that transports fluids and nutrients, much like a network of blood vessels. Now, researchers are realizing that this material may provide similar benefits to mammalian cells.
While modern efforts in regenerative medicine have employed synthetic or bacteria-produced cellulose, the Pelling lab sees no reason to reimagine millions of years of plant evolution. They use a common “decellularization” technique involving soap and water to remove the cells from fruits and veggies. What’s left behind is a naturally-vascularized cellulose scaffold that can then be repopulated with many types of cultured mammalian cells.
The idea for one of the lab’s first plant decellularization endeavors came to former undergraduate researcher Daniel Modulevsky during lunch. The fleshy inside of his colleague’s partially-eaten apple looked like it might provide a large, moldable structure to support mammalian cells. Online recipes suggested that McIntosh apples were particularly hearty, and so Modulevsky began peeling them, decellularizing them and coating them with cells. After promising preliminary results, he brought his lunch-time premonition to fruition and remained in the Pelling lab to complete his PhD in biology. The researchers have since carved their decellularized apple flesh into an ear-shaped scaffold for human cells. More recently, they’ve even implanted the apple scaffolds into living mice to foster connective tissue, collagen and networks of blood vessels.
Although the Pelling lab’s unusual ideas were initially met with resistance from the scientific community, Modulevsky is pleased to see that their apple scaffolds have since seeded many new research projects—from growing bone-like tissue in rats to creating habitats for roundworms, which are popular research subjects for biologists. “It's really cool to see how a small project has really taken off around the world,” he says.
At Boston College, biomedical engineer Glenn Gaudette is using similar decellularization techniques on spinach leaves, coating them in human heart cells to engender cardiovascular tissues. He was drawn to spinach in particular because its veiny structure is ideal for supplying oxygen and nutrients to heart cells, as well as for flushing out metabolic waste. He plans to sew the central vein of the decellularized spinach onto the heart’s main artery, the aorta, in order to facilitate blood flow to damaged heart muscles. The rest of the leaf would blanket the general area, expanding and contracting with each heartbeat. Eventually, he also envisions folding spinach leaves into the shape of a human heart and sprouting an entire organ.
Gaudette anticipates that less than five years of bench-side research likely remains before plant-based scaffolds can be used in clinical trials involving relatively straightforward tissues like skin. Before then, simple issues need addressing, such as ensuring that the soapy detergents used to decellularize the plants are fully washed away prior to implantation. And more serious concerns exist too. For instance, researchers need to determine how a patient’s immune system might respond to cellulose (although Gaudette’s unpublished work, as well as Pelling’s preliminary studies in mice and rats, has shown promising results). Gaudette thinks one strategy in humans might involve reverting a patient’s own cells back into stem cells, and cultivating them on the spinach scaffold prior to implantation. This might ultimately help the immune system accept the new tissue as part of the body.
According to Gaudette, there’s still work to be done, but researchers are getting closer. “It's fun to dream, right?” he says. “I think we have an opportunity to start a new industry.”
Like Pelling’s lab, Gaudette’s team has begun engineering edible, plant-based scaffolds capable of producing environmentally-friendly, lab-grown meat. While bread crumb scaffolds would be well-suited to what Gaudette calls ground “mush meat,” spinach scaffolds might provide the rigid matrix needed for more structured cuts like steak.
As researchers continue to scan the grocery aisles for the next scaffold innovation, it’s becoming clear that some plants are better suited to certain applications than others. For example, Gaudette’s colleagues are using bamboo to regenerate teeth because it’s tough and has a small diameter. Peaches, by contrast, are far too soft to support structures for grinding and chomping food.
Gaudette’s work on spinach scaffolds has become recommended reading for students in bioengineer Grissel Trujillo de Santiago’s biomaterials class at Tecnológico de Monterrey in Mexico. In the lab she heads with a colleague, Trujillo de Santiago is finding ways to 3D print living tissues. Like Gaudette and Pelling, she aims to engineer elegant ways to fabricate vascular systems that mimic human blood vessels. Unlike Gaudette and Pelling, though, her team is using water-filled networks called hydrogels rather than cellulose.
She’s intrigued by the possibility of employing plant-derived structures to grow both human tissues and edible meats. The latter application, in particular, would require scaffolds to be cost-effective and scalable, she says, to meet the demands of carnivores around the world.
In terms of medical uses, Trujillo de Santiago says the Pelling lab’s previous success implanting mice with apple scaffolds is promising. Besides testing the scaffolds in humans and ensuring our immune systems respond well to the plant-based material, she says researchers will need to demonstrate that their implants will function like the tissues they are intended to augment or replace.
Although Trujillo de Santiago has yet to experiment with plant-derived scaffolds herself, she is beginning to use plant viruses to create structures for mammalian cells. The viruses are harmless to mammals such as mice and humans, and come together to form a mesh-like material that helps anchor cells. As she puts it: “We have this portfolio of biomaterials in nature that we can use for human health.”
Back at the University of Ottawa, Pelling, Modulevsky and their colleague Charles Cuerrier have founded a company based on their most promising decellularized fruits and veggies. One of their techniques, which uses asparagus scaffolds to regenerate spinal cords in rats, was recently designated a breakthrough device by the U.S. Food and Drug Administration. Unlike many existing scaffolds that are designed to degrade over time, the Pelling lab’s asparagus inserts are less likely to be broken down by enzymes in the human body and release toxic byproducts. Although it will be a few years until their decellularized asparagus will be tested in humans, the researchers are optimistic.
Not every vegetable will lead to a breakthrough device, but Pelling says each new idea has value. “Your students—the ones who are willing to work in a lab like this—they're going through the exercise of discovery,” he says. “And when you stumble onto the random discovery that’s actually important, your whole team is trained and ready to execute.”
After the strict pandemic restrictions lifted, Holmes returned to campus with her colleagues. There, she continued concocting various soda bread recipes and baking them in the lab’s sterilization oven. She’s now nearing graduation, and intends to apply the open-minded approach she learned in the Pelling lab to a career in speech pathology. Her main take-away? “There's no such thing as a bad idea or an idea that's too far out there.”