By Alissa Poh

If humans had the regenerative capacity of planaria, executioners in centuries past—particularly those charged with quartering convicts already drawn and hanged—would have had a bad time. These minute aquatic flatworms are remarkably resilient: chop one in half and it becomes two. Or three, if sliced in thirds. Scottish nobleman Sir John Dalyell described it well in 1814: “No matter how I cut these animals, they could almost be called immortal under the edge of the knife.”

Not that regeneration was a dark horse by then. Says Howard Hughes Medical Institute and Stowers Investigator Alejandro Sánchez Alvarado, PhD, “Everyone wanted to know if it was broadly distributed across animal species” during the mid-1700s, after Swiss naturalist Abraham Trembley observed this feat in Hydra (tiny freshwater animals). “Trembley influenced what’s sometimes referred to as The Great Snail Controversy; many of his peers began decapitating garden snails and slicing salamanders just to see what might happen.” Amazingly, the victims not only survived but fully regenerated. Religious intellectuals of the day were troubled—how, they wondered, could this be possible for organisms possessed of “indivisible souls?”

Since then, scientific understanding has grown by leaps and bounds, but Sánchez Alvarado still wants to know why the capacity for regeneration varies so widely across animal species. After discovering that a particular species of planaria, Schmidtea mediterranea, fulfilled his criteria for a choice model system—diploid organism, small genome size, high regenerative capacity, existence of sexual and asexual strains—he proceeded to make it genetically malleable for regeneration-centric investigations, and is widely regarded in the field as both pioneer and leading authority.

If you build it, they will go forth and multiply

Operation Planaria at Stowers started small: Besides caring for the likes of whiptail lizards, zebrafish, and other laboratory animals, the Reptile & Aquatics Facility staff housed a handful of flatworms in Rubbermaid containers, in a single incubator occupying an unused procedure room. Then Sánchez Alvarado joined the Institute in 2011, persuaded to move not just most of his human crew from the University of Utah’s School of Medicine, but also his entire planaria colony.

“We ratcheted up almost to breaking point in terms of manpower needs,” says Diana Baumann, head of the Reptile & Aquatics facility. “So an expansion plan became necessary, because we didn’t want to be the factor limiting planaria-related scientific productivity.”

Rapid evolution followed, culminating in the creation of the first core facility anywhere dedicated to planaria. These days, a total of 185 sexual and asexual flatworm strains occupy not only plastic containers, but two whole rooms—separated by method of reproduction—with a third room set aside for research and development. A five-member team, supervised by Aquatics Specialist Shane Merryman, provides full-time planaria care.

Not only that, but the asexual planaria—which reproduce by sticking their tails downward and swimming in the opposite direction until, like overstretched rubber bands, they snap in two—have already made themselves at home in the most advanced aquatic animal housing around, courtesy of Merryman and his group. It’s called a recirculating system, where the water is continuously recycled after being filtered free of all possible contaminants. Baumann considers it “a fantastic time-saver that greatly improves the quality of care,” given the current scale of this core facility. Housing planaria in individual containers such as the aforementioned Rubbermaid containers, each with static water that requires manual changing, is feasible only when grooming particular strains for specific research purposes.

No off-the-shelf planaria recirculating system exists, Baumann adds, so what the asexual planaria are currently enjoying is the result of much careful research on Merryman's part. For instance, water courses downward from the topmost of three stacked tanks before getting recirculated, mimicking a river's flow—and the worms' natural habitat. Like all animals, these creatures move around, albeit slowly, so the system's pipes are oriented to minimize the number of runaway worms that reach the water-collecting sieves below the bottom tank.

Prior to joining Stowers, Sánchez Alvarado and his research group had identified conditions to rear sexually reproducing animals in captivity, but had yet to optimize procedures to overcome one key obstacle: why, of multiple egg capsules from their sexual strains, few were actually gravid, that is containing developing embryos. Baumann recalls adjusting salt concentrations in the worms’ watery home and adding crushed coral for calcium-enhancing purposes, among other troubleshooting efforts, before she went out on a limb and introduced them to a fresh water supply from the pond by her house. “Its surface runoff was completely free of insecticides and pesticides, and I thought the water might contain basic minerals these worms required,” she says. “They finally began reproducing in this source, which we called ‘BP water’ for the longest time before revealing that it stood for ‘Baumann Pond.’”

“We keep track of fertile eggs collected, and a recent monthly total came to almost 11,000,” says Merryman, whom Baumann credits for building a meticulous database that records all things planaria. Every species and strain is barcoded, complete with scanners. Volumes, numbers, and other specifics are mere mouse clicks away; just about any question regarding this core facility is answerable— except, perhaps, the exact worm count on a given day.

Meanwhile, the critters themselves live like royalty. Their loyal servants prepare planaria chow three times weekly, mashing calves’
livers shipped fresh from Chicago into a fine paste—“think making pâté en masse,” Baumann quips—which is then loaded into syringes and hand-dispensed throughout the tanks. Every egg produced by the sexual planaria is laboriously collected, and while they await their own recirculating system (currently being designed by Merryman), their water has to be manually prepared in sixty-gallon rounds.

“These are itsy-bitsy worms with outsized care requirements,” Baumann says. “But consistent, top-quality animal husbandry is what we’re here to provide.”

Happy, healthy worms = better science

Sánchez Alvarado is delighted that his sexual planaria have flourished at Stowers and even produced progeny equally capable of churning out fertile eggs—not necessarily a heritable trait from one generation to the next.

“Although we have not yet fully optimized sexual reproduction and production of large numbers of embryos in captivity, we are nonetheless at a point where it is possible to break ground in the interrogation of planarian embryogenesis.” As Sánchez Alvarado points out: “The embryogenesis of S. mediterranea has yet to be described in detail, because until now, it’s been difficult getting them to reproduce. Rather than trying to fit this process into any preconceived notions of developmental stages, we want to see if the transcription profiles of individual embryos, at different ages, can tell us more about the steps involved.”

As for regeneration, it continues to fascinate even those largely unenamored of science and has long made popular culture’s hit list of cool concepts: Take the Lizard, for instance, Spider-Man’s nemesis and product of genetic tinkering gone badly wrong; or the injury-defying Wolverine of X-Men fame. But while research like Sánchez Alvarado’s is key to increasing our understanding of many biological processes, including aging and tissue homeostasis, “it’s fanciful to imagine we’re within reach of restoring body parts,” he says. “We’re not even close. Should we get there, though, planaria will likely fit somewhere on the totem pole of creatures deserving credit.”