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Where the wild things are: Planarian flatworms

Planarian flatworms could unlock the secrets to how we may—someday in the future—regrow a damaged or missing limb or repair an organ, potentially transforming the lives of the generations to come.

13 December 2023

By Rachel Scanza, Ph.D.

In the 18th century, an intriguing animal feature captured the imagination of biologists. Despite their small and simple appearance, planarians, just barely the size of a grain of rice, could do something astonishing—they could regenerate.

These tiny worms, when decapitated, not only regrew their heads, but the heads regrew their bodies. Then, when scientists diced them into smaller and smaller pieces, each piece somehow knew what to do to become whole again.

Meet the planarian flatworm. Planarians flourish nearly everywhere on Earth with thousands of unique types found in diverse environments including freshwater, seawater, and on land. They also come in a variety of shapes, sizes, and colors, with land-dwelling worms measuring as much as 1 meter (approximately the length of a baseball bat), and psychedelic sea-dwelling flatworms exhibiting vibrant colors.

Freshwater planarians look like ordinary little worms. Populating streams, creeks, ponds, and puddles, these creatures happen to have extraordinary biological properties—namely, the ability to regenerate heads, tails, and entire bodies thanks to a large and ubiquitous supply of stem cells, which can develop into any cell type in the body.

Why study planarian flatworms?

While humans can regenerate certain cells—we are able to donate blood and part of a liver—if we lose a head, or limbs, or sustain damage to vital organs like the heart, brain, or spinal cord, our outlook is not nearly as promising. Perhaps planarians can help tell us why, or why not.

Photograph of planarian flatworms.

When viewed under a microscope, planarian flatworms are adorable. Like their moniker, they are flat and have two googly, cartoon-like eyes on top of their head. They are free-living, in contrast to other types of flatworms that are parasitic and live off a host organism.

In a sort of perpetual immaturity, planarian stem cells can receive signals from surrounding cells and tissues that direct them to mature and become any type of cell—an eye, a mouth, a tail, a head. These directions are akin to a musical score, instructing the appropriate genes to turn on and off for stem cells to differentiate, multiply, and facilitate regeneration.

Planarian flatworms’ ability to rebuild and repair themselves following insults like amputation or doses of irradiation that are lethal to mammals may hold keys for helping understand human conditions like organ failure and treatment-resistant cancers.

These tiny worms could unlock the secrets to how we may—someday in the future—regrow a damaged or missing limb or repair an organ, potentially transforming the lives of the generations to come.

Planarians at the Stowers Institute

The predominant planarian species studied at the Stowers Institute originated from an abandoned fountain nearly 5,000 miles away.

Twenty-five years ago, Stowers President and Chief Scientific Officer Alejandro Sánchez Alvarado, Ph.D., and his then postdoc Phil Newmark, Ph.D., collected the flatworms in Barcelona, Spain—scooping them from the fountain. These flatworms have an astonishing capacity for regeneration; a fragment that is a mere 0.5 mm in diameter can remold itself and in about two weeks regenerate a complete flatworm.

Fluorescent microscopy image of a planarian flatworm.

Nine species of planarians now live at the Institute in several facilities, with a miniature setup in the Sánchez Alvarado Lab affectionately called “The Ranch.” In the main planarian living quarters, worms cling to the sides of their transparent enclosures, or heap on top of a hunk of food like football players piled over a football. Sometimes, a worm hugging the side of its tank decides to let go and gently drifts before settling on the tank’s floor.

However, the main flatworm species both at the Institute and increasingly worldwide for regenerative studies is Schmidtea mediterranea, which comes in two varieties. These flatworms can be distinguished by their reproductive preference, with over 350,000 asexual and 13,000 sexually reproducing worms cared for by a specialized team.

Stowers planarians have custom housing and water. All planarians receive “Planaria Water,” a special recipe modeled from the water source at the original collection site of the asexual worms in Spain. Asexual planarians have their own room with several sets of three large tanks stacked on top of each other. Water flows from top to bottom allowing asexual worms to swim freely in a system resembling their natural environment.

Olivia Lynch, an associate scientist on the invertebrates team, works in the planarian facility. Custom housing enables the separation of different planarian flatworm species.

In a separate room, sexual planarians along with the other eight species—including a few labeled “unknown”— reside in a setup cleverly fashioned from commercially available supplies. Individual clear plastic cylinders snugly cushioned into PVC pipe dishes are arranged in four-by-four grids within industrial dishwashing racks, the type you would be more likely to encounter in a restaurant kitchen. Twenty-five floor-to-ceiling stacks with five racks per stack can accommodate up to 2,000 cylinders. The smaller living environments enable the planarian team to separate different species, generations, and crosses and to ease egg collection following copulation.

Schmidtea meditarranea shared worldwide

The exceptional care they receive at the Institute, including meals of locally sourced, organic calf liver in contrast to wild worm diets of insects, has propelled S. mediterranea’s status as the “go to” worm to investigate regeneration. The worms are shipped to researchers across the globe, a service offered by the Sánchez Alvarado Lab’s website Since the team first started documenting flatworm shipments in 2017, 114 colonies totaling around 175,000 worms have been shipped to research labs around the world, with 27 colonies (over 50,000 planarians) sent in 2023 alone.

Technology has advanced exponentially, allowing researchers the ability to probe and understand the biology of more unusual, unordinary research organisms. Scientists at the Stowers Institute and around the globe are actively investigating the molecular and genetic tools planarians use to accomplish their remarkable regenerative feats.

Watch: The Great Planarian Hunt

Learn more about our other (un)ordinary organisms

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