Severed sea cucumber appendages don’t seem to die

Organs, arms, appendages, and other complex tissues usually decay rapidly when they’re separated from their host. Over the years, biologists have seen some success with keeping them alive outside of the body—organ transplants depend on it—but it has always required germ-free environments and nutrient-rich mediums filled with growth factors. Now, though, scientists have discovered bits of tissue removed from a species of sea cucumber called Psolus fabricii can keep on living indefinitely if they’re left in ordinary seawater.

“This is naturally occurring tissue immortality,” said Sara Jobson, a researcher at Memorial University of Newfoundland and lead author of the study. “Having tissues that survive that easily is unheard of. We’ve never seen anything like this.”

The beginning of LiPfe

Psolus fabricii is a species of sea cucumber that lives in the cold waters of the Atlantic and Arctic oceans. Its bottom side, known as a sole, is soft and ringed by a band of tube feet that it uses to grip rocks. Once on a rock, it extends soft, branching tentacles into the water to feed on suspended particles. Because these sea cucumbers inhabit harsh environments, their feet and tentacles experience high rates of injury and loss. Evolution has therefore endowed these sites with an incredibly high capacity for regeneration.

While sea cucumbers can easily regrow these parts, they don’t have whole-body regeneration like flatworms and some starfish. Their severed bits don’t grow into new sea cucumbers. But it turns out they don’t die, either.

“We didn’t set out to find immortal tissues,” Jobson said. “Our lab focuses on sea cucumbers, and this sea cucumber has been used in other studies. One of my collaborators happened to notice that its amputated tissue just kept living, and it seemed to be healing and surviving and she didn’t do anything special to keep it. It was a fortuitous discovery.”

This fortuitous discovery quickly turned into an organized long-term experiment. The researchers took excised tube feet, groups of tube feet called ambulacra, and tentacles from P. fabricii and found all of them survived when placed in natural, non-sterile seawater.

“We examined all of them, but we primarily focused on tube feet,” Jobson said. When tube feet were severed, the wound margin was a mess of missing or fragmented epidermal and connective tissue. Within two days, the explants began shedding this damaged tissue. Internally, a large influx of coelomocytes, the sea cucumber’s immune cells, rushed from the inner connective tissues toward the damaged spot, apparently to facilitate organismal defense and regeneration.

By day six, the healthy tissue had curled inward, completely sealing the wound site; the severed organ was more or less restored to working order.

It turned out LiPfe explants weren’t just surviving; they were actively reorganizing their architecture to adapt to the new, severed state. First came the shrinking. During the first week, the tissue shrank by about 23 percent in diameter. Given more time, it stabilized and reversed this trend. Between 60 and 120 days post-excision, LiPfe grew back to their initial size, and after a year, they were 12 percent larger than when they were first cut from the host.

The researchers have introduced these tissues as a completely new class of living material they called LiPfe—living immortal P. fabricii explants. And as time went by, LiPfe put on quite a show.

The metamorphosis

The internals of a foot tube attached to a sea cucumber include a mix of epidermal tissue, connective tissue, a neural plexus, muscle tissue, and an inner lumen. The separated explants, though, got busy dismantling parts of themselves that were no longer useful. Muscle tissues, which initially made up 17 percent of the explant, were gradually invaded by coelomocytes that broke the muscle down into small pieces and destroyed its organization. After 180 days, the muscle tissue and the lumen had completely disappeared from the explant.

In their place, connective tissue expanded to become the dominant structure. The collagen fibrils within it began bundling together, creating strong bands or striations that looked similar to the vanished muscles. By the end of the first year, connective tissue accounted for 74 percent of the explant, while the epidermal tissue thinned out to occupy just 20 percent.

The outward appearance changed, too. The explants shifted in color from red or orange to a lighter white or pink. Red-pigmented cells clumped into small aggregates and migrated toward the center of the tissue, leaving the outer edges of the explants increasingly transparent.

In a year, LiPfe explants rebuilt themselves into alien-like translucent orbs with a large red cellular mass at their core.

They didn’t develop any orifices, though, or anything even remotely resembling a digestive system. “One of the first questions we had was how they were able to maintain cellular energy,” Jobson said.

Feeding the orbs

To test how LiPfe sustains itself, Jobson’s team exposed its explants to isotopically labeled amino acids and ammonium. By six days post-excision, the tissues saw a significant spike in the absorption of dissolved amino acids. LiPfe was directly sucking nutrients from the surrounding seawater to fuel its tissue repair and survival. And they could survive for a really long time.

The scientists noted that some of the tube foot explants survived for years just sitting free at the bottom of holding tanks, covered in a layer of particulate matter and surrounded by other living organisms. Some were completely buried under 10 millimeters of mud and still displayed the same morphology: round shape, transparent margin, and a red core. The only thing that could apparently harm them was proximity to decaying tissues of other species. “This made them struggle to survive,” Jobson said. “I think there were toxins or harmful materials their immune system could not cope with”.