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Catch and release: sea cucumbers might put a torn Achilles tendon back together again

Natural History, Nov, 2003 by Adam Summers

When football season Rolls around, a biomechanist's thoughts inevitably turn to connective tissue--and then, of course, to sea cucumbers. Most fans focus on cutbacks, open-field tackles and chop blocks, but I can't help but ponder the common casualties of these maneuvers: anterior cruciate ligaments (of the infamous ACL injury), hamstrings, and Achilles tendons. Anyone who has had to endure an injury in one of those body parts understands why they come to mind. Although tendons and ligaments--generally referred to as connective tissue--do stretch, they aren't nearly as elastic as rubber bands. In fact, they have a distressing tendency to tear or break, and when they do, they are devils to repair.

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Sea cucumbers, invertebrate animals of the phylum Echinodermata, might hold out some hope for the afflicted. Although they have no internal skeleton, sea cucumbers and other echinoderms do have a kind of connective tissue, but one whose qualities are quite unlike those of mammalian ligaments and tendons. Biochemists and biomechanists are studying the stuff, known as catch connective tissue, because it might lead to new and dramatically superior repairs for injuries such as a running back's torn ACL.

Tendon is made up mostly of collagen, a protein that spontaneously aggregates into long, thin structures known as fibrils. The fibrils interact with each other and with their surroundings to form a stiff and cohesive tissue. But the process is apparently irreversible and non-renewable, and so if physical strain sunders the fibril bonds, tearing the tendon, it is impossible to reform them, at least in living tissue. The standard treatment is to tie the ruptured ends together and let scar tissue bridge the gap. But the bridge between fibrils is not terribly effective, and the scar tissue forms unwanted adhesions to other surrounding tissue. As a result, the tendon never regains more than about 60 percent of its original strength.

Imagine, then, the implications of an ointment that could cleanly break bonds between collagen fibrils and form new ones. A surgeon could chemically undo the rest of the bonds between two partially disjoined fibrils in the torn ends of a tendon, add fibrils to the gap at the frayed ends, and finally stabilize the repair by reestablishing the bonds between new and old fibrils and the rest of the tissue in the matrix: no gap, no scar, no loss of strength.

The armchair anatomist would be hard-pressed to find similarities between a sea cucumber and any part of a quarterback. Lacking arms, legs, and head, the brown cuke looks more like a football than a football player. Without an internal skeleton, it has to propel itself across the seafloor with bands of minute, hydraulically powered tube feet.

Catch connective tissue, also called mutable connective tissue, is a dense, white, fibrous material that makes up the dermis, or body wall, of the sea cucumber. What grabs the biomechanist's interest is that it can change from stiff to flexible and back again with ease. For example, when you hold the sea cucumber's skin between your thumb and forefinger, at first it feels soft. Within moments, though, it hardens, retaining the indentations of your hand for some time. Catch connective tissue enables the foraging sea cucumber to be soft enough to flow into nooks and crannies in pursuit of food. But it could also enable the creature to "don" leathery armor rapidly when a predator threatens (though that possiblity has not been tested).

A mammal's tendon can do no such trick. But both mammalian and catch connective tissue do share common structures. Both are made up of collagen fibrils, and the fibrils of both are suspended in a gooey material that makes the tissues "viscoelastic." (The goo, called the extracellular matrix, is made of water, proteins, and compounds known as proteoglycans, with filaments known as micro fibrils suspended in it to serve as scaffolding.) A viscoelastic material acts partly like a solid and partly like a fluid. Under a rapidly applied load, the material reacts like a solid, deforming slightly but holding its shape, and pushing back as hard as it is pushed on. Under a force applied for a sustained period of time, the material reacts like a liquid, slowly taking whatever shape its surroundings impose. Silly Putty is a good example. A ball of the stuff bounces like a Super Ball when thrown, but it flows so well under steady, slow pressure that it takes a nice fingerprint.

The great trick of catch connective tissue, however, isn't just that it has both viscous (fluid) and elastic (solid) properties; it's that the tissue's viscoelasticity can change. Some cells in the dermis secrete a plasticizing protein that loosens the grip on the collagen fibrils, enabling them to slide past one another and making the overall tissue soft and pliable. Other cells release a stiffening factor that causes the fibrils to "catch" and make the dermis far stiffer.

Biomechanists Greg K. Szulgit of Hiram College in Ohio and Robert E. Shadwick of the Scripps Institution of Oceanography in La Jolla, California, were able to extract chemical derivatives of both the "stiffeners" and the "plasticizers" from the dermis cells. Those derivatives gave the investigators a tool for altering the material properties of pieces of dermis at will, and thus to address a key question about the mechanism for the mutable viscoelasticity: Does the mutability originate from changes in the solid collagen fibrils, or from changes in the viscous, gooey matrix in which they are suspended (or both)?

Catch and release: sea cucumbers might put a torn Achilles tendon back together again

Natural History, Nov, 2003 by Adam Summers

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