Like a starfish, an octopus can regrow lost arms. Unlike a starfish, a severed octopus arm does not regrow another octopus. But the biological secrets inside their arm regeneration feat do hold the promise of learning more about how we might better regenerate our own diseased or lost tissue. If not whole limbs, at least perhaps fresh nerves or organ segments.
Rare is the octopus with fewer than eight—at least partial—arms. Because as soon as an arm is lost or damaged, a regrowth process kicks off to make the limb whole again—from the inner nerve bundles to the outer, flexible suckers. Even lizards that lose their tails often regrow ones that are of poorer quality that the original ones. Not so with octopuses; once an arm is regrown, it is basically as good as new. And now we are finally just getting a look at the mechanisms behind this impressive process.
A new study examines the seemingly crucial role of a protein acetylcholinesterase (or AChE). We have AChE in our own bodies. It is known primarily for its presence in brain synapses and other nervous system intersections. But it also plays a role in cell proliferation and differentiation—as well as in cell death. And it seems to be unusually active in octopuses that are in the process of regrowing parts of a limb. And when it is active in the process seems to be the key. The findings are in the September special "Cephalopod Research" issue of the Journal of Experimental Marine Biology and Ecology.
Researchers studied six healthy (eight-armed) female common octopuses (Octopus vulgaris), captured off of the Italian coast. The octos were put under anesthesia and a small tip (just one to two centimeters) from each arm was removed. Once the animals awoke, they all seemed to carry on as before.
Only the biochemistry in their bodies had already started changing. Within three days, some cascade of chemical signals cued the formation of a "knob," covered with undifferentiated cells, where the cut had been made. And further molecular signals were responsible for the "hook-like structure" that was visible at the end of the arm in the second week. Around that time, a mass of stem cells and a hefty amount of blood vessels have arrived at the site. Yet by day 28, these features disappeared. And for the next hundred days or so, the arm tip grew back in to resemble the original one. What could be orchestrating all of these specific steps?
In tissue sampled that wasn't in the process of regenerating, AChE was mostly active only in the nerve cord. And in the damaged arms, the AChE activity stayed low—until about the third week after the surgery. Then, a time period during which new suckers and chromatophores (the color-changing structures in an octopus's skin) first appeared—along with muscles and nervous system components—the compound seemed to flood into action. By day 42, the AChE activity began to taper off, and by day 130, when the new arm tips had fully regenerated, it was just about back to normal levels.
These findings don't solve the mystery of such detailed tissue regeneration. But they could help make the octopus a new scientific model for researchers looking to study regeneration. They also point to more molecular medical work. "AChE protein may have an important influence in the process of arm regeneration," the researchers noted in their paper. And "It could be considered as a potential target to promote or regulate the regenerative process." Such a toehold could help us make new leaps into regenerative medicine. "By targeting the AChE activity at single specific regeneration states, it will be possible to study the regenerative process in its proceeding and regulate phases of the reparative pathway," they noted. Anyone need a new arm? Suckers optional.
Illustration courtesy of Ivan Phillipsen