Zebrafish and Flatworms: Lessons in Regrowth after Loss

This was originally published in April of 2017. 

“It’s a boy!”

“What??!!” Surprise and bewilderment on the new mother’s face. You see, we were convinced until that very moment that we were having a girl. We had, after all, been told so by people who should know. (You can read about our misadventures in naming our unexpectedly-male child in A Boy Without a Name.)

Our second boy is born, and the nurses hand the newly-swaddled baby to me, his father. I gaze at him lovingly, carefully, confusedly, and announce to my wife, “He’s got an extra thumb.”

“What??!!” Again the surprise and bewilderment on my wife’s face. The poor woman. Mere moments after pushing something the size of a largish melon out of a passage the circumference of a smallish tangerine, I’m springing sex changes and extra digits on her.

It was small, certainly, not a full-grown appendage, not much more than a nubbin, really, but there it was – an extra thumb, sticking out of the side of his regular thumb. The nurses noticed, and asked us it we wanted it removed. My wife and I conferred a moment, and while we couldn’t deny the advantages having three thumbs would convey on future hitch-hiking excursions, it also looked like something that would get caught on things – the sleeve of a onesie, the edge of a table – and twist, break, cause problems. So we said yes, please.

Now here’s the weird part – it grew back. Not to its original dimensions, but he still to this day has eleven fingernails.

Here’s a perhaps even weirder point. My wife, as an infant, lost the tip of her index finger to a slamming door. It, too, grew back. Perhaps not as shapely as before – it remains somewhat stubby and spatulate, rather ET-esque – but certainly there.

It’s long been known that humans (and other mammals) can spontaneously regenerate the tips of lost fingers and toes. Particularly, but not exclusively, if they are very young. Take, for, example, the case of an eight-year-old Washington girl who lost the tip of her middle finger when she stuck it in the spokes of her brother’s bicycle. Her parents rushed her to the ER, where it was sown back on.

“The girl came back in a few weeks with the old fingertip in a bag and a new one on her hand,” reports Dr. Christopher Allen, the girl’s orthopedic surgeon. “It was far better than anything that I could have given her with a graft or surgery.”

How does this happen? Researchers at Stanford University “have shown that damage to a digit tip is repaired by specialized adult stem cells that spend their lives quietly nestled in each tissue type. Like master craftsmen, these cells spring into action at the first sign of damage, working independently yet side-by-side to regenerate bone, skin, tendon, vessels and nerves.”

Wow. To me, that’s pretty incredible. But mice are even better at such regeneration, and even adult mice can regrow lost digits with ease.

Then, of course, there are lizards. Lizards are famous for being able to regrow lost tails, which are broken off (or in some species voluntarily ejected in a process called autotomy) in a bid to get whatever predator is attacking them to focus on the now-squirming tail while the rest of the lizard scuttles to safety.

Whenever humans, however, have tried to regenerate their own limbs using lizard DNA it has ended in disaster. Or at least it did for Dr. Curt Connors, whose forays into experimentation with lizard DNA to regrow his own arm ended with horrible and recurring transformations into The Lizard and frequent violent confrontations with Spider Man.

For salamanders and newts, digit regrowth is child’s play. Hack off a salamander’s whole leg and the thing will grow back, bone, muscle, nerves, and all. Scientists around the world are assiduously studying this phenomenon in order to see if this nifty trick can be replicated in humans. (See The Lizard for caveats.)

Zebrafish are also at the forefront of genetic research. Not only will a zebrafish regrow a fin sliced off by a researcher’s scalpel, but it will repair and replace (not to be confused with repeal and replace) badly damaged heart tissue, something that no mammal’s heart can accomplish. Mammalian hearts are not so resilient.

Starfish. Earthworms. Sea cucumbers. Flatworms. All have the capability of regeneration, some to the degree that lost bits and pieces will actually form new individuals. Hack a sea cucumber into, say, five parts, and you’ll end up with five sea cucumbers. If you happen to need that many.

And then there are the sponges. Sponges, you might be surprised to hear, are actually animals. Sure, they don’t do much, and they don’t have those flashy nervous, digestive, or circulatory systems favored by lots of other animals, but they do have a pretty amazing trick up their cells. Whizz a sea sponge in a blender, place it in a suitable environment, and it will reform itself, like some 1950’s B movie blob monster. While not strictly regeneration but rather reaggregation, it’s still a pretty unbelievable bit of self preservation.

And just about…now, you’re saying to yourself, “Okay, Matt, but what the hell does this have to do with anything? It might be interesting(ish), but what’s the point?”

Well, I’ve been rather preoccupied recently with the ideas of regrowth, regeneration, renewal. You see, my wife has been battling cancer for the past two and a half years. (If you’re behind the curve and want to get up to speed, you could check out On Colons, Cancer, and Courage and I Wish Cancer Would Get Cancer and Die.)

During her penultimate surgery she had a portion of her liver removed, as her colon cancer had metastasized into it. I don’t know if you know this, but the liver is, apparently, the only major internal organ in humans that will regrow. Remarkably quickly at that. A few weeks after having a chunk roughly the size of a child’s clenched fist sliced off, her liver would have returned to its former dimensions. Crazy, I know. (Her thyroid gland, which also came out in a different, unrelated bout of cancer, has no such powers of regeneration, and she’ll have to take hormone replacement medication from here on in.)

Well, the cancer is back, persistent, aggressive, ugly. (I’ll be writing more about that later.) Suffice it to say that while we’re still hopeful and upbeat, her chances of being with us five years from now are statistically slim. So. Renewal, regeneration, regrowth. They’ve been much on my mind of late. I don’t know how all of this will turn out, but if worse comes to worst her family and friends, all the people who love her dearly, are going to need large doses of all three.

Perhaps we’ll be sponges, broken in the blender of loss only to reaggregate into some semblance of our former selves. In any case, they really need to delve deeply into the zebrafish’s ability to repair broken heart tissue. The mammalian heart is not so resilient.

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