Talkin’ bout Regeneration: Why can’t humans regrow limbs?

The life of a seastar (read: starfish) is a pretty enviable thing. Aside from the artistically inspired body plan, seastars get to spend their days relaxing in the ocean, enjoying the ebb and flow of the tides. They watch all manner of interesting marine life pass by overhead without ever having to worry about jobs, bills, or crooked mechanics. However, if I had to pick one thing from a seastar’s life to really get jealous about it would have to be the ability to regrow limbs.

If, during their care-free oceanic existence, something comes along that wants to make a meal out of them seastars don’t have a lot of options. They are hardly built for speed and camouflage isn’t exactly in their wheelhouse. However, if something does get a hold of them, they have the ability to detach one of their limbs and hopefully escape with the other four. Over a period of a few months or up to a year or so they can regenerate the lost limb. Not only that, the limb they left behind (if it somehow managed to escape becoming octopus food) can regrow the rest of it’s body and form a second, genetically identical seastar.

Seastars are hardly alone in this ability. Earthworms, salamanders, snail firs, some fish, and even the occasional mouse have been known to regrow complete or partial limbs. Seastars often go one step further, though by voluntarily detaching their own limbs to reproduce asexually when lady seastars are not being especially receptive to their advances.

So what’s the deal? Why do all these other animals have this clearly convenient ability while us humans are left twiddling our phantom thumbs? Well, first off, we aren’t totally outside the club of regenerative species. Many children have lost the tips of fingers or toes only to turn up at the doctor’s office a few months later with everything back in place (minus fingerprints). The interesting thing in each case where this happens is that when the fingertip was cut off some of the nail bed was left intact; and it is this fact that gives some insight into how regeneration works.

It seems that to regrow body parts, the bits that are left behind in the wake of a serious injury need to know how to communicate. If you’ve ever tried to explain to someone how to build something specific out of Lego without actually doing the work yourself, you can begin to understand how challenging communication can get in the world of construction. At the cellular level, communication falls on the shoulders of stem cells left at the site of an injury. Scientists have discovered two kinds of stem cells that do this work: somatic stem cells and pluripotent stem cells. Unfortunately, in the world of limb regeneration, not all stem cells are created equal.

Somatic stem cells can close a wound, regrow some skin, and fill in the gaps with scar tissue but not a whole lot else. Pluripotent stem cells are the real professionals. They send signals known as Wnt communications throughout the body, getting bones and even nerves involved in the healing process. These are the cells that allow structures to reform. The bad news for humans and most other mammals is that, outside of the time we spend in our mothers’ wombs, we don’t get too many pluripotent stem cells. In fact, most of the ones we are left with by the time we are born are in our hair follicles and our fingernails... It’s all starting to come together now.

Scientists aren’t exactly sure why mammals seem so deficient in what are obviously handy (no pun intended) bodily resources. Some believe that our fast metabolisms that require constant food preclude us from hiding in a dark cave for a year while our limbs regrow. Instead we have to stop the bleeding and get back to hunting as soon as possible. Others believe that the complex body plans of mammals are too complicated to be regenerated in the same way that seastars' body parts can be.

A few optimistic souls say this might just be a temporary state of affairs. Sometime in the next few decades we may unlock the chemical communication secrets of pluripotent stem cells and set to work regrowing our own body parts. However, until that day comes, I wouldn’t get too close to any wood-chippers.