If I break an arm or a leg, my body will eventually heal it together. If I cut my finger, my body heals it.
So why don’t teeth heal? If I chip a tooth, my body doesn’t do anything. If I get a tooth knocked out, it’s gone forever. It doesn’t grow back. Why not? Why do our bodies heal most of our other simple injuries, but not teeth?
I’d especially like to hear from those who believe in evolution. Why haven’t we evolved to grow new teeth?
What, you’ve never heard of baby teeth? When those fall out, you get new ones.
I think that the reason that teeth don’t heal, is because most of the tooth is made of non-living cells. All the part that you can see is “already dead”.
But really, I can’t think of many body parts that, when removed, will regenerate themselves.
Teeth are generally acellular - that is, they are not composed of living (or dead, for that matter) cells, unlike, say, bone. The enamel and dentine for a tooth are laid down during development. As such, they don’t (indeed, can’t) heal, but in most animals (non-mammalian), they can be replaced. Most mammals, however, have only two generations of teeth
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As for why we don’t continually grow teeth, like sharks, for example, it probably has to do with our tooth structure. Mammals have what are called “thecodont” teeth - that is, teeth which lie in sockets. Socketed teeth are stronger and can withstand shearing forces (relative to the jaw), which is a good thing, since this allows us to chew (side-to-side jaw motion). The problem is, in order to affix the tooth so it doesn’t pop out of its socket, a type of cement has evolved to hold the tooth in place. Like much of the rest of the tooth, this cement is acellular, and there is no physical contact between the tooth proper and the jaw. So, we get the benefits of chewing our food, but the disadvantage of having teeth isolated from any living tissue (with the exception of the pulp and nerves running through the center of each tooth), which when lost, are lost for good (the second time around, anyway). In most cases, barring accident or excessive sugary snacks, the cement is strong enough to fix the tooth in place for the lifetime of the animal.
As one of those odd people who is part way through a third set of teeth (they grew in when I was about twelve) I can tell you one good reason why we don’t constantly replace teeth. Teeth grow from collections of cells called tooth buds located in the jaw. The future location of your teeth is set very early in embryonic development. The first set are no worries since the jaw is unformed and simply conforms to the teeth. The second set aren’t too bad since the jaw is far larger than when the first set grew in and there’s room for shuffling and sliding of the new teeth if they aren’t in quite the right spot.
The third set are a bitch.
The jaw is already filled with full sized teeth and if the bud ‘germinates’ even slighly beneath an existing tooth there’s no room for manouevering. It can’t push on either tooth hard enough to eject it and adult teeth lack the mechanism that causes them to be easily resorbed by the jaw the way baby teeth are. As a result the third set impacts, erupts out the side of the jaw and all sorts of crazy stuff. Definitely not a good thing from a survival perspective. I had four rear molars and an excess canine removed at the age of 13 and that wasn’t pleasant, and then a few years ago I had 6 (that’s right 6) wisdom teeth removed in a space of twenty minutes. Not real nice either. I dread to think what would happen if I ever got a fourth set. Evolutionarily speaking I suspect I’d be a complete dead end if it weren’t for modern dentistry.
There are of course animals whose teeth (some of them anyway) grow continuously throughout their lives and heal damage taht way. The problem with that solution is that the diet has to be fairly monotonous or the animal has to do a lot of gnawing to ensure that the tooth growth and rate of wear cancel out so teeth don’t grow so long as to become unuseable. It’s only really a solution for incisors or canines and not very applicable for apes.
I once heard a genetics researcher make vague claims that in the future, gene therapies would exist that could help us grow new teeth. I don’t think I want to be a guinea pig for that experiment.
Well, just to nitpick, it’s the other way around. Your permanent teeth come in and push out the baby teeth.
My son lost a few baby teeth and his permanent teeth are not coming in to replace them. He still has to wait until they come in on the typical time table.
Most of the time my baby teeth fell out THEN after a while my adult teeth grew in.
Only one time did my ADULT tooth push out my baby tooth.
What is the deal with Rabbits? I understand they have to gnaw to reduce their teeth or they grow too big and the rabbits can’t eat.
I’ve also heard from TV documentories that Elephants will die not from old age but their teeth rot and they can’t eat anymore thus starve.
IANAD, but here’s how I understand it. The root of the tooth is snuggled securely down in the socket. When an adult tooth comes in, it pushes out at the root. All it has to do is unseat the baby tooth from the socket. Then the baby tooth falls out while the adult tooth is still pretty deep inside the socket. It continues growing outward steadily, but there will be some delay between when the baby tooth falls out and the adult tooth finally appears above the gum.
Rabbits (and rodents in general) fall into the category of ‘mammals whose teeth do grow continuously’. At least, their incisors do. Although, what is really happening is that the teeth in these animals have very long crowns (the part of the tooth that in humans is the part above the gum-line), and the roots are deep within the jaw. As time goes on, the roots are pushed higher in the jaw, thus exposing more of the crown; the space left behind in the jaw is filled in with bone. By the time the crown is even with the gum-line, the animal is probably very old. Such teeth are known as hypsodont teeth. And, as you say, if the tooth does not get worn down sufficiently over time, the tooth winds up being longer than might be good for the animal.
Elephants have a slightly different method of doing the same thing. Their teeth lie in a groove, rather than a socket. Each groove is sloped upward going from the back of the jaw toward the front (thus, the groove is deeper at the back than at the front). Six teeth form at the back of this groove, and are gradually pushed forward; the space behind is again filled in with bone. As the tooth row is pushed forward, the front two teeth are being worn down in feeding. As the row reaches the front end of the jaw, the front-most tooth is ejected (by this time, the crown has usually been worn away completely). By the time all teeth have been worn down, again, the animal is probably of advanced age.
Oh my baby wisdom tooth came out but not another one grew back. nuts.