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A similar question but about teeth healing themselves is Do teeth have the capacity to heal?.


So I understand that teeth have the capacity to heal themselves to a certain degree. It appears to be that part of the reason they can't heal all of themselves is that they are not surrounded by cells. In that case why can't we grow teeth back?

If a tooth is grown in the gum, or somewhere else, it would be surrounded by cells and so should be able to grow back right?

It appears that sharks can grow their teeth back:

New teeth are continually grown in a groove in the shark’s mouth and the skin acts as a “conveyor belt” to move the teeth forward into new positions.

Why can't we do this?

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    $\begingroup$ Short answer: Teeth are formed prenatally in tissue destined to become bone (the mandibles), that is, in the embryo. Once they have formed, they can grow and develop, but new teeth do not form after mid-gestation. Shorter answer: that's how the genes for tooth formation expression are regulated. It's like asking why we don't grow a new leg if our leg is amputated. $\endgroup$ – anongoodnurse Apr 19 '17 at 14:28
  • $\begingroup$ @anongoodnurse why not put that as an answer? $\endgroup$ – another 'Homo sapien' Apr 19 '17 at 15:22
  • $\begingroup$ @another'Homosapien' - I don't know; "because" is not much of an answer. $\endgroup$ – anongoodnurse Apr 19 '17 at 18:25
  • $\begingroup$ @anongoodnurse it would be enough for an answer. I might edit that up too :) $\endgroup$ – another 'Homo sapien' Apr 20 '17 at 6:19
  • $\begingroup$ Possible duplicate of Why do some bad traits evolve, and good ones don't? $\endgroup$ – canadianer Apr 26 '17 at 17:38
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Unfortunately, the answer to your question isn't as well researched as other areas, I believe...let's add some complexity into this.

You're correct, teeth does "grow" in the gums. And it does start off surrounded by cells (look up: tooth bud, cap, bell stage)!

The tooth originates from an interaction between the ectomesenchyme (likely originating from the neural crest cells) and the epithelium of the mouth when it is forming while you're still an embryo in your momma's tummy.

Unlike sharks, which are homodonts (meaning, their tooth are all of one morphology), we are heterodonts (e.g. we have different shaped teeth - e.g. molars, canines, incisors, premolars). There are a couple of theories to why this occurs (look up: clone model, field model, odontogenic homeobox code - a combination of all these theories). Basically, a bunch of genes transcribe for proteins and signalling factors that create the complexity of your tooth shape and position in the mouth.

Basically, this means, our teeth are going to take longer and require more energy and effort to carve into (yes carve! lot's of the tooth shape is create by apoptosis of the regions around the "cusp" area when it is developing).

When your primary/baby teeth pop out, they serve functions such as developing speech, allowing you to eat, strengthening your jaw muscles (which in turn helps to develop your mandible and shape of the face), all this so you can have a great bite when you get older!

Luckily, sharks don't speak much underwater, so they probably don't give a poop about having properly lined up teeth. Their teeth grows everywhere. Proper bite for eating various nuts, fruits, meats and vegetables? NAH. WE JUST CHEW OFF LEGS. WE CAN ONLY SHRED STUFF. Sharks need strong ankylosed teeth (ankylosed = like an anchor? teeth are anchored into the mandible of the shark).

In contrast, humans have teeth in gomphosis (sockets), which gives our teeth some cushioning and sense of proprioception (the sense knowing how hard you're biting) when we bite on something extremely hard, to prevent us breaking our teeth - thus helping us preserve our teeth. Sharks don't have that, they just break their teeth.

Humans do finally replace their teeth, after our jaw grows enough for us to accommodate them at a genetically planned stage. This happens by tooth germs (cells) being bud off (like a flower yes :D) from the baby teeth. Note though, your 1st adult/permanent molars are already beginning to form when you're still in your momma's tummy.

I believe that the continuous creation of teeth requires more energy, but takes less time (sharks). For humans, one time creation of teeth requires less energy, but takes wayyyyy more time (and also has to be timed correctly with all our other growing bits).

TL;DR Evolutionary reasons and different functional uses of teeth between sharks and humans probably determine why we only have two sets of teeth (diphyodonts). That being said, it would be cool if we were like rats (have molars/incisors) though, which replace their teeth anytime its lost. Maybe one day we can evolve into it? Yay!


Some interesting points: You mentioned teeth "healing".

Your teeth is made up of enamel on the outside, which is NEVER replaced. It "heals" by remineralisation which is not exactly the same as growing new crystals on top of it. Kinda complex, won't go into it.

However, your teeth is always forming new dentine, which is below the enamel. In a way, your tooth really is growing back all the time. Because a new layer of dentine is deposited everyday (like...4 micrometers). This is because the dentine producing cells, called odontoblasts, are still around. In contrast, the enamel producing cells, called ameloblasts, are removed when your tooth has developed. I THINK, the reason is to make space in your enamel so it can be fully mineralised and completely filled up by hydroxyapatite. (Too much fluoride when young can block this removal of ameloblasts and its proteins like amelogenin and enamelin = forming holes, which cavitate resulting in fluorosis).

Hopefully all that answered your question. But, it was fun for me to rant anyway. Cheers.

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  • $\begingroup$ Nice effort on this answer! However, please provide some resources or citations to back-up your claims. We want all of our users to feel confident that the information they read on Bio.SE is well-researched and reflects scientific understanding and not simply opinion. Thanks! $\endgroup$ – theforestecologist Nov 9 '18 at 1:08
  • $\begingroup$ Actually it is quite well researched, Mammals are the only group of animals that grow only a limited number of teeth but socketed teeth and heterodont teeth are not limited to mammals. $\endgroup$ – John Jul 31 at 0:04
  • $\begingroup$ Also rats do not replace lost teeth, they have incisors which never stop growing, those are very different things. $\endgroup$ – John Aug 25 at 4:45
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Because mammals are weird, and spent way too much of their evolutionary history being tiny short lived creatures.

No seriously, Growing endless numbers of teeth (Polyphyodont) is the normal aka basal condition in vertebrates. Every other group of animals with teeth just keep replacing them forever. Mammals uniquely have lost this ability.

All mammals grow a fixed number of teeth, most are diphyodont growing their teeth in two distinct sets, a few mammals (elephants and kangaroo) spread their number of teeth into multiple series but the total number of teeth they produce is still fixed, they are sort of pseudo-polydont. Some whales produce a much larger number of teeth but the total number of teeth is still fixed.

Why are mammals weird, because the ancestors of all extant mammals were small short lived insectivores. If you are short lived insectivores wear is insignifiant on teeth and dislodging them is a minimal risk. Multiple sets of teeth is not a significant advantage. What is a significant advantage is tightly occluding teeth, that is teeth that fit together very tightly with multiple cusps, this shape is perfect for processing insects into digestible pieces, but it does not work well when teeth are replaced. The current thought is that mammals repurposed or lost the genes that allowed for new tooth bud formation. Many believe the genes were repurposed possibly into some of the genes that control tooth shape allowing the complex tooth structure mammals have. This is difficult to test but is supported by the fact the groups of mammals to increase the number of teeth, whales, armadillo, and some seals, also have extremely simply tooth structures and high numbers of anomalous teeth, that is teeth with seemingly no controls on their shape. Although I should note extra erroneous teeth occasionally occur in nearly every mammal line.

The exact changes have not yet been discovered and is an are with much interest. We know why pretty well even if we don't know how. Although there is some fascinating work being done by studying how reptiles continue to replace teeth throughout life. Certain basal cell lines persist in reptiles but completely differentiate in mammals during early development. Whales get around this by making a lot of new buds early on and holding on to them, but they still can't make new ones past a certain point.

Also related, why mammals have bigger nerves in their teeth.

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