Evolution of the Mammalian Tooth

   

The primary origin of teeth was once believed to be derived from the dermal denticles. However, there has been much speculation about this hypothesis in the last 15 years (Witten et al., 2014). Now there are two main but opposing hypotheses regarding the origin of the teeth - the outside-in hypothesis and the inside-out. The outside-in hypothesis claims that, as previously stated, the teeth evolved from the denticles. Contrarily, the inside-out hypothesis states that they derived from an endodermal origin (from the neural crest mesenchyme) (Witten et al., 2014). The latter hypothesis believes that these endodermal teeth evolved independently among the different classes of vertebrates. This post will focus more on the evolution of mammalian teeth from our synapsid ancestors.

Early mammals evolved from synapsid reptiles - mammal-like reptiles evolved around 300 million years ago and the basal mammals about 210 million years ago (Lucas, 2006). The mouth of the early reptiles was extremely similar to extant reptiles and includes large numbers of simple, conical teeth. 

Differences Between Reptilian and Mammalian
Dentition
 https://jurassiccoast.org/fossilfinder/focus-on-fossils/mammals/

 Their teeth had no roots and were kept in position by simple fusion to bone (Lucas, 2006). They lost teeth periodically throughout their life, and these teeth could be continuously replaced.

As they began to evolve from their synapsid ancestors, the early mammals developed fewer teeth, housed in bony sockets along the jaw bone and that only replaced once (had two generations) (Lucas, 2006). There was also the development of a clear differentiation of tooth types including incisors, canines and post-canines.

Modern humans, and many other extant mammals, have these three classes of teeth. The incisors, found at the front of the jaws, have a simple shape. In most mammals they have come to a point at the bottom, but in higher primates (humans included) they are flattened to create a working surface (Lucas, 2006). These help us grip and tear our food.

Types of Teeth
http://biology-igcse.weebly.com/human
-teeth-and-dental-decay.html

The canines are used for ripping and tearing tough foods,
such as meats. Therefore they are not found in animals that are
strictly herbivorous - cows, sheep etc.

The postcanines - referred to as the molars and premolars - are the teeth at the back of the mouth that help us grind our food into smaller components to aid in swallowing and digestion.

Human Teeth - How Our Teeth Differ and How to Care for Them

Baby Teeth https://www.mamanatural.com
/when-do-babies-start-teething/

Humans are born without any teeth at all. It isn't until we are almost a year old before our twenty baby teeth push through our gums and into our oral cavity. However, these teeth won't be in our mouth forever. These teeth, commonly referred to as our baby or milk teeth, will be shed and replaced while we are children. They begin to loosen and fall out when the lower adult teeth push on them from below.

We have 32 adult teeth and unlike most of the reptilians, once our adult teeth have formed, thats all we are going to get. Therefore proper dental hygiene is extremely important in order to keep our teeth cavity free. Proper dental care includes brushing the teeth, at least twice a day, and flossing. We can also try to avoid food and drink that can damage our enamel including citrus fruits, carbonated drinks, sour candies and more. Although our enamel is responsible for protecting the internal structures of the tooth, it is susceptible to degradation by these foods and once it's gone, it's gone forever.

The teeth are held in and surrounded by our gum tissue. Proper cleaning of the gums is also very important. If we do not keep our teeth free from plaque and bacteria our gums may become inflamed, leading to gingivitis. This can be avoided by both brushing and flossing of the teeth.

Pathology

There are many things that can go wrong when it comes to our teeth. This post describes just a few dental pathologies:

Supernumerary Teeth https://link-springer-com
.qe2a-proxy.mun.ca/content/pdf/10.1007%2F
978-3-642-36714-4.pdf

1. Hyperdontia/Supernumerary Teeth: as the name shows, this is a condition where the person grows more teeth than they are supposed to (Slootweg, 2016). These teeth may look like all the others or completely different. This condition often results in irregular eruption of the other teeth, but usually isn't a major issue unless there is a large number of extra teeth.

Concrescence https://link-springer-com.
qe2a-proxy.mun.ca/content/pdf/10.1007%
2F978-3-642-36714-4.pdf

2. Concrescence: fusion of the cementum of two or more teeth (Slootweg, 2016). The teeth appear normal in size but contain two regions of dental pulp and an excess of cementum material.  

3. Dental Caries/Cavities: a pathology that many people have likely experienced. Begins with the destruction of the enamel by microorganisms attached to the teeth. This destruction of the enamel allows enzymes to access and destroy the dentin below (Slootweg, 2016). These can be prevented with regular brushing and flossing.

Oligodontia https://link-springer-com.
qe2a-proxy.mun.ca/content/pdf/10.1007%
2F978-3-642-36714-4.pdf

4. Tooth Agenesis: congenital absence of all or many teeth, can involve the milk teeth and/or the adult. There are multiple forms including: A) anodontia - absence of all teeth, B) hypodontia - absence of 2-10 teeth or C) oligodontia where more than half the teeth are missing (Slootweg, 2016). The missing teeth are most often the molars. Tooth agenesis can be caused by a variety of factors - environmental chemicals/radiation, genetics, and can sometimes be involved with a syndrome such as Down syndrome (Slootweg, 2016).  

References

Abd-Elmeguid, A., & Yu, D. (2009). Dental Pulp Neurophysiology: Part 1. Clinical and Diagnostic Implications. Journal of the Canadian Dental Association. Retrieved October 16, 2018, from http://www.cda-adc.ca/jcda/vol-75/issue-1/55.html

Eroschenko, Victor P., and Di Fiore Mariano S. H. Di Fiores Atlas of Histology with Functional Correlations. Wolters Kluwer Health/Lippincott Williams & Wilkins, 2013.

Ferguson, M. W., Smith, M. M., & Teaford, M. F. (2006). Development, function and evolution of teeth. Cambridge: Cambridge University Press.
Witten, P., Sire, J., & Huysseune, A. (2014). Old, new and new-old concepts about the evolution of teeth. Journal of Applied Ichthyology. Retrieved October 17, 2018, from https://onlinelibrary-wiley-com.qe2a-proxy.mun.ca/doi/epdf/10.1111/jai.12532.


Lucas, P. (2006). Dental functional morphology: How teeth work. Cambridge: Cambridge University Press.

Slootweg, P. J. (2016). Dental Pathology A Practical Introduction. Berlin: Springer Berlin.

Thesleff, Irma, and Mark Tummers. “Tooth Organogenesis and Regeneration.” StemBook, 31 Jan. 2009, doi:10.3824/stembook.1.37.1.