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Mouth ulcers are red/white rings with a crater. They are sore and last for 7-10 days.

What I want to know is what they are 'made of' - i.e., what is the ring filled with, and why is the centre crater so sore? And why do these relatively benign and short-lasting ulcers have to hurt so much when they simply heal in a couple of days

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  • $\begingroup$ I'm especially interested in the latter half of this question, specifically in the case of minor oral injuries. What is the mechanism by which a small wound (e.g. from accidentally biting the inside of your lip) can develop into a wide open sore that's many times the size of the original injury? $\endgroup$ – SuperElectric Jun 7 '18 at 11:57
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The redden area around the grayish-white indentation of the canker sore isn't filled with anything. This circular area is simply inflamed. The grayish-white area in the center is not filled with puss or anything else as well. This is an area of dead cells but underneath this layer of dead cells the tissue red [1],[2],[3].

Canker sores, Aphthous stomatitis, is an ulcer. An ulcer is a break in the membrane. That is, you have an open sore in your mouth. As with any open wound, pain and discomfort will occur but the mouth is how we consume our food. Since we intake our daily bread in this manner, what we consume can cause the pain to increase; for instance,

  • acidic foods
  • spicy foods
  • abrasive foods

The cause of canker sores is not entirely known, but according to the Mayo clinic [2], possible triggers are

  • A minor injury to your mouth from dental work, overzealous brushing, sports mishaps or an accidental cheek bite
  • Toothpastes and mouth rinses containing sodium lauryl sulfate
  • Food sensitivities, particularly to chocolate, coffee, strawberries, eggs, nuts, cheese, and spicy or acidic foods
  • A diet lacking in vitamin B-12, zinc, folate (folic acid) or iron
  • An allergic response to certain bacteria in your mouth
  • Helicobacter pylori, the same bacteria that cause peptic ulcers
  • Hormonal shifts during menstruation
  • Emotional stress

Canker sores may also be caused by the following diseases as well [2]

  • Celiac disease, a serious intestinal disorder caused by a sensitivity to gluten, a protein found in most grains
  • Inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis
  • Behcet's disease, a rare disorder that causes inflammation throughout the body, including the mouth
  • A faulty immune system that attacks healthy cells in your mouth instead of pathogens, such as viruses and bacteria
  • HIV/AIDS, which suppresses the immune system
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