What's a mouth ulcer/canker sore "made of", and why do they develop?
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