How does tooth decay advance?

Decaying always starts on tooth surface, and advances to reach dentin layer. Dentin includes much more organic matter than enamel, and thus decaying rapidly spreads in this layer; but decay spreading speed changes significantly from one person to another. Another aspect to keep in mind is that dentin is actually part of tooth defence. At the end of the channels (dentinal tubules) described earlier are building and maintenance cells called odontoblasts; these are capable of blocking decayed dentin with repair tissue. This natural defence mechanism can easily become ineffective against rapidly spreading decay.