The permeability of the dentine, which resists this inward diffusion of acid, changes with age. Newly erupted teeth are more permeable and less mineralised allowing the rapid diffusion of acids. As such they may be more susceptible to rapidly progressing caries. Pulp dentine complex reactions to this stimulus are aimed at reducing the permeability of the dentine. The most common reaction depends upon a vital odontoblast process and is the deposition of apatite and whitlockite crystals within the dentinal tubules leading to dentine tubule sclerosis. In addition to this, tertiary dentine may also be laid down by the odontoblast within the pulp chamber.10