Mouse incisor. Coloured scanning electron micrograph (SEM) of a mouse (Mus musculus) incisor that has been left to grow. This tooth is comprised mainly of dentin, with enamel only forming on the surface facing the lips (far left). The act of eating wears down the tooth, causing the pulp cells to produce more dentin, which ensures that the nerve cells within the pulp are not exposed. Incisors erupt between 10 and 12 days after birth. The rate of eruption is equal to the rate of wear, so that the incisors remain at a constant size in adult mice. If the tooth is not worn down, it will continue to grow, forming a curve.