Tooth Development 4

Root development

This is a section through the skull of a rat highlighting a mandibular molar where the root is developing (blue arrow) (also present are a maxillary molar and an incisor). The crown shows enamel in various stages of development from the cusp tip where there is no remaining enamel matrix to the cervical margin where the enamel matrix is intact and yet to be degraded. At the cusp tip the enamel organ is greatly reduced.

The root sheath is a bilaminar structure derived from the inner and outer enamel epithelia. It has nothing analagous to the stratum intermedium or stellate reticulum (this may be significant in the lack of enamel production on the root). The root sheath induces the outer cells of the papilla adjacent to it to differentiate into odontoblasts and very rapidly to start to produce root dentine. The root sheath is always the same length regardless of how long the root is because once dentine formation is initiated the root sheath disintegrates at its coronal end (resulting in the rests of Malassez). Before disintegration the cells of the root sheath may deposit (enamel) matrix proteins on the dentine surface. This layer may be important in the subsequent induction of cementoblasts in the follicle adjacent to the root.