Epithelial root-sheath changes during molar formation in the mouse.

The ultrastructure of the epithelial root-sheath was examined in the first mandibular molar teeth of 11, 16 and 21-day-old mice. The changing morphology of the sheath was related to root maturity. An initial, predominantly bicellular layer progressively shortened in an apical direction, the outer layer more so than the inner. The changing morphology appeared to involve maintenance of the inner layer largely at the expense of the outer. Some inner-layer cells persisted at spaced intervals adjacent to forming acellular cementum. Some epithelial rests, distant from the root surface, appeared to be formed early in root development at the time of initial root-sheath severance from the cervical loop of the enamel organ. Epithelial rests located along the acellular cementum appeared to arise from inner, rather than outer, epithelial root-sheath cells and at later stages in root development.