Aspects of cell proliferation kinetics of the inner dental epithelium during mouse molar and incisor morphogenesis: a reappraisal of the role of the enamel knot area.

First lower E-14 and E-16 mouse molars and E-13 lower incisors were cultured in vitro and either sequentially or continuously labelled with BrdU (5-bromo-2'-deoxyuridine). The behaviour of the non-cycling inner dental epithelial cells emerging from the enamel knot area of the molars was analysed by 3D (three dimensional) reconstructions of serial sections. These cells, as well as slow cycling cells underwent a coordinated temporo-spatial patterning leading to their patchy segregation at the tips of the forming cusps. In incisors (in vitro and in vivo), non-cycling cells were also present in the inner dental epithelium of the enamel knot area. However, these cells were not redistributed during incisor morphogenesis. These non-dividing inner dental epithelium cells of the enamel knot area which are either redistributed or not according to the tooth type specific morphogenesis might represent the organizers of morphogenetic units (OMU), the cusps.