A subset of epithelial cells are nonproliferative throughout incisor placode and bud morphogenesis. Confocal fluorescence images of mouse embryonic mandible expressing keratin 17-GFP (green) and fluorescent cell cycle indicator for G1 cell cycle phase (red). (top view) Each figure is an optical section in xy plane, and the dotted line represents the prospective tooth area. (side view) Each figure is a projection in z plane, the epithelial border is delineated with a dotted line, and wide arrowheads represent the position of the xy-section in the z-axis. (A) K17-GFP was initially expressed throughout the oral epithelium in the superficial peridermal layer. At E11.5, the dental lamina appeared as a thickening of the epithelium. There was a diffuse stripe of nonproliferating Fucci G1 cells appearing along the dental lamina (marked by thin arrowhead in z-projection). K17-GFP expression became restricted to the forming placode cells between E12.0 and 12.5, and there was specific subset of cells in G1 in the placode, localized to the labial part of the thickening (arrowhead). At E12.5, invagination of the tooth bud began, and G1 cell focus became restricted at the anterior part of the bud. At E13.5, the early G1 foci remained in the dental cord, and new distinct G1 foci appeared in the tip of the bud (asterisk). Bars, 100 µm. (B) 3D surface rendering of the forming incisor tooth placode/bud shows the initial G1 focus at E12.0 and 12.5 (thin arrowhead) in the anterior part of the bud, while a new G1 focus appeared at the tip of the bud (asterisk). (C) A schematic representation of the G1 cell populations in the developing incisor tooth placode/bud. The early G1 focus in the anterior part of the forming tooth bud is separate from the G1 focus corresponding with the EK.