Fig. 6. Analysis of semi-thin cross-sections and electron microscopy. Transverse sections of the apical regions of mutant incisors grown under kidney capsules (A-C), and electron microscopic observation of a labial region (crown analog side) at an apical end (D). (E,F) Transverse section of upper second molar mesio-buccal root of PN w3 mice. (B,C,F) Higher magnifications of boxes in A and E. (A) This transverse section is very similar to that showing root (E,F) in a mouse molar. The ring is made of dentin, and the inside is dental pulp composed of odontoblasts and dental pulp cells. At the outer surface of dentin, the epithelial cell rests (B,C, arrowheads) and the invading dental follicle cells were apparent. (D) Electron microscopic analysis of the labial region of a mutant incisor. Cementum matrix is deposited at all dentin surfaces. Cementoblast-like cells (asterisk) are observed at all outer dentin surfaces. These cells have well-developed organelles and retain thick bundles of collagen fibers for periodontal ligament (arrows). A fragmented epithelial cell rest is also recognizable (D, arrowhead). cm, cementum matrix; d, dentin; od, odontoblasts; df, dental follicle; p, pulp; pl, periodontal ligament. Scale bars: 100 µm in A; 25 µm in B,C; 5 µm in D; 50 µm in E; 10 µm in F.