Malformed Tomes’ processes and defective amelogenesis in Col17−/− mice A–D: At the pre-secretory and early secretory stages, the EMJ separates pre-ameloblasts and pre-odontoblasts. A, C: The overall structures of pre-ameloblasts and pre-odontoblasts were similar in the Col17+/+ (A) and Col17−/− (C) mice in the pre-secretory to the early secretory stages at the light microscopic level. B, D: Ultrastructurally, from the pre-secretory to the early secretory stages, the basement membrane between ameloblasts and odontoblasts was blurred in the Col17−/− mouse (D), compared with more obvious, intact basement membrane structures in a Col17+/+ mouse (B). E–H: At the secretory stage, Tomes’ processes are formed and enamel matrix is produced by ameloblasts. E, G: In the secretory stage, the processes of ameloblasts were malformed and blurred (arrows) in the Col17−/− mouse (G), compared with well-organized lattice-like structures of the Tomes’ processes (arrows) in the Col17+/+ mice (E). The thickness of the enamel matrix seemed similar both in Col17+/+ (E) and Col17−/− (G) mice. At the secretory stage, Tomes’ processes were apparently hypoplastic in the Col17−/− mouse (H), compared with normal Tomes’ processes in the Col17+/+ mouse (F). I–L: In the maturation stage, disruption of the processes of ameloblasts (am) was more advanced in the Col17−/− mouse (K), compared with regular processes in the Col17+/+ mice (I). At the maturation stage, the electron density of the enamel matrix is remarkably lower in the Col17−/− mouse (L) than that in the Col17+/+ mouse (J). In addition, enamel rod structures are blurred in the enamel matrix of the Col17−/− mouse (L). am: ameloblast; em: enamel matrix; en: enamel; de: dentin; od: odontoblast; tp: Tomes’ processes. Scale bars: (A, C, E, G, I, K) = 30 μm; (B, D, F, H, J, L) = 3 μm.