Figure 1. Buccal-lingual sections of a two-week-old rat mandible through the first molar tooth germ (M1) stained with Azan (A-C) or treated for detecting GDF-5 (D,G,J), -6 (E,H,K), and -7 (F,I,L) transcripts. (A) Whole view of M1 before eruption. Small rectangles of B and C indicate the areas enlarged in B and C, respectively. (B) Obliquely oriented thin collagen fibers (stained blue) show initial signs of attachment to the root surface as matrix fibers of acellular extrinsic fiber cementum. (C) Collagen fibers are poorly oriented in the apical region. (D-F) Cells expressing the GDF genes (stained brown) are aligned on the surface of the alveolar bone (ab) and scattered in the periodontal ligament (arrowheads). (J-L) In the apical region, few GDF-positive cells are detectable only on the alveolar bone surface. (G-I) Control with sense probes showed no positive reactions. ab, alveolar bone; d, dentin; pul, pulp. Bar = 500 µm (A), 50 µm (B-L).