Fig. 1. Fibroblastic PDL cells express ALPase activity and genes for type I collagen, periostin and Runx2/Osf2, but not genes for osteocalcin and BSP. (A) ALPase staining of mouse mandibula. (B) Higher magnification of the inset in A. (C-G) In situ hybridization studies. The antisense probes used were C, type I collagen; D, periostin, E, Runx2/Cbfa1/Osf2; F, osteocalcin (OCN); and G, BSP. (H) A schematic drawing of the tissue section used. B-G show a tissue section roughly corresponding to the schematic drawing shown in H. Signals in C-G disappeared when the corresponding sense probes were used (data not shown). Note that expression of the periostin gene, originally cloned as an osteoblast-specific marker, in the fibroblastic PDL cells is much higher than in the osteoblasts lining the surface of alveolar bone (D). The expression level of Runx2/Cbfa1/Osf2 gene was comparable in PDL, osteoblasts and odontoblasts, but was zero in the cementoblasts (E). The expression of the OCN gene is highest in the odontoblasts, followed by osteoblasts and then by cementoblasts. Expression of PDL cells is undetectable (F). BSP expression is detected in the cementoblasts and osteoblasts but not in PDL cells (G). Original magnification: A, x8; B-G, x40. See text for details.