Vascularly oriented differentiation gradient of osteoblast precursor cells in rat periodontal ligament: Implications for osteoblast histogenesis and periodontal bone loss

Paravascular distribution of fibroblastlike cells was determined, under conditions of arrested osteogenesis (resorbing or resting bone surface), in periodontal ligament (PDL) of maxillary first molars from 89‐day‐old male rats. As identified by nuclear volume, less differentiated precursor cells (A) and committed osteoprogenitor cells (A') were predominantly localized within 20 μm of the nearest major blood vessel (NMBV). G1 stage preosteoblasts (C cells) and G2 stage preosteoblasts (D cells) tended to be more numerous away from NMBV. Since the osteoblast (Ob) histogenesis sequence is A → A' → C → D → Ob, the present data were consistent with an osteogenic differentiation gradient radiating from blood vessels. It was concluded that preosteoblast formation (A' → C) occurs in a relatively low cell density (4–5 cells/1000 mm2) area ≥20 μm from NMBV. More than 30 μm from the vessel wall, C cells enter DNA synthesis and become D cells, which divide and migrate toward the bone surface, forming 2 osteoblasts each. This is the first integrated description of the cell proliferation differentiation and migration process associated with osteoblast histogenesis.