Figure 2. Microscopic appearance of dental-periodontal interface in HPP and control specimens (A-E) and dentin mineral content (F). (A,B) Light microscopy of a control (A) and an HPP tooth (B). Note the absence of acellular cementum (c) in HPP. The arrow marks a periodontal ligament cell that has migrated into the orifice of a dentinal tubule. d = dentin; p = periodontal ligament. (C,D) EM showing interface between dentin (d) and periodontal ligament (p) of a HPP tooth. Acellular cementum is absent. Note the presence of a thin, electron-dense, amorphous layer covering the interface. Occasionally, individual collagen fibrils seem to cross the interface (arrow; shown at higher magnification in D). (E) Cellular cementum (cc) around the apex of an HPP tooth. (F) Relationship between HPP tooth age and mineral content in the 4 dentin compartments (C, R1-3; expressed as µg Ca/µg hydroxyproline).