Peritubular Dentin: Protein and Elemental Composition Characterized by TOF–SIMS

Peritubular dentin (PTD) surrounds the dentinal tubules in coronal dentin contiguous to the lesser mineralized collagen-rich intertubular dentin (ITD). Prior studies suggest that PTD has a protein and mineral composition different than ITD and perhaps a different crystalline structure. Objectives: PTD is difficult to isolate from ITD without degradation. Our objective was to develop a method for in situ comparison of their structures and composition. Method: Coronal bovine dentin slices were cut perpendicular to the tubule direction from bovine molars, polished, mounted and scanned with Ga+ at 25 kV by time of flight-secondary ion mass spectrometry (TOF-SIMS). Positive and negative secondary ions were accelerated into the MS, and detected with the time sequence to give the SIMS spectrum, mapping their surface distribution in situ. Ion compositions were determined from their charge and masses. Results: The PTD and ITD were clearly distinguishable. PTD was rich in positive ions from glutamic acid, serine and methionine, and Ca2+, Mg2+, Na+, K+. Collagen-associated proline, hydroxyproline and glycine were absent from the PTD but concentrated in the ITD. Besides being rich in phosphate ions, the negative ions spectrum showed the PTD contained highly concentrated organic materials. This was supported by SEM examination of deproteinated fractured teeth in which apatite crystals retain their structure. Examination of the deproteinated teeth in the TOF-SIMS showed that while the PTD was still rich in Ca2+, the Mg2+ and amino acid fragments were removed. Conclusions: This study provides a new approach to study the intact PTD and ITD. We can speculate that the protein of the PTD is surprisingly abundant, and rich in glutamic acids and serine. Mg2+ ions appear to be associated with the PTD protein, not the apatite crystals. Collagen is not present in the PTD but is tightly interdigitated at the PTD-ITD boundary. DE-01374 (AV)