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Peritubular dentin (PTD), a highly mineralized annular ring surrounding each odontoblastic process within the dentin, is an enigmatic component in vertebrate teeth. To characterize its structure and composition, we have coupled in situ scanning electron microscopic (SEM) and time-of-flight secondary ion mass spectrometric (TOF-SIMS) analysis of the surface composition of intact bovine coronal dentin with the isolation of intact PTD from hypochlorite-treated dentin and its subsequent TOF-SIMS and direct chemical analysis. The isolated PTD is shown to be a mineralized but porous structure complexed with a high-molecular mass calcium-proteolipid-phospholipid-phosphate complex, which cannot be extracted from the dentin prior to demineralization. The TOF-SIMS and direct amino acid analysis data confirm that the PTD protein is rich in glutamic acid but does not contain collagen. Phosphatidylcholine, phosphatidylserine, and phosphatidylinositol are present, along with a mannose-rich glycan and chondroitin-4- and chondroitin-6-sulfate glycosaminoglycans. PTD apatite, well described in the literature, must therefore form in this noncollagenous proteolipid-phospholipid complex without the intervention of collagen; nevertheless, as shown by SEM, the apatite is formed in small platy crystals, as in the bulk of the intertubular dentin (ITD). We hypothesize that the porous nature of the PTD and its proteolipid-phospholipid complexes may be involved in regulating communication between the ITD and internal PTD tubule fluids and the odontoblasts, similar to the involvement of such lipid complexes in neural, brain, and nuclear transport functions. Thus, the PTD should not be considered solely as a passive structural element in some teeth but as part of the system that allows for the vital function of the dentin.

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Peritubular dentin (PTD) is a hypermineralized phase within the dentinal tubules in some vertebrate teeth as an interface between the intertubular dentin (ITD) and the cell processes. Our aim has been to understand the composition, structure and role of PTD as a mineralized tissue. We have utilized the technique of time of flight secondary ion mass spectrometry (TOF-SIMS) to map the distribution of positive and negative inorganic ions as well as organic components in the fully mineralized, intact PTD structure in bovine tooth cross-sections, and correlated these with scanning electron microscopy (SEM) in standard and backscatter modes. In recent work, we developed a procedure to freeze fracture the teeth and separate PTD from the less dense ITD by the use of aqueous sodium phosphotungstate step density gradients, after degrading the ITD collagen with NaOCl. Here, PTD-containing fragments were characterized by SEM and TOF-SIMS surface structure analysis. The TOF-SIMS data show that the isolated PTD does not contain collagen, but its surface is rich in glutamic acid-containing protein(s). The TOF-SIMS spectra also indicated that the intact PTD fragments contain phospholipids, and chemical analyses showed phosphatidylserine, phosphatidylinositol and phosphatidylcholine as the principal lipid components. In SEM sections, untreated PTD shows as a smooth collar around the tubule, but after digestion with ethylenediamine to remove all organic components, the porous nature of the mineral phase of small, thin platy apatite crystals becomes evident. Thus, the organic matrix of PTD appears to be a proteolipid-phospholipid complex.

A realistic picture of a cell is that of a highly viscous, condensed gel-like substance, crowded with macromolecules that are mostly anchored to membranes and to intricate networks of cytoskeletal elements. Theoretical and experimental approaches to investigating crowding have not considered the role of diffusion through a crowded medium in affecting the selectivity and specificity of reactions. Such diffusion is especially important when one considers interfaces, where at least one reactant must move through the medium and reach the interface. Here, we address this issue by directly investigating how diffusion through a gel medium affects the competition between a single specific reaction and a large number of weak nonspecific interactions, a process that is typical of reactions occurring at interfaces. We present an approach for achieving orientation-controlled interactions based on the configuration-dependent diffusion rate of the reacting molecule through a gel medium. The effectiveness of the method is demonstrated by the high selectivity obtained both in the adsorption of DNA to a surface and in DNA hybridization to preadsorbed single-strand oligomer on a surface.