ABSTRACT
Peritubular dentin (PTD) is a relatively dense mineralized tissue that surrounds the tubules of coronal tooth dentin. It is composed mainly of crystals of carbonated apatite together with a small amount of collagen. Its mode of formation has been investigated by studying the relatively dense particles isolated from a powdered preparation. Electron microscopic examination of the PTD particles, including 3-dimensional image reconstruction and electron diffraction, shows that the organization of the crystals of PTD is very similar to that of the adjacent intertubular dentin (ITD). The latter contains relatively large amounts of collagen and the carbonated apatite crystals are closely associated with the collagen matrix. The proteins present in the PTD particles are soluble after decalcification and stain with Stains All. The principal protein has higher molecular weight and a quite different amino acid composition than the phosphophoryns of the intertubular dentin. The interface between the PTD and the ITD shows structural continuity. These data show how two distinct carbonated apatite-based mineralized tissues can be organized and formed contiguously within the same organ by utilizing different sets of matrix proteins.
Subject(s)
Dentin/ultrastructure , Adult , Chromatography, Ion Exchange , Dentin/chemistry , Electrophoresis, Polyacrylamide Gel , Humans , Image Processing, Computer-Assisted , Indicators and Reagents , Microscopy, Electron , Microscopy, Electron, Scanning , Proteins/isolation & purificationABSTRACT
Phosphophoryns (PPs), a family of Asp and Ser(P)-rich dentin proteins, are considered to be archetypal regulators of several aspects of extracellular matrix (ECM) biomineralization. We have cloned a rat incisor PP gene, Dmp2, from our odontoblast cDNA library and localized it to mouse chromosome 5q21 within 2 centimorgans of Dmp1, another tooth-specific ECM protein. The carboxyl-terminal region of Dmp2 protein (60 residue % Ser, 31 residue % Asp) is divided into two domains, one with unique repetitive blocks of [DSS]n,3=14, the other with [SD]m = 2,3. Conformational analysis shows the phosphorylated form of the [DS*S*]n repeats to have a unique structure with well defined ridges of phosphates and carboxyls available for counter ion binding. The [S*D]m domains have different phosphate and carboxylate interaction edges and thus different calcium ion and apatite surface binding properties. These two domains and the colocalization of Dmp1 and Dmp2 genes at a position equivalent to the dentinogenesis imperfecta type II location on human 4q21 all suggest that the PPs are indeed involved in some aspect of ECM mineralization.