Blood circulation as source for osteopontin in acellular extrinsic fiber cementum and other mineralizing tissues

Osteopontin (OPN) is one of the major non-collagenous proteins in root cementum and other mineralized tissues. Although most of this mineral-seeking protein is thought to be produced by local tissue cells, some of it might enter the mineralizing matrix from the blood. To test this hypothesis, we followed the distribution of a single dose of purified porcine or rat 125I-labeled OPN injected i.v. in rats, in mineralizing and non-mineralizing tissues and in subcutaneously implanted collagenous implants. The animals were killed 30 or 48 hrs after injection. Tissues (calvaria, tibia, lower and upper jaws) were harvested and processed for radioautography and biochemical analysis. Tissues as well as calcifying collagenous implants proved to have taken up radiolabel. In EDTA extracts of long bones, the majority of the radiolabel was demonstrated to be associated with intact OPN. The iodinated protein was also found in the acellular extrinsic fiber cementum (acellular cementum) layer investing the continuously growing incisors, in laminae limitantes, cement lines, and in forming bone near the mineralization front. Further, the label was present in the circumpulpal dentin of the incisors, and some of it appeared to have been incorporated into developing enamel. It is concluded that OPN in acellular cementum and other mineralizing tissues may-at least partially-originate from sources outside the direct environment following its transportation via serum.