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 ceme ntum and other mineralized tissues. Although most of this mineral-seeking p rotein is thought to be produced by local tissue cells, some of it might en ter the mineralizing matrix from the blood. To test this hypothesis, we fol lowed the distribution of a single dose of purified porcine or rat I-125-la beled OPN injected i.v. in rats, in mineralizing and non-mineralizing tissu es and in subcutaneously implanted collagenous implants. The animals were k illed 30 or 48 hrs after injection. Tissues (calvaria, tibia, lower and upp er jaws) were harvested and processed for radioautography and biochemical a nalysis. Tissues as well as calcifying collagenous implants proved to have taken up radiolabel. In EDTA extracts of long bones, the majority of the ra diolabel was demonstrated to be associated with intact OPN. The iodinated p rotein was also found in the acellular extrinsic fiber cementum (acellular cementum) layer investing the continuously growing incisors, in laminae lim itantes, cement lines, and in forming bone near the mineralization front. F urther, the label was present in the circumpulpal dentin of the incisors, a nd 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 environme nt following its transportation via serum.