Gene expression and immunohistochemical localization of decorin and biglycan in association with early bone formation in the developing mandible.

We investigated the expression of the small proteoglycans, decorin and biglycan, which are associated with osteoblast differentiation, and how this relates to the expression of osteocalcin and bone sialoprotein (BSP) early in the formation of bone in the rat mandible by immunohistochemistry and in situ hybridization. The mandibles of rat fetuses were collected on embryonic days 14 (E14) to E18. In situ hybridization showed that gene expression of decorin, biglycan, osteocalcin and BSP was not apparent in the developing mandible at E 14, but was expressed by newly differentiated osteoblasts at E15. The expression of these mRNAs increased linearly as the number of osteoblasts increased in specimens from E16 to E18. Immunohistochemistry showed that newly differentiated osteoblasts expressed biglycan moderately, decorin weakly, and osteocalcin and BSP faintly. The unmineralized bone matrices among the osteoblasts showed prominent staining for decorin, weak staining for osteocalcin and BSP, and very weak staining for biglycan. When the intercellular matrix was mineralized at E16, the mineralized bone matrix showed more prominent staining for osteocalcin and BSP, but lacked staining for decorin and biglycan. The same staining profile was observed during the subsequent phases of bone formation at E17 and E18. These results indicate that decorin, biglycan, osteocalcin and BSP are expressed at the gene and protein level by newly differentiated osteoblasts before the onset of matrix mineralization and that they could play a role in the earliest stages of bone formation. Negative proteoglycan staining in the mineralized bone matrix suggests that a loss of, or a sharp decrease in proteoglycans occurs concomitant with bone matrix mineralization.