Fibromodulin is expressed by both chondrocytes and osteoblasts during fetal bone development

Fibromodulin, a keratan-sulfate proteoglycan, was first isolated in articul ar cartilage and tendons. We have identified fibromodulin as a gene regulat ed during BMP-2-induced differentiation of a mouse prechondroblastic cell l ine. Because expression of fibromodulin during endochondral bone formation has not been studied, we Examined whether selected cells of the chondrocyti c and osteoblastic lineage expressed fibromodulin. Fibromodulin mRNA was de tected in conditionally immortalized murine bone marrow stromal cells, oste oblasts, and growth plate chondrocytes, as well as in primary murine calvar ial osteoblasts. We, therefore, investigated the temporo-spatial expression of fibromodulin in vivo during endochondral bone formation by in situ hybr idization. Fibromodulin was first detected at 15.5 days post coitus (dpc) i n the perichondrium and proliferating chondrocytes. Fibromodulin mRNA was a lso detected at 15.5 dpc in the bone collar and periosteum. At later time p oints fibromodulin was expressed in the primary spongiosa and the endosteum . To determine whether fibromodulin was expressed during intramembranous bo ne formation as well, in situ hybridization was performed on calvariae. Fib romodulin mRNA was present in calvarial osteoblasts from 15.5 dpc. These re sults demonstrate that fibromodulin is developmentally expressed in cartila ge and bone cells during endochondral and intramembranous ossification. The se findings suggest that this extracellular matrix protein plays a role in both endochondral and intramembranous bone formation.