SPATIAL AND TEMPORAL EXPRESSION OF FIBRIL-FORMING MINOR COLLAGEN GENES (TYPE-V AND TYPE-XI) DURING FRACTURE-HEALING

Skeletal development involves the coordinated participation of several types of collagen, including both major and minor fibrillar collagens . Although much is known about the major fibrillar collagens, such as types I and II, less is known about the minor fibrillar collagens, and their role in the repair and regeneration of bone has not been extens ively studied. To clarify the role of minor fibrillar collagens in fra cture repair, we examined the spatial and temporal expression of mRNAs for pro-alpha 2(V) collagen and pro-alpha 1(XI) collagen in healing f ractures in the rat by in situ hybridization and compared their patter ns of expression with those of mRNAs for pro-alpha 1(I) collagen, pro- alpha 1(II) collagen, and osteocalcin. A strong signal for pro-alpha 2 (V) was detected in the periosteal osteoprogenitor cells, whereas oste ocalcin mRNA was strongly expressed only in the deep layers of the har d callus. The distribution of the pro-alpha 2(V) signal was correlated with that of pro-alpha 1(I) but was mutually exclusive of that of pro -alpha 1(II). The expression of pro-alpha 1(XI) mRNA was synchronously regulated with that of pro-alpha 1(II) during chondrogenesis in the s oft callus. In the hard callus, pro-alpha 1(XI) signal was found in os teoblastic cells at the site of intramembranous and endochondral ossif ication. These cells simultaneously expressed pro-alpha 2(V), although they were negative for pro-alpha 1(II). These findings suggest that t he alpha 2(V) collagen chain participates in the formation of the nonc artilaginous fibrillar network in the hard callus and preferentially c ontributes to the initial stage of the intramembranous bone formation. Recent reports have revealed that type-XI collagen, which had been cl assified as a cartilage-type collagen, is not necessarily specific for cartilage. The present results advanced this recognition and demonstr ated a coexpression of alpha 1(XI) mRNA and alpha 2(V) mRNA in the non cartilaginous tissues in the fracture callus; this suggests the presen ce of tissue-specific and stage-specific heterotrimers consisting of a lpha 1(XI) and alpha 2(V) collagen chains and the association of such hybrid trimers with the major fibrillar collagens in the process of fr acture healing.