EXPRESSION OF COLLAGEN-I, COLLAGEN-II, COLLAGEN-X, AND COLLAGEN-XI AND AGGRECAN MESSENGER-RNAS BY BOVINE GROWTH-PLATE CHONDROCYTES IN-SITU

The cells responsible for skeletal growth are the chondrocytes of the cartilaginous growth plate. These cells differentiate through a series of maturational stages, establishing different zones in the growth pl ate. Among the major functions of these cells is the production of app ropriate extracellular matrix, primarily composed of collagens and pro teoglycans. To determine whether matrix synthesis varies with respect to maturational stage and in which cell populations different collagen s are expressed, bovine growth plates were analyzed by in situ hybridi zation to mRNA and by Northern blot hybridization. The most abundant c ollagen mRNA in the growth plate was type-II collagen. This mRNA was p resent at relatively low levels in the most immature cells of the grow th plate but increased several-fold as cells entered the proliferative stage and remained high through subsequent phases of maturation. Type -XI collagen mRNA and mRNA for the cartilage-characteristic proteoglyc an, aggrecan, were codistributed with the type-II collagen mRNA; howev er, both were present in much smaller quantities. Type-X procollagen m RNA was localized to chondrocytes late in their maturation and was exp ressed at levels similar to the expression of type-II collagen. In sit u hybridization of serial sections revealed that growth plate chondroc ytes in their more mature stages contain both type-II and type-X colla gen mRNA. Type-I collagen mRNA was not observed in growth plate chondr ocytes at any maturational stage; rather, it was localized to a morpho logically distinct population of cells attached to calcifying cartilag e septa in the region of vascular invasion. These data indicate that t he genes for major matrix constituents synthesized by the growth plate in some cases are expressed differentially at different stages of cel lular maturation and in other cases are expressed coordinately. The pa ttern of mRNA expression suggests possible mechanisms of gene regulati on.