Human Osteoarthritic Clones Represent a Proliferative Population of Mesenchymal Progenitor Cells

Introduction: Osteoarthritis affects nearly 27 million people in the United States alone. Current treatment options for OA range from pain management to total joint replacement surgery. There are currently no drugs on the market that are able to slow the progression, or reverse the affects of osteoarthritis. This necessitates a better understanding of the biology of osteoarthritis, which will lead to the development of better treatment options for OA. Advanced human OA is characterized by loss of extracellular matrix, fibrillated cartilage, and clusters of chondrocytes called “clones”. There is variability among the types of chondrocytes that make up these clones. Some are characterized by an increase in proteoglycan staining and matrix synthesis while others are characterized by decreased proteoglycan staining and phagocytosis of matrix components. Some researchers describe clones consisting of large, rounded chondrocytes with a euchromatic nucleus and other describe clones consisting of secretory cells and degenerating chondrocytes. There is also some debate as to whether chondrocytes are able to migrate and regroup into these clones, or if they are a result of chondrocyte proliferation. In this study, we aim to further characterize the properties of the chondrocytes that compose osteoarthritic clones. We provide evidence that these chondrocytes represent a population of mesenchymal progenitor cells that have proliferative potential. We also show that the transcription factor Runx1 is present in these chondrocyte clones.