Articular cartilage has a very poor regeneration potential following injury, and former dogma held that it contains no progenitor cells. However, in recent years, articular cartilage progenitor (ACP) cells, which have the ability to produce stable articular cartilage in vitro, have been isolated and cloned from the tissue. However, we have found a subset of ACPs that undergo hypertrophic chondrogenesis, indicative of an endochondral chondrocyte phenotype. This study aimed to understand where this subset of ACPs reside in the tissue.