Does the Superficial Zone of Articular Cartilage Contain Progenitor Cells?

Introduction. The regeneration capacity of cartilage in general is very limited. Cartilage does not contain blood vessels and is thought to consist of only one cell type (chondrocytes) lacking progenitor cells. The chondrocytes, laying isolated in a dense extracellular matrix, have very limited proliferation potential. Despite the absence of repair or regeneration of damaged articular cartilage, cartilage structures in the head and neck region do show some healing capacity. These cartilage structures are covered with a layer of perichondrium. The neocartilage formed originates from proliferation of progenitor cells in the inner layer (cambium layer) of perichondrium. Articular cartilage is not lined by perichondrium and thus lacks this internal progenitor cell source. We here present two sets of different, unrelated experiments that lead us to formulate the hypothesis that the superficial layer of articular cartilage might possess perichondriumlike properties. Materials/Methods. In the first set of experiments explants of rabbit ear cartilage were cultured for 3 weeks with the entire perichondrium intact, the entire perichondrium removed or only the outer layer of the perichondrium removed. In the second set of experiments full-thickness articular cartilage explants of calves (6 months old) were cultured for 4 weeks under standard culture conditions. In an additional group, explants were treated with 30 U/ml collagenase the first 48 h of the culture period. Culture medium consisted of DMEM/Ham’s F12 medium with 10% FCS and 25 μg/ml l-ascorbic acid. The explants were stained with HematoxylinEosin, Alcian Blue and Thionin. Immunohistochemical staining for collagen type II was performed using antibody II-II6B3. Results. Explants of cartilage with intact perichondrium as well as explants with the entire perichondrium removed, never showed formation of new cartilage. On the other hand, explants where the outer (fibrous) perichondrium-layer was selectively removed thereby exposing the inner layer of the perichondrium that contains progenitor cells, demonstrated outgrowth of new cartilage. The new cartilage tissue clearly originated from the inner perichondrial layer (Fig 1). The experiments using untreated articular cartilage explants revealed comparable images. In our culture of full thickness articular cartilage, progenitor cells from the bone marrow were not present. Another cell source suggested by Hunziker to be able to repair articular cartilage defects is the synovial lining. This type of cells was also not present in our culture set-up. We therefore were surprised that the cultured cartilage explants expressed formation of new cartilage tissue attached to the explant in half of the cultures (Fig 1).