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We have previously reported the isolation of a hydrophobic, type-II Collagenbinding glycoprotein of molecular weight 31,000 (31,000-mol-wt protein) from chick chondrocyte membranes (Mollenhauer, J ., and K. von der Mark, EMBO Eur. Mol. Biol. Organ . l., 2 :45-50) . The function of this protein in anchoring pericellular type II collagen to the chondrocyte surface was inferred from its ability to bind native type-II collagen either when detergent solubilized or when inserted into liposomes . In the present study we have used specific antibodies to localize this protein, which we now call anchorin CII, to the surface of chondrocytes in both cartilage sections, and in cell culture . In immunofluorescence studies of isolated chondrocytes we observed a dense, punctate distribution of anchorin CII on the cell surface when chondrocytes were enclosed by a pericellular type II collagen matrix . Removal of the pericellular matrix with trypsin also removed anchorin CII . The membrane protein character of anchorin CII was indicated by the demonstration of antibody-induced patching and capping on the chondrocyte surface at 22°C and 37°C, respectively . In monolayer culture, the amount of anchorin CII appeared reduced on flattened chondrocytes lacking a pericellular type II Collagen matrix but was prominent upon intercellular cell processes . Fab' fragments prepared from either anchorin CII antiserum or an antiserum directed against the entire chondrocyte membrane inhibited the attachment of chondrocytes to a type II collagen substrate . In each case, the inhibition of attachment was neutralized by preincubation of Fab' fragments with purified anchorin CII .In hyaline cartilage, chondrocytes are embedded in an extracellular matrix consisting predominantly of chondroitin sulfate proteoglycan, type II collagen, several minor collagens, and glycoproteins. Type II collagen represents about 80-90% of the total cartilage collagen (1, 2). Minor cartilage collagens include molecules consisting of la, 2a, and 3a chains (3) and a class of collagen fragments characterized by intramolecular cystine bridges, M-collagens (4-7); they represent about 10-15% of the cartilage collagen (for review see reference 8).Collagen provides the three-dimensional framework of the cartilage matrix in which the proteoglycans are imbedded and contributes essentially to the mechanical and physiological properties of cartilaginous tissues . However, type II collagen has also been shown to be influential in chondrogenic differentiation (9-12), and in the regulation of the chondrocyte phenotype (for review see 13,14) .Immunohistochemical and clectronmicroscopical studies on isolated chondrocytes (15)(16)(17)(18)(19) have demonstrated a close association of type II collagen with the chondrocyte surface . The precise nature ofthis cell:matrix association, however, is still unclear . Chondronectin, an adhesion factor that enhances 1572 the binding of chondrocytes to a collagen substrate, has been isolated from serum (20,21). Whether this component is intim...

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Diffusion chambers containing bone marrow cells from adult rats were implanted intraperitoneally into rat hosts and cultured in vivo for up to 64 days. Biochemical and histological analyses of the contents of the chambers demonstrate that a connective tissue consisting of bone, cartilage and fibrous tissues is formed by precursor cells present in marrow stroma. The amounts of osteogenic tissue and DNA are directly correlated with time of implantation and with number of cells inoculated. In the chambers there is initial formation of fibrous tissue which is strongly reactive to collagen type III, laminin and fibronectin. In areas of osteogenesis which appear later within this fibrous anlage, expression of collagen type III, laminin and fibronectin decrease and collagen types I and II increase in association with bone and cartilage respectively. Where osteogenesis does not develop, fibrous tissue continues to express collagen type III. The sequential expression of the different extracellular matrix components is similar to that previously observed during osteogenic differentiation in embryonic and adult developmental systems. It is concluded that the formation of fibrous and osteogenic tissues in diffusion chambers by precursor cells present in adult marrow, resembles the normal developmental process.

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