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The attachment of chondrocytes to collagen substrates is stimulated by serum but not by fibronectin. The active material in serum was partially purified and was shown to be a protein by its sensitivity to trypsin and heat and its chromatographic properties. This factor, which we have named chondronectin, is distinct from fibronectin and does not stimulate fibroblast attachment. Because material with similar attachment-enhancing activity is produced by chondrocytes and is extractable from cartilage, c ondronectin may be a chondrocyte-specific attachment protein.Collagen substrates promote the attachment (1), growth (2), and differentiation (3, 4) of celb in culture and presumably in vivo. It is now known that fibroblasts and some other cell types do not bind directly to collagen (1, 5). Rather, fibronectin, a large glycoprotein produced by fibroblasts (6) and many other cell types (7-12), binds these cells to collagen (13). Fibronectin is also present in serum (12) at a high concentration (:t200-300 Mg/ml) and attaches freshly trypsinized cells to their substrates.Fibronectin-free serum as well as pure fibronectin can be prepared by affinity chromatography on immobilized collagen (14, 15) and used to test the requirements of various cells for attachment to collagen.Fibronectin, however, does not mediate attachment of all cell types to collagen. Epidermal cells, for example, adhere preferentially to type IV collagen by a process that is not stimulated by fibronectin (16). Many hours are required for epidermal cells to attach, suggesting that they synthesize their own attachment factor, although this has not yet been shown.Although fibronectin is produced by limb mesenchymal cells (7, 17), it is not produced by differentiated chondrocytes in cartilage (7,17,18 (20), type III from fetal calf skin (21), and type IV from a murine sarcoma (22).Chromatographic Procedures. Affinity chromatography. Fibronectin was removed from serum by adsorption to a collagen-Sepharose 4B column (14, 15). In these experiments, the adsorbant was first equilibrated with F12 medium and then incubated with fetal calf serum (GIBCO) for 1 hr at room temperature. Unbound material was washed from the column with F12 medium, and bound fibronectin was eluted with 1 M KBr in 0.05 M Tris-HCI, pH 5.3, containing 0.025 M 6-aminohexanoic acid. The isolated fibronectin fraction was dialyzed against fresh F12 before use. DEAE-cellulose column chromatography. Chondrocyte attachment activity was precipitated from serum with ammonium sulfate (30% of saturation). The precipitated material was redissolved in 6 M urea, dialyzed against 0.05 M Tris*HCI, pH 7.4, and chromatographed at 4°C on a DEAE-cellulose column equilibrated with the same buffer. Bound material was eluted from the column with a linear gradient of 0 to 1 M NaCl. Fractions (5 ml) were collected and aliquots of each fraction were dialyzed against F12 and assayed for attachment-enhancing activity for chondrocytes and for CHO cells. RESULTSCharacteristics of Chondrocyte Attachment. The ro...

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When demineralized bone matrix powder is implanted subcutaneously in the rat, the early responses involve the appearance and proliferation of mesenchymal cells at the site of implantation, followed by cartilage and bone formation. The ability of cells to migrate to the implant suggests that chemotaxis may be a critical event in this process. Therefore, using the modified Boyden chamber assay, we tested extracts of demineralized bone matrix for chemotactic activity. We have identified and partially purified, on molecular sieve chromatography, a heat labile and trypsin-sensitive protein (Mr = 60,000-70,000) that is a potent chemoattractant for mouse calvaria, osteoblast-like cells (MMB-1), but not for monocytes (putative osteoclast precursors). These findings suggest that chemotactic protein(s) have a significant role in the recruitment of osteoprogenitor cells to a site of bone repair.

The National Institutes of Health (NIH) has been a leading advocate of the zebrafish as a model organism for the study of vertebrate development, physiology, and disease. Genomic tools, developed with the support of NIH funding, have made zebrafish even more attractive as a genetic system and have stimulated research using this model. The NIH continues to provide support for new and existing community resources, such as the Zebrafish International Resource Center and the Zebrafish Information Network, research tool development and mutant screens, and a wide array of investigator-initiated studies that are using zebrafish to elucidate many aspects of vertebrate biology.