Fibronectin has been localized by indirect immunofluorescence during the various phases of endochondral bone formation in response to subcutaneously implanted demineralized bone matrix. Its histologic appearance has been correlated with results of biosynthetic experiments. (a) The implanted collagenous bone matrix was coated with fibronectin before and during mesenchymal cell proliferation. (b) During proliferation of mesenchymal precursor cells, the newly synthesized extracellular matrix exhibited a fibrillar network of fibronectin. (c) During cartilage differentiation, the fibronectin in the extracellular matrix was apparently masked by proteoglycans, as judged by hyaluronidase treatment. (d) Differentiating chondrocytes exhibited a uniform distribution of fibronectin. (e) Fibronectin was present in a cottony array around osteoblasts during osteogenesis. (f) The developing hematopoietic colonies revealed fibronectin associated with them. Therefore, it appears that fibronectin is ubiquitous throughout the development of endochondral bone and bone marrow.

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