Dessau W, Sasse J, Timpl R, Jilek F, von der Mark K
J Cell Biol. 1978 Nov;79(2 Pt 1):342-55. doi: 10.1083/jcb.79.2.342.
Fibronectin, the major cell surface glycoprotein of fibroblasts, is absent from differentiated cartilage matrix and chondrocytes in situ. However, dissociation of embryonic chick sternal cartilage with collagenase and trypsin, followed by inoculation in vitro reinitiates fibronectin synthesis by chondrocytes. Immunofluorescence microscopy with antibodies prepared against plasma fibronectin (cold insoluble globulin [CIG]) reveals fibronectin associated with the chondrocyte surface. Synthesis and secretion of fibronectin into the medium are shown by anabolic labeling with [35S]methionine or [3H]glycine, and identification of the secreted proteins by immunoprecipitation and sodium dodecyl sulfate (SDS)-disc gel electrophoresis. When chondrocytes are plated onto tissue culture dishes, the pattern of surface-associated fibronectin changes from a patchy into a strandlike appearance. Where epithelioid clones of polygonal chondrocytes develop, only short strands of fibronectin appear preferentially at cellular interfaces. This pattern is observed as long as cells continue to produce type II collagen that fails to precipitate as extracellular collagen fibers for some time in culture. Using the immunofluorescence double-labeling technique, we demonstrate that fibroblasts as well as chondrocytes which synthesize type I collagen and deposit this collagen as extracellular fibers show a different pattern of extracellular fibronectin that codistributes in large parts with collagen fibers. Where chondrocytes begin to accumulate extracellular cartilage matrix, fibronectin strands disappear. From these observations, we conclude (a) that chondrocytes synthesize fibronectin only in the absence of extracellular cartilage matrix, and (b) that fibronectin forms only short intercellular "stitches" in the absence of extracellular collagen fibers in vitro.
纤连蛋白是成纤维细胞主要的细胞表面糖蛋白，在分化的软骨基质和原位软骨细胞中不存在。然而，用胶原酶和胰蛋白酶解离胚胎鸡胸骨软骨，然后进行体外接种，可重新启动软骨细胞合成纤连蛋白。用针对血浆纤连蛋白（冷不溶性球蛋白[CIG]）制备的抗体进行免疫荧光显微镜检查，可发现纤连蛋白与软骨细胞表面相关。用[35S]甲硫氨酸或[3H]甘氨酸进行合成代谢标记，通过免疫沉淀和十二烷基硫酸钠（SDS）圆盘凝胶电泳鉴定分泌蛋白，显示纤连蛋白合成并分泌到培养基中。当软骨细胞接种到组织培养皿上时，表面相关纤连蛋白的模式从斑块状变为丝状。在多边形软骨细胞的上皮样克隆形成的地方，只有短的纤连蛋白链优先出现在细胞界面。只要细胞继续产生II型胶原，且在培养一段时间内未能沉淀为细胞外胶原纤维，就会观察到这种模式。使用免疫荧光双标记技术，我们证明合成I型胶原并将这种胶原沉积为细胞外纤维的成纤维细胞以及软骨细胞显示出不同的细胞外纤连蛋白模式，其在很大程度上与胶原纤维共分布。在软骨细胞开始积累细胞外软骨基质的地方，纤连蛋白链消失。从这些观察结果中，我们得出结论：（a）软骨细胞仅在没有细胞外软骨基质的情况下合成纤连蛋白；（b）在体外没有细胞外胶原纤维的情况下，纤连蛋白仅形成短的细胞间“缝线”。
