IS THE ARTICULAR CHONDROCYTE A TERMINALLY DIFFERENTIATED CELL?

关节软骨细胞通常被认为是终末分化的 这是由于其在体内复制的有限倾向。患者 然而，在骨关节炎中，关节软骨细胞表现出 克隆分裂，合成型，X胶原，并呈现 类似于终末分化的肥大生长板 软骨细胞此外，从几个关节软骨细胞收获， 动物物种和体外培养的分裂响应于促有丝分裂 存在于血清中的信号，并且也被证明可以合成X型 在一定的培养条件下。这些观察提示 关节软骨细胞可以不是终末分化的细胞，但 而是悬浮在分化中间阶段的细胞， 可以诱导进一步分化时，放置在一个允许的 环境，或当暴露于特定的病理刺激时。的事实 体内的关节软骨细胞只表现出 骨关节炎条件下的肥大表型表明， 表型可能参与软骨损伤的发病机制， 尝试修复。尽管有几篇关于X型胶原蛋白的报道 关节软骨细胞合成，但其机制知之甚少 这一现象的发生。一个可能的因素是 是甲状腺激素。虽然有丰富的 甲状腺激素与生长终末分化相关的证据 板软骨细胞，目前没有数据支持或反驳的作用 甲状腺激素在关节X型胶原合成中的作用 软骨细胞我们已经建立了一个化学定义的大鼠生长模型 板软骨细胞分化，其中细胞被培养为三个 在无血清条件下培养三维细胞沉淀。添加甲状腺 激素对这个系统的作用导致细胞迅速分化为 停止分裂、表达X型胶原肥大软骨细胞 和碱性磷酸酶mRNA，并显示高水平的碱性磷酸酶， 磷酸酶活性。我们建议使用这个系统来测试 假设关节软骨细胞是终末分化的 通过追求以下具体目标，(1)成功 大鼠关节软骨细胞三维培养方法建立 在体外化学限定的条件下维持;（2）确定 如果大鼠关节软骨细胞表达甲状腺激素受体亚型， 体内;（3）确定关节软骨细胞是否培养在 在化学限定条件下的体外对甲状腺激素有反应 通过表达与肥大细胞相关的标志物的治疗 软骨细胞表型

The articular chondrocyte is often considered a terminally differentiated cell due to its limited tendency to replicate in vivo. In patients with osteoarthritis, however, articular chondrocytes demonstrate the ability to clonally divide, synthesize type, X collagen, and assume an appearance similar to the terminally differentiated hypertrophic growth plate chondrocyte. Moreover, articular chondrocytes harvested from several species of animals and cultured in vitro divide in response to mitogenic signals present in serum, and have also been shown to synthesize type X collagen under certain culture conditions. These observations suggest that the articular chondrocyte may not be a terminally differentiated cell, but rather a cell suspended in an intermediate stage of differentiation which can be induced to differentiate further when placed into a permissive environment, or when exposed to a specific pathologic stimulus. The fact that articular chondrocytes in vivo have only been shown to express this hypertrophic phenotype under osteoarthritic conditions suggests that this phenotype may be involved in the pathogenesis of cartilage injury and attempted repair. Despite several published reports of type X collagen synthesis by articular chondrocytes, little is known about the mechanism by which this phenomenon occurs. One possible factor which may be involved in signalling this response is thyroid hormone. Although there is abundant evidence linking thyroid hormone to terminal differentiation of growth plate chondrocytes, no data currently exist to support or refute a role for thyroid hormone in type X collagen synthesis by articular chondrocytes. We have developed a chemically defined model of rat growth plate chondrncyte differentiation in which cells are cultured as a three dimensional cell pellet under serum free conditions. Addition of thyroid hormone to this system results in the rapid differentiation of cells into hypertrophic chondrocytes which cease dividing, express type x collagen and alkaline phosphatase mRNA, and demonstrate high levels of alkaline phosphatase enzymatic activity. We propose to use this system to test the hypothesis that the articular chondrocyte is a terminally differentiated cell by pursuing the following specific aims. (1) To successfully establish three dimensional cultures of rat articular chondrocytes maintained under chemically defined conditions in vitro; (2) To determine if rat articular chondrocytes express thyroid hormone receptor isoforms in vivo; and (3) To ascertain whether articular chondrocytes cultured in vitro under chemically defined conditions respond to thyroid hormone treatment by expressing markers associated with the hypertrophic chondrocyte phenotype.