Myofibroblasts orchestrate wound healing processes, and if they remain activated, they drive disease progression such as fibrosis and cancer. Besides growth factor signaling, the local extracellular matrix (ECM) and its mechanical properties are central regulators of these processes. It remains unknown whether transforming growth factor-β (TGF-β) and tensile forces work synergistically in up-regulating the transition of fibroblasts into myofibroblasts and whether myofibroblasts undergo apoptosis or become deactivated by other means once tissue homeostasis is reached. We used three-dimensional microtissues grown in vitro from fibroblasts in macroscopically engineered clefts for several weeks and found that fibroblasts transitioned into myofibroblasts at the highly tensed growth front as the microtissue progressively closed the cleft, in analogy to closing a wound site. Proliferation was up-regulated at the growth front, and new highly stretched fibronectin fibers were deposited, as revealed by fibronectin fluorescence resonance energy transfer probes. As the tissue was growing, the ECM underneath matured into a collagen-rich tissue containing mostly fibroblasts instead of myofibroblasts, and the fibronectin fibers were under reduced tension. This correlated with a progressive rounding of cells from the growth front inward, with decreased α-smooth muscle actin expression, YAP nuclear translocation, and cell proliferation. Together, this suggests that the myofibroblast phenotype is stabilized at the growth front by tensile forces, even in the absence of endogenously supplemented TGF-β, and reverts into a quiescent fibroblast phenotype already 10 μm behind the growth front, thus giving rise to a myofibroblast-to-fibroblast transition. This is the hallmark of reaching prohealing homeostasis.

中文翻译：

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在工程裂隙中的组织生长过程中，拉伸力驱动可逆的成纤维细胞到肌成纤维细胞的转变。

肌成纤维细胞协调伤口愈合过程，如果它们保持激活状态，就会推动疾病进展，如纤维化和癌症。除了生长因子信号外，局部细胞外基质 (ECM) 及其机械特性是这些过程的中心调节器。尚不清楚转化生长因子-β (TGF-β) 和张力是否协同作用于上调成纤维细胞向肌成纤维细胞的转变，以及一旦达到组织稳态，肌成纤维细胞是否会发生凋亡或通过其他方式失活。我们使用了从宏观工程裂隙中的成纤维细胞体外生长的三维微组织数周，发现随着微组织逐渐闭合裂隙，成纤维细胞在高度紧张的生长前沿转变为肌成纤维细胞，类似于闭合伤口部位。正如纤连蛋白荧光共振能量转移探针所揭示的那样，增殖在生长前沿被上调，并且新的高度拉伸的纤连蛋白纤维沉积。随着组织的生长，下面的 ECM 成熟为富含胶原蛋白的组织，主要包含成纤维细胞而不是肌成纤维细胞，并且纤连蛋白纤维处于降低的张力下。这与细胞从生长前沿向内逐渐变圆相关，α-平滑肌肌动蛋白表达降低、YAP 核易位和细胞增殖。总之，这表明肌成纤维细胞表型通过张力在生长前沿稳定，即使没有内源性补充的 TGF-β，并在生长前沿后面 10 μm 处恢复为静止的成纤维细胞表型，从而引起肌成纤维细胞到成纤维细胞的转变。这是达到预愈合稳态的标志。