Fibroblasts close a void in free space by a purse-string mechanism

How stromal cells fill voids in wounded tissue remains one of the most fundamental questions in regenerative medicine. Fibroblasts are known to fill voids by depositing extracellular matrix (ECM) proteins while migrating towards the wound site; however, their ability to adopt an epithelial-like purse-string behaviour remains unexplored. Here, we fabricated an artificial wound with a deep void space to investigate fibroblasts behaviour during gap closure. We found that fibroblasts can form a free-standing bridge on deep microvoids and consequently close the void through the purse-string contraction, which was previously believed to be exclusively an epithelial wound closure mechanism. The results also revealed that the fibroblast gap closure in our fabricated 3D artificial wound depends on myosin II-mediated contractility and intercellular adherent junctions. Our study reveals that stromal cells can gain the structural features of epithelial cells, namely, intercellular contractile rings, to fulfil their functions under the specific microenvironmental conditions of tissue repair. Furthermore, fibroblasts can close artificial wounds with gap widths up to 300 m, approximately twice as large as the critical epithelial gap closure size on non-adherent substrates. Fibroblasts exhibited a zip-up gap closure mechanism with a geometrical size effect. These findings reveal a new mechanism for gap closure by stromal cells during wound healing and pave a way to groundbreaking therapeutic strategies for tissue repair.