Shi Yunfei, Varner Victor D, Taber Larry A
Department of Biomedical Engineering, Washington University, Saint Louis, MO 63130, USA.
Phys Biol. 2015 Jan 30;12(1):016012. doi: 10.1088/1478-3975/12/1/016012.
Cytoskeletal contraction is crucial to numerous morphogenetic processes, but its role in early heart development is poorly understood. Studies in chick embryos have shown that inhibiting myosin-II-based contraction prior to Hamburger-Hamilton (HH) stage 10 (33 h incubation) impedes fusion of the mesodermal heart fields that create the primitive heart tube (HT), as well as the ensuing process of cardiac looping. If contraction is inhibited at or after looping begins at HH10, however, fusion and looping proceed relatively normally. To explore the mechanisms behind this seemingly fundamental change in behavior, we measured spatiotemporal distributions of tissue stiffness, stress, and strain around the anterior intestinal portal (AIP), the opening to the foregut where contraction and cardiac fusion occur. The results indicate that stiffness and tangential tension decreased bilaterally along the AIP with distance from the embryonic midline. The gradients in stiffness and tension, as well as strain rate, increased to peaks at HH9 (30 h) and decreased afterward. Exposure to the myosin II inhibitor blebbistatin reduced these effects, suggesting that they are mainly generated by active cytoskeletal contraction, and finite-element modeling indicates that the measured mechanical gradients are consistent with a relatively uniform contraction of the endodermal layer in conjunction with constraints imposed by the attached mesoderm. Taken together, our results suggest that, before HH10, endodermal contraction pulls the bilateral heart fields toward the midline where they fuse to create the HT. By HH10, however, the fusion process is far enough along to enable apposing cardiac progenitor cells to keep 'zipping' together during looping without the need for continued high contractile forces. These findings should shed new light on a perplexing question in early heart development.
细胞骨架收缩对众多形态发生过程至关重要，但其在心脏早期发育中的作用却鲜为人知。对鸡胚的研究表明，在汉伯格-汉密尔顿（HH）第10阶段（孵化33小时）之前抑制基于肌球蛋白II的收缩，会阻碍形成原始心管（HT）的中胚层心脏区域的融合，以及随后的心脏环化过程。然而，如果在HH10开始环化时或之后抑制收缩，融合和环化则相对正常进行。为了探究这种行为上看似根本性变化背后的机制，我们测量了前肠门（AIP）周围组织硬度、应力和应变的时空分布，前肠门是前肠的开口，收缩和心脏融合在此发生。结果表明，沿着AIP两侧，硬度和切向张力随着与胚胎中线距离的增加而降低。硬度、张力以及应变率的梯度在HH9（30小时）时增加至峰值，随后下降。暴露于肌球蛋白II抑制剂blebbistatin会减弱这些效应，这表明它们主要由活跃的细胞骨架收缩产生，有限元建模表明，所测量的力学梯度与内胚层相对均匀的收缩以及附着中胚层施加的约束相一致。综合来看，我们的结果表明，在HH10之前，内胚层收缩将双侧心脏区域拉向中线，在中线处它们融合形成HT。然而，到HH10时，融合过程已经进展到足以使相邻的心脏祖细胞在环化过程中继续“拉链”式连接在一起，而无需持续的高收缩力。这些发现应该会为心脏早期发育中一个令人困惑的问题带来新的启示。
