Department of Bioengineering, Stanford University, Stanford, CA 94305.
Department of Applied Physics, Stanford University, Stanford, CA 94305.
Proc Natl Acad Sci U S A. 2018 Oct 30;115(44):E10333-E10341. doi: 10.1073/pnas.1802934115. Epub 2018 Oct 11.
By definition of multicellularity, all animals need to keep their cells attached and intact, despite internal and external forces. Cohesion between epithelial cells provides this key feature. To better understand fundamental limits of this cohesion, we study the epithelium mechanics of an ultrathin (∼25 μm) primitive marine animal , composed essentially of two flat epithelial layers. With no known extracellular matrix and no nerves or muscles, has been claimed to be the "simplest known living animal," yet is still capable of coordinated locomotion and behavior. Here we report the discovery of the fastest epithelial cellular contractions known in any metazoan, to be found in dorsal epithelium (50% shrinkage of apical cell area within one second, at least an order of magnitude faster than other known examples). Live imaging reveals emergent contractile patterns that are mostly sporadic single-cell events, but also include propagating contraction waves across the tissue. We show that cell contraction speed can be explained by current models of nonmuscle actin-myosin bundles without load, while the tissue architecture and unique mechanical properties are softening the tissue, minimizing the load on a contracting cell. We propose a hypothesis, in which the physiological role of the contraction dynamics is to resist external stresses while avoiding tissue rupture ("active cohesion"), a concept that can be further applied to engineering of active materials.
根据多细胞的定义，所有动物都需要保持细胞的附着和完整，尽管会受到内部和外部力量的影响。上皮细胞的黏合提供了这个关键特征。为了更好地理解这种黏合的基本限制，我们研究了一种超薄（约 25 μm）原始海洋动物的上皮力学，它主要由两个扁平的上皮层组成。由于没有已知的细胞外基质，也没有神经或肌肉，被认为是“已知最简单的活体动物”，但仍然能够进行协调的运动和行为。在这里，我们报告了在任何后生动物中发现的最快的上皮细胞收缩，这种收缩存在于 背侧上皮中（在一秒钟内，顶端细胞面积收缩 50%，比其他已知的例子至少快一个数量级）。实时成像揭示了突发的收缩模式，这些模式主要是单个细胞的偶发性事件，但也包括在组织中传播的收缩波。我们表明，细胞收缩速度可以用无负载的非肌肉肌动球蛋白束的现有模型来解释，而组织架构和独特的机械性能则使组织软化，最大限度地减少收缩细胞的负载。我们提出了一个假设，即收缩动力学的生理作用是抵抗外部应力，同时避免组织破裂（“主动黏合”），这个概念可以进一步应用于活性材料的工程设计。
