Simple hydrostatic skeletons such as the Hydra's consist of two stacked layers of cells perpendicularly oriented. Although this crisscross architecture can be recapitulated in vitro, little is known on the formation of such multilayers starting from a monolayer. In the present article, we show that bilayering of myoblasts results from the organization and activity of the cells originally in the monolayer which can be described as a contractile active nematic. As expected, most of the +1/2 topological defects that are associated with this nematic order self-propel. However, a subpopulation of these defects remains immobile. Perpendicular bilayering occurs exclusively at these motionless defects. Indeed, cells located at the head of these defects converge toward the (immobile) core and accumulate there until they start migrating on top of the tail of the first layer while the tail cells migrate in the opposite direction under the head cells. Since the cells keep their initial orientations, the two stacked layers end up perpendicularly oriented. This concerted process leading to a bilayer is dependent on the apical secretion of Extra Cellular Matrix (ECM) by the cells. Indeed, we evidence the presence of ECM between the cell layers and at the apical surface of the topmost layer. ECM molecules are oriented in the direction of the cells that produce them, which may guide the migration of the subsequent cell layers on their apical side.

中文翻译：

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细胞片的交叉多层

简单的流体静力学骨架，例如 Hydra 的骨架，由两层垂直排列的堆叠单元组成。尽管这种纵横交错的结构可以在体外重现，但对从单层开始形成这种多层结构知之甚少。在本文中，我们表明成肌细胞的双层是由最初在单层中的细胞的组织和活动引起的，可将其描述为可收缩的活性向列。正如预期的那样，大多数与此向列顺序相关的 +1/2 拓扑缺陷会自行推进。然而，这些缺陷的一个亚群仍然不动。垂直双层仅发生在这些静止缺陷处。确实，位于这些缺陷头部的细胞会向（固定的）核心聚集并在那里积累，直到它们开始在第一层尾部的顶部迁移，而尾部细胞在头部细胞下方以相反的方向迁移。由于电池保持其初始方向，因此两个堆叠层最终垂直定向。这种导致双层的协同过程取决于细胞对细胞外基质 (ECM) 的顶端分泌。事实上，我们证明了细胞层之间和最顶层的顶端表面存在 ECM。ECM 分子朝向产生它们的细胞的方向，这可能会指导后续细胞层在其顶端的迁移。由于电池保持其初始方向，因此两个堆叠层最终垂直定向。这种导致双层的协同过程取决于细胞对细胞外基质 (ECM) 的顶端分泌。事实上，我们证明了细胞层之间和最顶层的顶端表面存在 ECM。ECM 分子朝向产生它们的细胞的方向，这可能会指导后续细胞层在其顶端的迁移。由于电池保持其初始方向，因此两个堆叠层最终垂直定向。这种导致双层的协同过程取决于细胞对细胞外基质 (ECM) 的顶端分泌。事实上，我们证明了细胞层之间和最顶层的顶端表面存在 ECM。ECM 分子朝向产生它们的细胞的方向，这可能会指导后续细胞层在其顶端的迁移。