LAUSR

In cell biology textbooks, cytoplasm is often depicted as a homogeneous color that bathes the nucleus and organelles, leaving the impression that it consists of a liquid solution without structure or physical properties. In reality, cytoplasm is crowded with macromolecules (1) and organized by cytoskeletal networks (2), which endow it with both viscous and elastic properties (2), but the biological significance of these properties is often unclear. In PNAS, Xie et al. (3) use magnetic tweezers to displace the mitotic spindle in sea urchin embryos and find that it springs back when displaced. Remarkably, the elasticity that maintains spindle position does not depend on microtubules and only partly depends on actin, suggesting that the crowded nature of cytoplasm may contribute. During cell division, eukaryotic cells assemble a mitotic spindle whose position determines cleavage geometry and sometimes, the developmental fate of daughter cells (4). In symmetric divisions, the spindle is positioned at the center of the cell. How it achieves this central location has been studied in multiple organisms. In most metazoans, the poles of the spindle are defined by radial arrays of microtubules called asters. Microtubules are nucleated at the aster center, and their growth is bounded by dynamic instability, leading to a maximum mitotic aster radius of ∼30 μm (5). If the cell radius is shorter than this, the spindle is positioned dynamically throughout mitosis by microtubules that touch the cortex, as in Caenorhabditis elegans embryos (6). If longer, as in Xenopus embryos, the spindle forms at a central location defined by microtubules in the preceding interphase (7, 8). In such large cells, the spindle has to remain in position throughout mitosis without connections to the cortex, and the mechanisms that keep it there have been unclear. The sea urchin embryos analyzed by Xie et al. (3) have a radius of ∼48 μm and a mitotic aster radius of ∼25 μm, so they are in the large cell regime and provide a system to probe the mechanisms used to maintain spindle location. Fig. 1. Viscoelastic forces from bulk cytoplasm maintain the spindle position. (A) The experimental procedure for displacing mitotic spindles with magnetic tweezers. (B) Viscoelasticity modeled by spring–dashpot models. In small cells where microtubules touch the cortex, microtubules constitute the main elastic component. In large cells, in contrast, bulk cytoplasm generates viscoelastic restoring forces. (C) Cytoplasmic components implicated in force generation. Passive components resist perturbations, while active components generate force using chemical energy.