The nucleolus is a membraneless organelle embedded in chromatin solution inside the cell nucleus. By analyzing surface dynamics and fusion kinetics of human nucleoli in vivo, we find that the nucleolar surface exhibits subtle, but measurable, shape fluctuations and that the radius of the neck connecting two fusing nucleoli grows in time as r(t) ∼ t1/2. This is consistent with liquid droplets with low surface tension ∼ 10−6 Nm−1 coalescing within an outside fluid of high viscosity ∼ 103 Pas. Our study presents a noninvasive approach of using natural probes and their dynamics to investigate material properties of the cell and its constituents.
Exchange bias is a property of widespread technological utility, but whose underlying mechanism remains elusive, in part because it is rooted in the interaction of coexisting order parameters in the presence of complex magnetic disorder. Here, we show that a giant exchange bias housed within a spin-glass phase arises in a disordered antiferromagnet. The magnitude and robustness of the exchange bias emerges from a convolution of two energetic landscapes-the highly degenerate landscape of the spin-glass biased by the sublattice spin-configuration of the antiferromagnet. The former provides a source of uncompensated moment, while the latter provides a mechanism for its pinning, leading to the exchange bias. Tuning the relative strength of the spin-glass and antiferromagnet order parameters reveals a principle for tailoring the exchange bias, with potential applications to spintronic technologies.
Liquid-liquid phase separation (LLPS) has been recognized as one of the key cellular organizing principles and was shown to be responsible for formation of membrane-less organelles such as nucleoli. Although nucleoli were found to behave like liquid droplets, many ramifications of LLPS including nucleolar dynamics and interactions with the surrounding liquid remain to be revealed. Here, we study the motion of human nucleoli in vivo, while monitoring the shape of the nucleolus-nucleoplasm interface. We reveal two types of nucleolar pair dynamics: an unexpected correlated motion prior to coalescence and an independent motion otherwise. This surprising kinetics leads to a nucleolar volume distribution, p(V)∼V-1, unaccounted for by any current theory. Moreover, we find that nucleolus-nucleoplasm interface is maintained by ATP-dependent processes and susceptible to changes in chromatin transcription and packing. Our results extend and enrich the LLPS framework by showing the impact of the surrounding nucleoplasm on nucleoli in living cells.
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