Surface Fluctuations and Coalescence of Nucleolar Droplets in the Human Cell Nucleus*

The nucleolus is a membraneless organelle embedded in chromatin solution inside the cell nucleus. By analyzing the surface dynamics and fusion kinetics of nucleoli in live human cells, we find that the nucleolar surface exhibits subtle, but measurable, shape fluctuations and the radius of the neck connecting two fusing nucleoli grows as r(t)~t^1/2 [1]. This is consistent with liquid droplets with low surface tension ~10^-6 Nm^-1 coalescing in a fluid of higher viscosity ~10³ Pa s, i.e. chromatin solution. We find the neck velocity, dr/dt, is comparable to the velocity of chromatin solution [2]. Surprisingly, nucleolar coalescence occurs in an active fluid, yet can be described by coalescence theory for passive liquid droplets, suggesting the measured quantities might be effective quantities. Our study presents a noninvasive approach, using natural probes to investigate material properties of the cell as well as to understand the physical interactions between nucleoli and chromatin solution [1,3].
1. Caragine CM et al, Phys. Rev. Lett. (2018)
2. Zidovska A et al, Proc. Natl. Acad. Sci. (2013)
3. Caragine CM et al, eLife (2019)