Abstract
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)∼t^{1/2}. This is consistent with liquid droplets with low surface tension ∼10^{-6} N m^{-1} coalescing within an outside fluid of high viscosity ∼10^{3} Pa s. Our study presents a noninvasive approach of using natural probes and their dynamics to investigate material properties of the cell and its constituents.