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Innovations in imaging and genetic engineering are coming together to probe the biophysics of cytoplasm inside living animals.

Cells can regulate their mass density. Here, the authors demonstrate how eukaryotes establish and maintain a lower density in the nucleus than in the cytoplasm via pressure balance, and how deviations emerge during pathophysiological states like senescence.

Kletter et al. show that cell state-specific cytoplasmic density controls spindle architecture and scaling in neural differentiation, suggesting that the physical properties of the cytoplasm are a det...

Nature Cell Biology - Kletter et al. show that cell state-specific cytoplasmic density controls spindle architecture and scaling in neural differentiation, suggesting that the physical properties...

Understanding of the dynamics inherent to biological matter is crucial for illuminating the physical mechanisms underlying cellular processes. In this study, we employ bright-field differential dynamic microscopy (DDM) to investigate density fluctuations inherent in a cell-free model of eukaryotic cytoplasm. Our measurements reveal subdiffusive fractional Brownian motion and non-Gaussian displacement distributions, highlighting cytoplasmic heterogeneity. We introduce an empirical model that combines fractional Brownian motion with an inverse Gaussian distribution of diffusivities to describe the observed non-Gaussianity. Validated through Monte Carlo simulations, this model allows us to estimate the fractional diffusivity and exponent effectively. By altering macromolecular composition, the addition of energy, and assembly of a cytoskeleton, we identify three independent mechanisms that result in similar fractional exponents yet distinct diffusivities. We find that energy addition leads to non-stationary dynamics, in contrast to the stationary behavior observed under passive conditions. Presence of microtubules introduces a secondary dynamical timescale, which we describe using a two-state fractional Brownian motion model to differentiate between cytosolic and microtubule network associated contributions. Our findings demonstrate the effectiveness of DDM as a label-free tool for quantifying viscoelastic and heterogeneous properties of the cytoplasm and provide insights into how physical and biochemical factors, including cytoskeletal organization, govern subcellular dynamics. ### Competing Interest Statement The authors have declared no competing interest.

SUMMARYThe development of multicellularity represents a key evolutionary transition that is crucial for the emergence of complex life forms. Although multicellularity has traditionally been studied in...