LAUSR

Significance Volume regulation is key in maintaining important tissue functions, such as growth or healing. In this process, the role of efflux of cellular fluids is difficult to capture due to the lack of apt technologies. Here, we use a tool based on Brillouin light scattering (BLS) that uses the interaction of a laser light with inherent picosecond timescale density fluctuations in the sample. We induced gradual volume decrease in multicellular spheroids using osmotic perturbations. BLS revealed a nonlinear increase of the tissue compressibility due to the subsequent increased biopolymer crowding within the cells. Our findings should inspire research regarding volume regulation and cellular crowding in tissues and stimulate the emergence of models for cell mechanics at short timescales. Volume regulation is key in maintaining important tissue functions, such as growth or healing. This is achieved by modulation of active contractility as well as water efflux that changes molecular crowding within individual cells. Local sensors have been developed to monitor stresses or forces in model tissues, but these approaches do not capture the contribution of liquid flows to volume regulation. Here, we use a tool based on Brillouin light scattering (BLS) that uses the interaction of a laser light with inherent picosecond timescale density fluctuations in the sample. To investigate volume variations, we induced osmotic perturbations with a polysaccharide osmolyte, Dextran (Dx), and compress cells locally within multicellular spheroids (MCSs). During osmotic compressions, we observe an increase in the BLS frequency shift that reflects local variations in the compressibility. To elucidate these data, we propose a model based on a mixing law that describes the increase of molecular crowding upon reduction of the intracellular fluids. Comparison with the data suggests a nonlinear increase of the compressibility due to the dense crowding that induces hydrodynamic interactions between the cellular polymers.