Multicellular spheroids (MCS) can be used as a model of in vivo tumors due to their ability to mimic several features of solid tumors (top left). To test the effect of solid stress on tumor growth, Dolega et al. used dextran molecules of varying sizes to induce osmotic water flow (yellow arrows). They found that small dextran molecules (blue circles, left) can enter the pores of extracellular matrix (ECM, green strings), thereby applying osmotic pressure (red arrows) on individual cells. Large dextran molecules (green circles, right) cannot enter the extracellular matrix, so they exert osmotic pressure on the whole spheroid (red arrows), mimicking the solid stress in in vivo tumors. d = diameter of dextran molecules; ρ is the spacing between the fibers in the extracellular matrix.