Fig. 4

MSC stiffness correlates with chondrogenic potential and is influenced by cell number within aggregates. a Images of suspended cells in RT-DC at day 7 of chondrogenic differentiation for different ACNs. b RT-DC scatter plots of cell size and deformation at day 7 for different ACNs. Each scatter plot summarizes more than 1000 cells per condition. Isoelasticity lines in gray highlight areas of equal elastic Young’s modulus. Color code indicates red (maximum) to blue (minimum) cell density. c Contour plot showing 50% (dashed) and 90% (solid) of maximum event density for aggregates at day 7 in different conditions: 70 k (red), 150 k (yellow), 250 k (dark green), 350 k (dark blue), and 500 k (blue). d Statistical analysis comparing elastic Young’s modulus and cell size to 70 k ACN condition. For day 2, cells were pooled from three to four aggregates to establish a mean for a particular condition, and day 7 shows data from experimental replicates analyzed by linear mixed models. Error bars represent the standard deviation of the distribution (day 2, one pooled sample) and standard error of the mean of the replicates (day 7, three pooled samples). The purple band represents the range of values for undifferentiated MSCs (*p < 0.05, **p < 0.01, ***p < 0.001)