Figure 6. Muscle stem cell differentiation is modulated by substrate stiffness.

A, B. Adult Quiescent MuSCs (Pax7+) residing on the fiber periphery receive an activation signal and divide to form two MuSCs, two early myoblasts (MyoD+), or asymmetrically divide to form one of each. Substrates that are too stiff [40] or too soft [78] favor differentiation while asymmetric substrates support asymmetric division [74]. Early myoblasts further differentiate to form late myoblasts (MyoG+) or undergo apoptosis, the later of which is more prevalent on stiff substrates [40]. Late myoblasts fuse to form contractile myotubes or myofibers. Substrate stiffness above or below 12 kPa inhibit the ability of myoblasts to undergo terminal differentiation [77]. The effect of stiffness is indicated by the line color: softer than healthy muscle (cyan), healthy muscle (green), and stiffer than healthy muscle (red). These muscle tissue stiffness values are plotted on the kPa scale to show healthy, diseased and developing muscle.

C. After 4 weeks in differentiation media C2C12 myoblasts grown on collagen strips demonstrate a narrow range of optimal stiffness for differentiation as depicted by the presence of sarcomere striations. Quantification shows substrate stiffness between 8–15 kPa maximizes differentiation and encompasses the stiffness of healthy muscles. Adapted from [77].