This aspect of skin keeps it physically strong and flexible, thanks in large part to skin’s key support system: the extracellular matrix or ECM. This is a densely packed structure (a “matrix”) of collagen and other support proteins that exist outside the cells (the prefix “extra-” means outside). When skin’s physical function is healthy, it provides a cushion against mechanical insult (such as when skin crinkles when you smile or frown), allowing skin to bounce back to a smooth, firm state.