How does the cross-linking theory explain the loss of elasticity in skin and tendons?

The cross-linking theory explains the loss of elasticity in skin and tendons as a result of chemical changes in collagen, a fibrous protein essential for structural integrity.12

As an organism ages, the number of cross-linkages within and between collagen molecules increases. This process transforms collagen from a more soluble, flexible form into a stable, insoluble, and increasingly rigid structure. This accumulation of cross-links leads to crystallinity and rigidity in tissues, which manifests as a general loss of elasticity and increased body stiffness.12

The chemical nature of this process is often compared to the tanning of leather, where similar cross-linking occurs. Beyond reducing flexibility, these structural changes decrease the permeability of tissues, hindering the movement of essential nutrients, hormones, and antibodies. While collagen cross-linking is a primary factor in the rigidity of skin and tendons, the loss of elasticity in other areas, such as blood vessels, is also attributed to the fragmentation of elastin.21