What is the purpose of the cuticle layer of a hair strand?

The purpose of the cuticle layer of a hair strand is primarily to protect the inner layers of the hair. The cuticle is composed of overlapping, scale-like cells that form a protective barrier surrounding the cortex and medulla, which are the inner parts of the hair. This barrier helps to shield the hair from environmental damage, such as pollution and UV rays, as well as from physical damage caused by styling and chemical treatments. By serving this protective function, the cuticle layer plays a crucial role in maintaining the overall health and integrity of the hair.

While the other options mention factors like color retention, moisture provision, and shine enhancement, these are more closely related to the effects of the inner layers of the hair or from external products rather than the primary function of the cuticle itself. The cuticle does influence the hair's overall appearance and condition indirectly, but its fundamental role is centered around protection.