Keratin

As skin cells mature and move toward the surface, they fill with keratin and flatten to form the protective outer layer. This keratin-rich shield helps hold moisture in and keep irritants out. Because sweat passes through keratinized skin to reach the surface, the protein is part of the environment that fluid travels through. Keratin also makes up the thickened skin on palms and soles, regions that sweat heavily. When this skin stays wet for a long time, the keratin layer can soften, which changes how sturdy the surface feels. The same protein forms hair, which is relevant where glands open into hair follicles. It is produced by the skin cells themselves as they move upward and die off. The thickness of a keratin layer often reflects how much friction or pressure an area receives. This is why the soles, which bear weight and sweat, carry an especially tough keratinized surface.

The firm, thick skin on the soles is heavily keratinized, which is why heavy plantar sweating sits on a tough surface. That toughness is also why softened, macerated sole skin is so noticeable, since the normally hard keratin layer becomes pale and pliable. A callus is simply extra keratin built up where repeated pressure has thickened the surface.

Keratin fills the cells of the stratum corneum, forms the barrier of the epidermis, and also builds the hair shafts that apocrine glands open beside.