Why is the stratum corneum water resistant but not waterproof?

Answer 1

The stratum corneum is the outermost layer of the epidermis, composed of large, flat, polyhedral, plate-like envelopes filled with keratin, which is made up of dead cells that have migrated up from the stratum granulosum.[1] From the Latin for horned layer, this skin layer is composed mainly of dead cells that lack nuclei. As these dead cells slough off on the surface in the thin air-filled stratum disjunctum, they are continuously replaced by new cells from the stratum germinativum (basale). In the human forearm, for example, about 1300 cells/cm2/hr are shed. This outer layer that is sloughed off is also known as the stratum dysjunctum.

Cells of the stratum corneal contain keratin, a protein that helps keep the skin hydrated by preventing water evaporation. These cells can also absorb water, further aiding in hydration, and explaining why humans and other animals experience wrinkling of the skin on the fingers and toes ("pruning") when immersed in water for prolonged periods. In addition, this layer is responsible for the "spring back" or stretchy properties of skin. A weak glutenous protein bond pulls the skin back to its natural shape.

The thickness of the stratum corneum varies according to the amount of protection and/or grip required by a region of the body. For example, the hands are typically used to grasp objects, requiring the palms to be covered with a thick stratum corneum. In a similar manner, the sole of the foot is prone to injury, and so it is protected with a thick stratum corneum layer. In general, the stratum corneum contains 15 to 20 layers of dead cells. The stratum corneum has a thickness between 10 and 40 μm.

In reptiles, the stratum corneum is permanent, and is replaced only during times of rapid growth, in a process called ecdysis or moulting. The stratum corneum in reptiles contains beta-keratin, which provides a much more rigid skin layer.