The stratum corneum is the outermost layer of the epidermis, and comprises the surface of the skin. It is composed mainly of dead cells that lack nuclei. As these dead cells slough off, they are continuously replaced by new cells from the stratum germinativum.
Cells of the stratum corneum contain keratin, a protein that helps keep the skin hydrated by preventing water evaporation. In addition, these cells can also absorb water, further aiding in hydration and explaining why humans and other animals experience finger and toe wrinkling (colloquially called "pruning") when immersed in water for prolonged periods.
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. Similarly, the sole of the foot is prone to injury, and so it is protected with a thick stratum corneum layer.
In reptiles, the stratum corneum is permanent, and is only replaced during times of rapid growth, in a process called ecdysis or moulting. The stratum corneum in reptiles contains beta-keratin which provides a more rigid skin layer.
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