Critical Role of Evolution

Without the stratum corneum, a highly effective waterproof layer, the body would rapidly dry out and die. But the skin, at the surface between inside and out, is maximally vulnerable to trauma.

Evolution’s brilliant solution is to constantly replace the stratum corneum, with a feedback mechanism so that if there is any damage, replacement rate is rapidly stepped up. Various cell ‘layers’ are described in the epidermis, but in reality the epidermis is dynamic. There is a constant flow of new cells produced above the dermo-epidermal junction that flatten to form the stratum corneum when they reach the top, giving a new stratum corneum every month.

Ultraviolet Protection

Ultraviolet (UV) radiation from the sun can cause dermal damage and promote skin cancer. Melanocytes, positioned above the dermo-epidermal junction, produce melanin in response to UV, resulting in temporary darkening, a tan. The high concentration of melanin in the deeply black skin of many African peoples provides very effective protection, whereas white skin in Northern climes allowed meagre sun exposure to be sufficient for vitamin D production.

Heat Regulation

The blood flow through the dermis can be rapidly altered by valves regulating blood flow through capillaries in the upper dermis or by short-circuiting blood through dermal arterio-venous anastomoses.

If the core body temperature goes up, say during strenuous exercise, the amount of blood near the surface is massively increased, so heat radiates away from the body. The skin looks redder (flushed) because there is so much blood near the surface. If the core temperature remains too high, the sweat glands are turned on and the latent heat of evaporation results in some cooling.

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