Architecture of the skin

An understanding of the biology of skin pigmentation requires some knowledge of the structure of the skin. There is a tendency to regard skin merely as the integument for the otherwise intricate and intriguing machinery of the human body. Yet, in terms of its multiplicity of cellular and fibrous constituents, the skin is not only the largest and most versatile organ of the body but also, with the possible exception of the brain, the most complex.

Dermis

The skin (Fig. l.l(a)) has two major components, the dermis and the epidermis. The dermis is basically a connective tissue layer comprising collagen, elastic and reticular fibres. It is traversed by a rich network of blood and lymphatic vessels. It also contains structures originally derived from the epidermis – the sweat and sebaceous glands, the hair follicles, and the hairs themselves. Attached to the hair follicles are minute bundles of smooth muscle (arrector pili), the contraction of which during cold or fear produces the phenomenon of ‘goose-flesh’. The dermis is supplied with sensory nerve endings (mediating the sensations of touch, heat, cold and pain) and with sympathetic nerves which regulate the activity of the sweat gland, arteriole and arrector pili.

Epidermis

The epidermis is a thin layer (about 0.10–0.15 millimetres in thickness) devoid of either blood or nerve supplies. It is composed of two distinct cell populations – epithelial cells or keratinocytes (also known as Malpighian cells) and pigment cells or melanocytes.