Introduction to skin and hair biology

The skin is the second largest organ in the body after the skeleton and of primary importance, integral to the survival of mammalian life, but often overlooked when examining the health and welfare of an individual. The heterogeneous skin structure is derived from the ectoderm and mesoderm of an embryo giving rise to the epidermis and dermis respectively. Within these generalized layers are specialized appendages also derived from the ectoderm and/or mesoderm including sensory nerves, sweat glands and hair follicles. The skin as a whole rests on subcutaneous tissue largely composed of a loose mesh of collagen fiber, fat cells and muscle tissue.

The average adult has over 3,000 square inches of skin surface area and the fat-free skin accounts for at least 6 percent of an individual's total weight. In other words, the fat free skin of a 150 pound person weighs at least 9 pounds. The density of structures in the skin vary considerably depending on the body location, but on average one square centimeter of skin contains about 10 hair follicles and 15 sebaceous glands, 100 sweat glands, half a meter of blood vessels, 2 meters of nerves, with 3,000 sensory cells at the ends of nerve fibers, 200 nerve endings to record pain, 25 pressure receptors for the perception of tactile stimuli, 2 sensory receptors for cold, and 12 sensory receptors for heat.

With all these different properties the skin has a diverse range of functions:

• Support - The skin acts as a flexible physical support and covering for underlying tissues.
   

• Temperature - Through its extensive blood supply and sweat glands, the mammalian skin is able to maintain the constant temperature of a homoeothermic.
   

• Excretion - Waste materials such as salts and water are removed from the body via the skin's sweat glands.
   

• Vitamin formation - Photochemical action in skin produces vitamin D. The skin is our primary source.
   

• Sensory function - Through the extensive network of sensory receptors we have sensations of pressure, texture, temperature and pain.
   

• Pigmentation - Melanin pigments protect against the excesses of ultra violet light.
   

• Protection - The epidermis prevents desiccation of the internal organs and so provides the fundamental requirement for mammalian land colonization - freedom from water dependence. It prevents absorption of unwanted and potentially dangerous chemicals.
   

• Immunological defense - The epidermis, particularly the stratum corneum (the outer most keratinized skin layer), provides a passive defense against entry of opportunistic pathogenic organisms. Skin also performs an active role in immunity through immunological surveillance.

In short, the skin acts as a barrier and is the primary organ through which we interact with the external world.

Significant in many of these properties is the hair follicle appendage. The key role of hair is to provide protection against heat loss. Hair traps air adjacent to the skin to provide an invisible, insulating layer. Several mammalian species produce special dense winter coats with added heat trapping properties. Otters have hair to trap a layer of air around their bodies keeping them warm and making them more buoyant while they swim. Many species go through molting cycles in tune with seasonal changes.

Hair can provide indications of sexual development through onset of secondary sexual characteristics from development of a mane on a male lion to beard development in humans. Hair may also be of importance in attracting mates and may be based on color - such as silver back mountain gorillas - distribution or quality, all indicators of the general health and vitality of an individual. Alternatively, it may aid in camouflage for survival where mute tones or dappled color blend with an animal's environment. Hair fiber also helps with protection forming a tough barrier helping protect the epidermis from minor abrasions and/or from ultra violet light. Specialized hair such as eyebrows and eyelashes protect the eyes by channeling or sweeping away fluids, dust and debris. Nasal hair plays an important role in trapping air borne foreign particles before reaching the lungs. Hair fiber may also increase the surface area for faster evaporation of sweat from neighboring apocrine glands. Some hair follicles have a highly developed nerve network around them and provide sensory, tactile information about the environment. Consequently, the hair follicle is of great importance to the survival of mammals.

Although its importance for humans has diminished it is still significant - not just biologically, but also through cosmetic and commercial considerations. The secondary functions of hair are now of primary importance for humans. Hair styles are used to make a statement, to identify the individual with a particular faction of society, and/or to attract a mate. Hair is the foundation for a multi million dollar industry focused on presenting, augmenting, and preserving scalp hair plus removing unwanted body hair.