Chapter 1
Your Skin, The Largest Organ

- Layers of the skin
- Facultative pigmentation
- Constitutive pigmentation

Skin covers all body surfaces. The skin of an average adult weighs 8-10 pounds and has an average area of about 22 square feet. The purpose of this outer covering for the body is to protect against injury, infection, heat, cold, and store water, fat and vitamins. The human skin is rejuvenated about once every four weeks.

Thinking of your skin as an organ, rather than something that we can use and abuse, puts things in proper perspective. Your skin is a wonderfully resilient organ and for the most part can survive virtually any form of punishment. The skin is the body's boundary, tough enough to resist all sorts of environmental assaults, yet sensitive enough to feel a breeze.

A versatile organ, skin creates the first line of defense against possible invasion by bacteria and germs, while maintaining the body's internal environment to within a few degrees of normal throughout our lifetimes. The skin also secretes fluids that lubricate it and barricade toxic substances, while maintaining this environment. The skin can absorb some soluble substances

The Skins Function
The skin is divided into three layers, the epidermis or outer layer which produces the tan; the dermis or middle layer which contains collagen and other materials vital to the skin's strength, its ability to repair itself and fight off infections; and the subcutaneous tissue or bottom layer which serves as insulation, a food reserve and binds the skin to your body. The layers of the epidermis which are involved in the tanning process are the horny (outer) layer and the germ (inner) layer. Cells from the germ layer are constantly reproducing and pushing old cells up through the horny layer where in approximately one month they are sloughed off. At the base of the epidermis, cells called melanocytes (about 5% of the epidermal cells) exist. These are the pigment cells involved in the tanning process. The melanocytes use the amino acid tyrosine to produce melanosomes (dark brown granules of pigment) which contain melanin that, when oxidized by UVR, provide the adaptive coloration of the skin. When exposed to ultraviolet radiation, the melanocytes release extra melanosomes thus making the skin darker and completing melanogenesis which is defined as the UVR-induced production and oxidation of melanin, i.e., the process of developing facultative pigmentation, better known as cosmetic tanning. Facultative Pigmentation is simply the level of an acquired tan developed by an individual exposed to ultraviolet light where as Constitutive Pigmentation is our natural skin color.

Every individual has only a given amount of melanin which is determined by an individual's skin type. Although a person may gradually increase the amount of melanin production through tanning, the person cannot change from one skin type to another.

One function of the skin is to protect its underlying tissues from invisible radiation i.e. that produced by the sun. The sun emits three kinds of ultraviolet (UV) rays, UVA, UVB and UVC. Although invisible, you can see the results of ultraviolet rays in such things as the growth of plants and the tanning of our skin.

UVC is the shortest, most harmful wavelength of ultraviolet rays, but is virtually stopped by the Earth's ozone layer and pollution. UVB is the medium wavelength and although overexposure can cause erythema (sunburn), a controlled amount is necessary to initiate tanning in the skin.

UVA is the longest wavelength and is responsible for the completion of the tanning process. Tanning is actually the body's natural defense mechanism to protect itself from the sun's rays.

The outer surface of dead cells (horny layer) is the first shield against any invader. These cells, called keratinocytes or skin cells (about 90% of the epidermal cells), arise from the living dividing basal cells (named for their location at the base of the epidermis). New cells rise, pushed from the base by rapidly dividing basal cells. These new cells produce greater and greater quantities of a protein called keratin. The fibrous keratin accumulates within the cells until it nearly replaces their living cellular machinery. This journey to the surface takes approximately four to five weeks. Now they have withered, died and bound themselves firmly to one another, forming a tough nearly impermeable outer shell to the epidermis. Perpetual shedding of this horny layer prevents many microbes from penetrating the skin. As the epidermis goes about the business of renewing the horny layer, it sheds the dried out cells at a rate of one million every forty minutes. This horny layer becomes thicker and tougher in response to UV to protect the skin from overexposure. The remaining 5% of cells found in the epidermis are mostly made up of Langerhans and Merkel cells. Langerhans cells, also known as "immune cells," help fight-off organisms trying to invade the body. Merkel cells, known as "touch receptors," relay touch sensations to the dermis as contact nerve endings.

Ultraviolet B initiates the tanning process by stimulating the melanocytes, releasing melanin into the surrounding cells. As these melanin granules migrate to the skin's surface, there is a chemical reaction that occurs between the tyrosine, the melanin and the UVA rays that turns the skin a light brown or brown giving us the tanned appearance.

The degree of coloring achieved depends on the amount of melanin one has, the duration of the exposure and the individual's reaction to the ultraviolet rays.

The sun is not selective in the proportions of UVA and UVB emitted. Therefore the skin is vulnerable to too much UVB which can cause sunburn, as well as other types of damage to the skin.

Another system at work in the epidermis is our immune system. The epidermis houses special cells that join the immune system in defense against disease. Langerhans cells give agents of the immune system information regarding the nature of foreign substances entering the body through the skin. Extremely high doses of UV can damage Langerhans cells, preventing them from sending the appropriate warning signals to the immune system. Lymphocytes are also located in the epidermis. They are the other defender in this delicate cellular world. Lymphocytes are also damaged by prolonged overexposure to UV.

This highly complex inner world of the skin mandates responsible treatment by its owner as well as those of us entrusted with the cosmetic care of this largest of human organs.