What is Skin?  Cosmetics Unmasked

Your skin is the largest organ in your body, accounting for about 16 per cent of your total body weight, and it has an area of 18, 000 square centimeters (2, 880 square inches) for an average adult. It is responsible for maintaining the correct body temperature; it senses heat, cold, contact and pain; it is an excretory organ, discharging waste matter such as urea; it helps to maintain the correct levels of water in your body fluids; it protects you from external dirt, microbes, and chemicals, and keeps you safe from rain, wind, snow, and the sun.

Beneath the two main layers of the skin is the subcutaneous layer, which separates your skin from the underlying muscles. This layer consists mainly of fat cells which help to insulate your body. Above the subcutaneous layer is the dermis, the main layer of skin.

The dermis is composed of living cells and contains tiny blood vessels called capillaries, nerve endings that detect temperature and touch, sweat glands that keep you cool, erector muscles that make your hair stand on end, collagen fibres that give your skin elasticity, hair follicles that supply the growing hairs with food and oxygen, and sebaceous glands that lubricate your hair shafts, hair follicles and skin.

When applied to undamaged skin, few cosmetic ingredients can reach the dermis. If they did, they would probably damage the living cells and cause irritation, sensitization, and allergies, and from there they would enter your bloodstream and may cause harm elsewhere in your body.

On top of the dermis is the outer layer of skin, called the epidermis. The epidermis consists of five layers but only one of them, the deeper layer, is distinct. This layer is called the basal layer or stratum germinativum. It consists of living cells that obtain food and oxygen from the dermis, and cells called melanocytes, which contain melanin, a brown pigment that protects you from ultraviolet radiation and gives your skin its colour.

As the basal cells reproduce, they are pushed upwards toward the surface of the skin, taking about three weeks to reach the surface where they are eventually shed ("desquamated"). During these three weeks the cells gradually die and the substances inside the cell slowly turn to keratin, a tough, fibrous protein which is also found in hair and nails.

By the time the cells reach the horny outer layer of the skin that you can see (the "stratum corneum"), they consist mainly of keratin. keratin are flat like paving slabs, and are almost completely dehydrated.

This outer layer is usually the only layer you ever hear about. many cosmetic manufacturers proudly boast that their products can penetrate the stratum corneum - a credible, scientific-sounding phrase that rolls easily off the tongue. Fortunately, you have three more protective layers of skin cells to prevent these chemicals from reaching your living skin cells.

Reference: Cosmetics Unmasked: Dr Stephen & Gina Antczak