The skin is our principle organ of beauty, touch, pleasure, and sensuality. Your skin is the largest organ of your body, accounting for 12% to 16% of your body weight and covering 12 to 20 square feet. With age the amount of subcutaneous (under-the-skin) fat cells are reduced resulting in a looser look to your skin.

The approximate chemical composition of your skin is:

Water 70.0%
Protein 25.5%
Lipids 2.0%
Trace Minerals 0.5%
Other 2.0%

Your skin's barrier has a difficult dual function. First it must protect the body against invasion from microorganisms and against losing fluid and drying out. However, this same skin barrier must still be open and permeable enough to allow an exchange of warmth, air and fluids. It also must act as the sensory organ for our delicate sense of touch. The skin regulates the body temperature by evaporating water. Your skin replaces itself about every 27 days and continuously produces a horny protective cover of hardened proteins (keratinization) while shedding the outermost layer of dead cells (exfoliation).

The Protective Skin Barrier

The skin barrier consists of (1) the hard outer skin proteins formed from dried and flattened skin cells, (2) the acid mantle of sebum and perspiration, and (3) the underlying mixture of cells and protein fibers.

Structure of Skin: Epidermis, Dermis, and Subcutis

Technically skin consists of three layers : the epidermis ("overskin”) or top layer, the dermis ("skin”) or middle layer and the subcutis (“underskin”) or bottom layer. The outer surface of the skin, the epidermis, is comprised of hard, flattened dead cells. Beneath this are living cells which are somewhat larger, and at deeper layers of the skin, the skin cells are larger and more round. At the bottom layer, there are new cells growing and pushing upward to the surface. As cells are pushed to the surface, they become flattened and lose most of their water content through pressure and dehydration.

Skin Layers

The Epidermis

The epidermis is the thinnest skin layer at a maximum 1 millimeter or as thin as a pencil line. It is thickest on the soles of the feet and the palms of the hands and its thinnest on our eyelids. The epidermis also produces the hair, toenails and fingernails. The epidermis consists of three interwoven types of cells: the which make the protein keratin, the melanocytes which produces the suntanning pigment melanin (which protects us from ultraviolet radiation and determines your skin color), and the Langerhans cells which are part of the immune system and intercept foreign substances that try to pass through the skin.

The epidermis itself forms four distinct layers with an average thickness of 0.25 mm - the top outer layer called the corneal layer and made from dead, hard, tough cells that form the hard skin surface. The three lower layers are called the stratum germinativiumwhich, from bottom to top, are the basal layer, which produces millions of new cells at the bottom of the epidermis every day. Squamous cells lie just below the hard outer surface of the skin. The new cells start as soft, columnar cells then undergo a differentiation process and are transformed into the hard, flat cells of the outermost layers as they are pushed to the surface.

The most important protective substances in the outer layer of skin are the keratin proteins and the skin lipids. The outer layer is a formation of 15-20 layers of horny cells (like the horns of cattle) that are embedded in a matrix of skin lipids. Every several days these horny cells is sloughed off layer by layer and replaced by new cells from below.

The epidermis has numerous nerve endings that make the skin a sensory organ which detects warmth, cold, light, taste and touch. Your skin shows emotions such as when fear causes the skin to grow pale and embarrassment causes a red blush.

The Dermis

The dermis is a thick, supple and sturdy layer of connective tissue and makes up about 90 percent of the skin's thickness. The dermis a dense meshwork of collagen and elastin fibers, two connecting proteins. This meshwork supports tiny lymph and blood vessels that allow the skin to breathe and be nourished as well as the nerves, muscle cells, sweat and sebaceous glands, and hair follicles. This layer contains the special cells that repair the skin, such as the fibroblasts that synthesize the skin proteins like collagen and elastin.

The sebaceous glands make the special oils for the skin and hair. Normal functioning of the sweat and sebaceous glands is essential for healthy skin. Together these glands provide the "acid mantle", the natural covering which protects the skin. The sebaceous glands in this layer are found at the base of each hair follicle. They secrete an oily substance called sebum which waterproofs the hair and lubricates the skin. Overproducing sebaceous glands, which often occurs in adolescence, leads to the formation of blackheads and pimples.

The dermis also contains the sweat glands (which are vital to cool the body) plus apocrine glands of the armpits, the ear canals, the nipples, and around the genitals that secrete chemicals called pheromones. Pheromones act as smells that increase interpersonal bonding and sexual attraction. At the lower (inner) portion of the dermis the hair follicles are anchored. The dermis is the skin layer that absorbs most of the substances that penetrate into the skin.

The Subcutis and Subcutaneous Adipose Tissue

The subcutis is the deepest layer of the skin, composed primarily of fat. The subcutaneous layer manages the skin's functions of feeding, excreting and heat exchange. The key cells are fat cells or adipocytes that provide energy, serve as a heat insulator for the body, and act as a shock absorber to protect underlying tissue against mechanical trauma and helps give your skin its resilience. Among mammals, only humans and marine mammals such as whales and dolphins have this subcutaneous layer of fat. Sweat glands originate in this layer and excrete waste matter through perspiration. This sweat controls the body's temperature by evaporating and cooling the skin surface. "Goose-bumps" occur when the fine layer of muscles found in this layer contract.

Understanding Your Skin: Firmness, Elasticity and Moisture

Keeping Your Skin Young and Beautiful