Flawless skin is a thing of beauty. We coddle it, we nourish it, we try to improve it. Yet, we regularly dis it as "only" skin, misunderstood and undervalued. It's a shame. Man has never made anything better as sensor, shield, and communicator.

Skin is the great protector. Its outer layer, the epidermis, is thinner than Saran Wrap; it is stain-resistant and waterproof. Tightly woven epidermal cells form a sturdy barrier to hold moisture in as well as keep unwanted water out. On the surface, dead, compacted, and sloughing cells add toughness, a kind of see-through coat of armor. With a cleverness the military would envy, the epidermis brims with stem cells ready to spin out reinforcements as needed, and pigment-producing melanocytes to deflect skin's No. 1 enemy--the sun. Its rays are especially damning to the skin's middle layer, the dermis.

Ah, the dermis. Suffused with collagen, the dermis brings firmness--and when collagen is broken down, wrinkles and sags. A layer of subcutaneous fat lies below, softening skin texture. Lacing through the skin are blood vessels, hair follicles, sweat and oil-producing glands, all bathed in a plasma soup of chemical messengers, hormones, and roaming white blood cells. Amid all this, abundant neural connections run to and from the brain and other organs. But finely tuned sensory nerve endings scan and process our surroundings: We caress softly the rose petal but recoil from the pain of its nearby thorns.

Indeed, the skin is a powerful interface between the mind, the body, and the external world. The emerging concept of a neuro-immuno-cutaneous-endocrine network recognizes the skin as an almost independent, untamable intelligence. Think of the blush that inadvertently reveals the mind's secrets. The goose bumps that warn when something is amiss. Or the crawling skin that shows your fear. The skin also has a rich life as an endocrine organ, manufacturing hormones like vitamin D for the rest of the body, and steroids and thyroid hormone for its own use. Though it's not always clear why, the skin makes many of the neurotransmitters and hormones found in the brain.

Critical outpost. This mind-skin interchange is more than meets the eye. For example, the brain can mysteriously make you itch, without an external cause to scratch or swat. And emotional stress can interfere with the protective functions of the epidermis or can activate immune or inflammatory reactions deeper within. The skin is also a critical outpost of the immune system, laden with specialized white cells that gobble up invading microbes and trigger a bodywide immunologic response. So effective is the skin in this way that researchers are challenging the age-old practice of vaccinating people with a needle directly into muscle--which lacks this immunologic power--proposing instead a far less painful prick of the skin, more akin to a tuberculosis test. It seems to do the job as well or better--at a fraction of the dose.

Skin is gender-sensitive as well. Though men and women have similar skin, some sex differences leave women at a distinct disadvantage. Perhaps in an evolutionary throwback to a time when women nested while their hunter-gatherer men braved the outdoors, women's skin is less prepared to brace the elements, being thinner than men's and less oily. Since thinner, drier skin is more prone to damage from the sun or the smoke of cigarettes, women so exposed are more apt to wrinkle. Women also sweat less than men do and are thus more likely to suffer heat stroke. Indeed, the frilly parasols of heavily clad Victorian ladies did double duty by imparting a cooling shade as well as sun relief.

Sex hormones shape skin, too. Estrogen increases collagen and skin moisture and promotes wound healing, while testosterone stimulates oil production and facial hair. Men enjoy both hormones, since skin is able to convert testosterone to estrogen. Women, too, benefit from both, since ovaries produce a small amount of testosterone. Until menopause, that is, when loss of sex hormones accentuates the crow's feet. Estrogen-deprived skin thins, loses collagen, and slows down its cell renewal. Hormone replacement softens the blow, but occasional risks can outweigh benefits.

Some bemoan the lack of skin research in studies of menopause. Wulf Utian, head of the North American Menopause Society, thinks that's because skin problems are not seen as life threatening, "forgetting, of course, the quality-of-life issues." That may change as the importance to our well-being of this largest of all organs gets under more people's skin.

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