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You've already flipped to the chapter on Botox, haven't you? Did you check out the section on what to do for sagging skin and then come back here to the beginning? I don't blame you. Lasers, needles, and antiaging antidotes definitely sound more exciting than an anatomy lesson at first. But knowing how your skin works is not just fascinating (and I don't say that just because I'm a doctor), it is integral to understanding how to care for your complexion, to make it healthier, happier, and ultimately beautiful. So avoid the temptation to skip this section completely and move on to all the juicy stuff about fillers and high-tech products. Since knowledge is power, this is truly the empowerment chapter. With this foundation, everything that follows -- about protecting your skin, about cancers and other specific skin conditions, even about cosmetic procedures and products -- will make sense. You'll know why many of us have oily T-zones, why most substances can't penetrate into the skin, and why a tan is a bad thing.

After buying a new appliance, a television, or a computer, who actually reads the instruction manual? Usually we just wing it and then dig up the information later when something breaks down or a problem arises. These next few pages are the crucial part of the manual that shows you where everything is and how all the moving parts work. Knowing how your skin operates will help you understand why it reacts the way it does to things such as the sun, what we eat, how well we sleep, and what we put on it.

Try to forget all the boring diagrams and complicated scientific descriptions from your old high school textbooks. The daily workings of the skin are more akin to a thrilling adventure story, filled with bad guys (the sun, infections, and more), special agents (the cells), and high-tech communication systems. Though the skin may appear to be just a pretty, or sometimes problematic, wrapper for the body, there's a lot going on just beneath the surface.

With its intricate framework of layers, vascular system, different kinds of cells and glands, the skin is a miniuniverse. It's amazing the way the body communicates through the microstructures that are set up. When I read my kids Horton Hears a Who!, the Dr. Seuss tale of an elephant that discovers a fully functioning yet microscopic planet on a speck of dust, I'm reminded of the amazing, dynamic composition of the skin. If there were a city as sophisticated and efficiently run as your body, I'd want to live there.

Your body is constantly under attack from outside forces, and the skin is its greatest defender. It functions like Gore-tex, a high-tech outer sheath that protects us from temperature extremes, wind, and ultraviolet rays from the sun. It's a two-way barrier that not only retains water in the body but also acts as a water-resistant raincoat against too much water coming in. It's also our best shield against injury (bruises, cuts, and scrapes) and infection.

This incredible packaging contains us and keeps the body in balance by regulating temperature and providing insulation from heat and cold. For example, all the hair follicles are attached to tiny muscles that contract as a response to cold air, elevating hair on the body and trapping air around us, keeping the body warm like fur. When it's hot outside, cooling mechanisms kick into gear as the nervous system triggers the blood vessels and sweat glands to release heat as fast as possible. Overheated blood is pumped away from the heart and out to the skin, where the heat dissipates (which is why your skin becomes flushed). Sweating cools the skin by bringing moisture to the surface, where it evaporates in the air -- instant air-conditioning.

As if all this weren't enough, the skin has the added role of synthesizing vitamin D, which is necessary for the absorption of bonebuilding calcium. The nutrient can also be obtained from milk and vitamin D-fortified juice and foods, as well as salmon, sardines, and fish oil. (Vitamin D supplements aren't easily absorbed into the system, so they don't work as effectively.) Luckily, a more palatable primary source of the vitamin is the sun. Vitamin D is known as "the sunshine vitamin" not because the sun has the nutrient in it but because UV rays actually convert a chemical found in the epidermis into vitamin D.

There are three fundamental layers of the skin: the epidermis, the dermis, and the subcutaneous fat layer. There are four epidermal layers, the first of which is the stratum corneum. When you touch your skin, what you're feeling is almost thirty layers of dead keratin cells (a protein that also makes up hair and nails). All these inert cells, called keratinocytes, overlap like thin shingles on a roof, with pores (the ducts for hair follicles and sweat glands) interspersed among them. These tough keratinocytes shed approximately every twenty-eight days, depending on your skin's regenerative process. New cells that are formed in the lowest level of the epidermis push upward to the surface, constantly replacing the old ones. When your skin flakes, what's coming off are thousands of those dead cells. In fact, every minute we shed about forty thousand keratinocytes.

Just beneath the Gore-tex of the stratum corneum lies the brick wall of the epidermis. The "bricks" are squamous cells (durable keratinocytes that will eventually move up to the stratum corneum and be sloughed off) held together with rope-like bridges. The mortar is filled with fatty ceramides, which act as glue between the cells. Cells are "aquaphilic," meaning they allow water-soluble molecules to enter but won't let oil pass through them. Fatty ceramides are "lipophilic," allowing oil and fat substances to enter. This oil-and-water-don't-mix concept is one of the many barriers to substances (including most cosmetic ingredients) moving farther than this layer.

The first tier of the brick wall of the squamous cells is called the granular layer, where the soft keratin of the skin is made (as opposed to the hard keratin of nails and hair). Next is the spinous or squamous cell layer, the thick middle tier of the epidermis. Under that is the basal layer, where basal cells (which eventually become the keratinocytes on the skin's surface) generate. As the cells divide and mature, they move up, pushing older cells up to the surface, where they ultimately shed. When the basal cells reach the spinous layer, they are referred to as squamous cells. (In the skin, basal and squamous cells are both forms of keratinocytes; their names simply indicate where they are in the epidermis and in the cell maturation process.) As the keratinocytes reach the stratum corneum, they are cut off from the nourishment in the dermis and become a flatter, harder protein (keratin), until they die and flake off completely. This is the life cycle of the epidermis.

Beneath these elaborately woven levels of the epidermis lies the cement-like basement membrane, which glues the epidermis to the dermis. Lying under all those strata, the dermis seems as if it would be deeper down, but it's only one millimeter past the surface of your skin -- less than the thickness of your thumbnail! But as you can see, getting there is no easy feat. Just imagine, this is the place all the cosmeceuticals re trying to reach, and they have to get through all those impressive, highly sophisticated tiers, not to mention the cement of the basement membrane, to get there. It's like trying to reach a castle behind obstacles like multiple barbed-wire fences, high brick walls, and moats full of killer crocodiles -- a nearly impossible challenge. The dermis is where all the action is.

The dermis is where collagen and elastin are found, which is one reason it's the coveted destination for most of the active ingredients found in cosmetic products. In fact, the protein collagen makes up 70 percent of the dermis. It is a dense filler, much like the Styrofoam we use to pack fragile things. Elastin is connective tissue, the fine rubber bands, so to speak, that hold the foamy collagen in place so everything can flex and move. The beautiful scaffolds of collagen and elastin anchor the precious, vulnerable structures in the dermis in place and protect them from injury. Almost twice as thick as the epidermis above it, this rich layer is packed with nerve receptors that trigger sensation in our skin, blood vessels that transport oxygen and nutrients, sweat and oil glands, and hair follicles.

When we see pores on our skin, we're looking at either hair follicles or glands. The sebaceous, or oil, glands are attached to the hair follicles, which explains why the T-zone of the face, where hair follicles re the most dense, tends to get oily in most of us. The apocrine glands, which develop during puberty, are also attached to hair follicles in the genital area and the underarms and secrete sweat and body odor linked to sexual pheromones. Eccrine, or sweat, glands are spread out all over the body and help regulate temperature.

Just below the dermis is the aptly named subcutaneous fat layer, which covers the muscles. It provides shock-absorbent padding for the body and an insulating layer to conserve heat. The fat stored here also serves as an energy source. Coursing through this level are big ropes of collagen to keep the fatty tissue quilted in place and bigger blood vessels that feed the smaller vessels in the dermis above.

Like the bustling, invisible world of Who-ville, the universe of the skin is inhabited by millions of microscopic workers -- the cells. In biology the concept of "structure is function" is vital, and it's exquisitely illustrated here. The entire body is built like a Lego set, with each piece linking to others very specifically. Every cell's structure is designed for a precise purpose. It's an efficient lock-and-key system in which a certain action in the body is unlocked, or activated, by one particular kind of cell that has just the right key.

Melanocytes, for example, have the crucial job of producing melanin, the pigment that absorbs ultraviolet light from the sun. Ultraviolet wavelengths can damage or destroy the DNA in cells, causing mutations that can turn into cancers. Once the skin is exposed to sunlight, the melanocytes try to cover all the cells with melanin, like sun hats that block UV rays. Ironically, though most of us feel that a suntan is a beautiful sign of health, it's actually a visible response to skin trauma, since the pigment has been produced to shield our cells' DNA from danger.

Sensing any kind of danger in the skin, the Langerhans cells, located in the epidermis and the dermis, are like the air traffic controllers of the immune system. They send out special agents -- immune cells such as T cells and B cells -- to capture foreign invaders such as bacteria and viruses, and fight off injuries like cuts and scrapes. They constantly monitor the environment of the skin for unsafe situations and send immune cells on their missions to bring back information about any trespasser. Then the body can decide to amass a great force of inflammatory cells to fight off the attacker by creating an allergic reaction or forming scar tissue.

Housed inside the hair follicle are stem cells, "pluripotential" cells that have the capacity to develop into any kind of cell they're needed to become. They can turn into collagen, skin, or immune cells if necessary, but once they have specialized they can't change again. Stem cells are similar to wild cards played in a game of poker, able to be converted into whatever incarnation will serve us best. In fact, scientists are now investigating the use of skin stem cells as an alternative to human embryonic stem cells in research that has the potential to cure illnesses such as Alzheimer's and Parkinson's and many different cancers. So the skin, this amazing and dynamic organ, will have latent possibilities in exciting medical research that promises to save lives in the future.

The fibroblast cells' extremely important function is to make collagen and elastin (the darlings of the cosmeceutical world). The skin's healing process is driven by these cells, which race to repair skin by making a scar or replenishing collagen to rebuild the structural integrity of the skin. It's probably no surprise to learn that as we age these cells get lazy and slow down production of this coveted protein. But we'll get to all the cold, hard facts about aging skin a bit later.

For now, ponder these numbers: the average square inch of skin (about the diameter of a quarter) can have approximately 65 hair follicles, 650 sweat glands, 20 blood vessels, more than a thousand nerve endings, and 60,000 melanocytes.

Our skin is a miniuniverse indeed, and one we should treat with quite a bit of wonder and complete consideration.

Copyright © 2009 by Dr. Ellen Marmur