What is the epidermis?

Human skin is composed of several layers. The epidermis is considered the very top or outer layer of skin. This layer is visible to the human eye and acts as a protective coating. Its major function is to protect us from influences of the outside world.

The skin areas that are closest to the light above are also known as the layers of the epidermis. The epidermis has a love-hate relationship with the outside world: It pulls in water, heat, and light, and pushes away bacteria, dirt, germs, and toxins.

• Keratinocytes are the primary players here. These plump cells form at the base of the epidermis and flatten out as they rise to the surface, die, and eventually flake off. Ironically, these dead cells, collectively known as the stratum corneum, make up what we call our skin -- the birthday suit we touch, wash, dry, pamper, and protect (even if we sometimes pierce or tattoo it).
• Keratinocytes produce keratin, which is the same tough protein that’s in hair and fingernails; on the skin’s surface, it helps form a roadblock against would-be intruders. - - Tough-guy immune system forces called Langerhans’ cells are located here too; they detect foreign substances. More than that, recently these cells have gained greater recognition as extraordinary players in immunity. In late 2005, researchers at Yale School of Medicine demonstrated that Langerhans’ cells in the skin, which had been thought to alert the immune system to invaders, instead dampen the skin’s reaction to infection and inflammation. We now view these cells not just as sentinels or stimulators of immune reactions as previously thought, but more as environmental peacekeepers. The skin is constantly challenged by the environment, but most challenges are not dangerous and do not warrant an immune response. Your Langerhans’ try to keep the peace before warranting a full-fledged response.
• Melanocytes churn out melanin, which determines the color of your skin, that is, whether you’re dark or pale. It’s the pigment that protects skin from too much ultraviolet light by darkening it after repeated UV exposure, at least in some people. Some types of melanin (Irish-Americans know this all too well) are simply too light to provide any UV protection.

From The Mind-Beauty Connection: 9 Days to Less Stress, Gorgeous Skin, and a Whole New You by Amy Wechsler.

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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 top layer of the skin, the epidermis, is the waterproofing layer. It keeps everything on the outside from entering the body and it keeps you from drying out. It contains fatty acids that are natural antibiotics, killing bacteria and preventing them from invading the body. It is also a barrier to viruses and fungi. The outer layer of the epidermis is made up of dead cells that mechanically protect the skin. This layer thickens in areas that are chronically irritated, such as the hands of a construction worker.

When the epidermis is broken, as in a scrape or a burn, liquid oozes out and quickly clots into a scab. This scab is a mechanical barrier that keeps the tissue from drying out. It also helps keep germs from getting into the body and causing infection. Epidermis contains the brown pigment melanin, a natural sun block. Melanin absorbs ultraviolet light and prevents it from injuring the DNA.