The Dermis

The dermis is where the action is functionally. The dermis contains sweat glands, hair follicles (each with its own tiny little muscle so that your "hair can stand on end"!), nerve endings and so on. There are several different types of nerve endings:

• Heat sensitive
• Cold sensitive
• Pressure sensitive
• Itch sensitive
• Pain sensitive

All these different nerve endings let you sense the world. They also help you protect yourself from burns, punctures and the like by warning you when something is damaging your skin.
The epidermis is your interface to the world, and it is actually quite interesting. It has two main layers, the inner of which is living and the outer of which is dead. The dead skin cells of the outer layer are what we can actually see, and they are constantly flaking off and being replaced by new cells being pushed outward.

The living, inner layer is called the malpighian layer. The malpighian layer creates the dead cells that we can see. It is in direct contact with the dermis, which feeds and supports it. The malpighian layer is our focus of attention actually, because it is here that the sun affects the skin during tanning. The malpighian layer is itself layered like this:

• In direct contact with the dermis is the basal layer. If you have ever heard of a basal cell carcinoma (cancer), this is where it starts.
• Above the basal layer is the spinous layer.
• Above the spinous layer is the granular layer.

Above the granular layer is the stratum corneum. The stratum corneum is the outer layer of dead cells -- the cells that we see as our skin. The cells in this layer are filled with a protein called keratin. Keratin is a very interesting protein because it is tough -- horns, hair, hoofs, fingernails and feathers all gain their strength from keratin. The same stuff that your fingernails are made of actually forms your visible skin (but in a much thinner and more flexible layer). That is what makes your skin so tough. In parts of the body that get a lot of wear, like the palms and the feet, the stratum corneum is thicker to handle the abrasion.
Living among the basal cells in the malpighian layer is another type of cell called a melanocyte. Melanocytes produce melanin, which is a pigment that is the source of tanning. The melanocytes are actually where a tan comes from. Here is what the Encyclopedia Britannica has to say about melanocytes:

"The appearance of the skin is partly due to the reddish pigment in the blood of the superficial vessels. In the main, however, it is determined by melanin, a pigment manufactured by dendritic cells called melanocytes, found among the basal cells of the epidermis. Their numbers in any one region of the body, which range from about 1,000 to more than 2,000 per square millimetre, are roughly the same within and between races; the blondest whites have as many as the darkest blacks. Colour differences are due solely to the amount of melanin produced and the nature of the pigment granules. When the skin becomes tanned on exposure to sunlight, the melanocytes do not increase in number, only in activity." ("Integumentary Systems, Pigmentation", Britannica CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)

Not only do melanocytes produce a tan, they are also responsible for the form of cancer called melanoma. Melanoma is caused by UV radiation damage to melanocytes. Repeated exposure to UV can cause cancerous mutations.

By Marshall Brain (HowStuffWorks.com)