The living, inner layer of the skin 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.