When the skin becomes tanned on exposure to sunlight, the melanocytes do not increase in number, only in activity." ("Integumentary Systems, Pigmentation",CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)
Melanocytes are also responsible for the form of cancer called melanoma. Melanoma is caused by ultraviolet û @UV û radiation damage to melanocytes. Repeated exposure to UV can cause cancerous mutations.
In non-Caucasian races, melanin production is continuous, so the skin is always pigmented to some degree. In these races the incidence of skin cancer is much lower because cells are constantly protected from UV radiation by melanin.
Melanocytes actually produce two different pigments: eumelanin (brown) and phaeomelanin (yellow and red). Redheads happen to produce more phaeomelanin and less eumelanin, which is why they don't tan very well. In albinos, the chemical pathway that produces melanin does not function because an enzyme called tyrosinase is missing. Therefore albinos have no melanin in their skin, hair, or irises.
So now we can begin to understand both tans and sunburns. When you get a tan, what is actually happening is that the melanocytes produce melanin pigment in reaction to the ultraviolet light present in natural sunlight. Ultraviolet light stimulates melanin production. Melanin pigments absorb the UV radiation in sunlight, effectively protecting the cells from radiation damage. Melanin production takes a fair amount of time, though, which is why most people cannot get a tan in one day. You have to expose yourself to UV for a short period of time to activate the melanocytes; then, they produce melanin over the course of a number of hours. By repeating this process over five to seven days, the pigment builds up in your cells to a protective level.
Melanocyte-stimulating hormone (or @MSH) is produced by the pituitary gland. MSH flows through the bloodstream and reaches the melanocytes, encouraging them to produce more melanin (for example, a person injected with a large dose of MSH will get darker). The pituitary gland is tied into the optic nerve, which means that it can sense light. The more light your eyes see, the greater the activity of your pituitary gland. If you have ever raised chickens for eggs, you know that in the winter egg production falls way off. You solve this problem by providing light in the chicken coop. The extra light stimulates the pituitary gland in chickens, which causes the gland to produce a hormone essential to egg laying.
