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There are special cells in the skin called melanocytes. They synthesize
melanin from an amino acid, tyrosine. (Amino acids make up proteins; there are about
20 different ones). Melanocytes can be stimulated by a hormone in the pituitary gland
called melanocyte stimulating hormone (MSH). I don't know how much biology you've had,
but melanocytes are derived from an interesting embryonic tissue called the neural crest,
which also gives rise to a lot of different types of neurons, so embryologically
melanocytes are related to neurons. If melanocytes become malignant, it becomes a very
bad form of cancer, called melanoma (often called "skin cancer", although there are
other forms of skin cancer).
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