|MadSci Network: Anatomy|
This is a great question…. The friend who told you its due to pigmentation in the skin is correct (they do NOT come from dirt in the skin).
Moles originate from pigmented cells called melanocytes. Almost everyone has moles, at least a couple. Moles usually develop in childhood or in the teen years of life although some people keep getting moles during their entire lifetime and a few people are born with them. Moles are usually brown in color (due to the melanin pigment) and are a variety of sizes and shapes, can be flat or raised from the skin, and can even contain body hairs. Moles may darken when pregnant, exposed to sunlight, or when undergoing certain medical treatments. It is now thought that many moles develop due to sun exposure (but certainly not all).
From the Merck Manual:
Moles usually are easily recognized by their typical appearance. They do not itch or hurt, and they are not a form of cancer. However, moles sometimes develop into or resemble malignant melanoma, a cancerous growth of melanocytes. In fact, many malignant melanomas begin in moles, so a mole that looks suspicious should be removed by a doctor and examined under a microscope. Changes in a mole—such as enlargement (especially with an irregular border), darkening, inflammation, spotty color changes, bleeding, broken skin, itching, or pain—are warnings of malignant melanoma. People with more than 10 or 20 moles have a somewhat increased risk of melanoma and should be checked every year by a doctor. If a mole proves to be cancerous, additional surgery may be needed to remove the skin surrounding it. *Most moles, however, are harmless and do not require removal. Depending on their appearance and location, some moles may even be considered beauty marks.*
Online references: http://www.merck.com/mmhe/sec18/ch215/ch215b.html
Hope this was helpful.
Brigham and Women’s Hospital, Boston, USA
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