Skin is the largest organ of the body, covering and protecting from outside attacks by physical trauma, chemical reactions, or microbial invaders. Skin of the average adult human exceeds 22 square feet yet is generally no thicker than 2 millimeters, or 1/16 of an inch. It regulates body temperature through sweating, detects information from objects by touch, and acts as a sun shield to protect the body from harmful ultraviolet rays while also initiating the process of vitamin D production during sun exposure.
Skin is comprised of two major tissue layers, the epidermis and dermis. The epidermis contains three types of dendritic cells: melanocytes, Langerhans, and Merkel cells. Melanocytes produce skin’s primary pigment called melanin. There are two types of melanin, eumelanin, which is brownish or black, and pheomelanin, which is reddish or yellow. Both are synthesized from tyrosine, one of the 20 amino acids used by cells to synthesize proteins. Tyrosinase is the main enzyme responsible for increased melanogenesis, leading to conditions such as freckles, solar lentigines, acanthosis nigricans (dark discoloration in body folds and creases), melasma, hyperpigmentation, and skin cancers such as melanoma.
The number of melanocyte cells is consistent in all skin types regardless of their ethnicity; however, the number of active melanocytes that produce melanin pigment varies greatly. In other words, it is not the number of melanocytes within the epidermis that influence skin pigmentation, but the genetically pre-programmed activity of the melanocyte cells that produce individual skin color. Variations in genes can be traced to geographic origin and correlate to skin, hair, and eye color.
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