The color of a person's skin is due to the manufacture and deposition of a pigment called melanin, which is produced in two forms: pheomelanin (a yellowish pigment) and eumelanin (a dark brown/black pigment). Both of these types of melanin are manufactured by melanocytes—cells in the skin located many layers below the surface at the bottom part of the epidermis. Melanocytes express a unique enzyme, tyrosinase, which functions to convert the simple amino acid tyrosine into melanin. To make and disperse this pigment throughout the skin, melanocytes use a three-step "manufacturing" process:
Activation—chemical signals coming from other cells in the skin cause the melanocyte to "activate" tyrosinase and begin melanin synthesis.
Synthesis—melanin is manufactured, "packaged" into elliptically shaped containers called melanosomes, and relocated to the outer regions of tentaclelike projections of melanocytes called dendrites.
Expression—melanin-filled melanosomes are transferred from the ends of dendrites to keratinocytes (surrounding cells in the skin) for shipment to the surface of the skin.
Every day, keratinocytes move from the bottom region of the epidermis to the surface of the skin. If they have picked up melanosomes, keratinocytes carry them to the skin's surface where the melanin in the melanosomes gives the skin a pigmented appearance. In spite of the large variations in skin color that exist among people from different parts of the world, everyone has exactly the same number of melanocytes in their skin. The difference in the amount of melanin in a person's skin is due primarily to (1) how active the melanocytes are, (2) how much melanin they make, and (3) how efficiently they transport melanin-filled melanosomes to keratinocytes throughout the skin.
There are many factors, environmental and other, that determine the activation state of melanocytes in the skin and, therefore, how much melanin will be made. Of course, one of the most well-known factors that activate melanocytes is UV radiation from the sun or tanning beds. Exposure to UV light leads to the production of a large number of inflammatory mediators and hormones in the skin, which then activate melanocytes by binding to specific receptors on these cells. The end result of this activation event is increased melanin production—commonly called tanning. While other environmental stressors that cause the skin to produce inflammatory mediators can lead to increased skin pigmentation, factors such as age, pregnancy, and birth control pills can also cause increased melanin production. This enhanced skin pigmentation is often uneven and blotchy and tends to get worse over time.
With the knowledge of how skin pigmentation is regulated, it should be possible to develop topical products that can effectively control the level of melanin produced and deposited in the skin. Research has shown that certain ingredients can help slow the production of stressors or inflammatory mediators in the skin while others can target the ability of inflammatory mediators and hormones to activate melanocytes. Certain ingredients also work to (1) minimize the activity of tyrosinase, (2) discourage the transfer of melanosomes to keratinocytes, and (3) influence the type of melanin made to be the lighter, less visible pheomelanin instead of the highly visible brown/black eumelanin. By targeting multiple steps along the melanin synthesis pathway, these ingredients provide more effective control of skin pigmentation.
Bryan B. Fuller