These melanocytes or “melanin factories” are located just below a layer of surface skin cells called keratinocytes.
1. Activation Phase (StageⅠ-StageⅡ)
·The melanocyte stimulating hormone (MSH) activates melanin production.
·MSH binds to a hormone receptor on the melanocyte.
·This binding activates the melanocyte to turn on the synthesis of melanin.
In the initial phase (activation) of the pigmentation process, a hormone initiates melanin production within the melanocyte. Other factors such as stress, hormones, and UV damage can also initiate melanin production.
2. Synthesis Phase (StageⅢ)
Three main chemical reactions exist within the synthesis phase:
·Tyrosinase, an enzyme essential to melanin synthesis, functions to convert tyrosine, an amino acid, to another amino acid called DOPA.
·Tyrosinase then converts DOPA into dopaquinone, a secondary chemical.
·Dopaquinone, in a final reaction, is converted into one of two types of melanin: light melanin (phaeo-melanin) or dark melanin (eu-melanin).
All three reactions occur inside the melanosome located within the melanocyte.
Light melanin (phaeo-melanin) is produced when the environment is abundant in certain amino acids. Dark melanin (eu-melanin) is produced in the absence of these amino acids.
In the production phase (synthesis), melanocytes produce melanin inside an even smaller factory called a melanosome. After a series of complex chemical reactions, the melanosomes are full of melanin and ready for delivery.
3. Expression Phase (Stage Ⅳ)
Expression is the process through which “packaged” melanin is brought to the surface of the skin and made visible to the eye.
·Once melanin has been produced and packaged into melanosomes, the melanocyte initiates a delivery process and pushes the melanosomes out to the ends of its numerous dendrite-like arms.
·As melanosomes reach the ends of these dendrite-like arms, they are transferred from the melanocyte to the adjacent layer of keratinocytes.
·These melanin-filled keratinocytes then make their way to the surface of the skin and melanin becomes visible.
In a final distribution phase (expression), melanosomes are transported to the keratinocytes and to the surface of the skin where the melanin is visible to the naked eye.