Melanin is an amino acid-derived biological pigment, mainly synthesized by melanocytes, and the amount of synthesis is not only regulated by genetic information, but also influenced by a variety of internal and external factors, often considered to be related to ultraviolet light, age, hormonal changes, inflammation and other factors: 1. Ultraviolet light: closely related to the synthesis of melanin. If the human body is exposed to sunlight for a long time, UV radiation can cause increased melanocyte activity, which leads to increased melanin biosynthesis and darkening of the skin; 2. Age: With age, the body’s metabolic capacity is poor, melanocytes will decline, and even some cells die. When the necrotic melanocytes keep accumulating, it will trigger pigmentation, i.e. senile pigmentation on the skin of the elderly; 3. Hormonal changes: If the human body has an imbalance of hormones in the body due to irregular life, the estrogen level will be significantly increased. And estrogen level is related to the synthesis of melanin, that is, estrogen acts through its receptors, thus promoting the proliferation of melanocytes, resulting in abnormal melanin secretion; 4, inflammation: patients whose skin is infected and allergic due to inflammation, after healing, pigmentation usually occurs. This is mainly due to the fact that inflammation products can stimulate melanocytes, which in turn leads to an abnormal increase in melanin synthesis, resulting in epidermal pigmentation.