Final answer:
Melanin is the pigment responsible for the color of hair and skin, and it is produced by melanocytes. Exposure to UV rays leads to increased melanin production, which can darken the hair.
Step-by-step explanation:
When considering the darkening of hair through repeated applications, we can look at the role of melanin in this process. Melanin, which is the pigment responsible for the color of hair and skin, is produced by melanocytes. These cells are found in the hair papilla and in the stratum basale of the epidermis.
Melanin comes in two primary forms: eumelanin, which provides black and brown colors, and pheomelanin, which provides red coloration. Hair gets its color from the type and concentration of melanin that is genetically determined. Exposure to UV rays from the sun or a tanning salon can cause an increase in melanin production.
This increase is the body's natural way to protect the DNA of epidermal cells from UV damage and the breakdown of folic acid. Melanocytes in the skin produce melanin, which is then transferred to keratinocytes. Hair and skin color variations among individuals result from different levels of melanin production.