The first grey hairs are typically visible around theage of 34. This inevitable and natural phenomenon is called "canities". It is characterised by a discolouration of the hair, which turns white over time. We are not all equal when it comes to the onset of grey hair. Numerous factors impact the age at which grey hair appears, and it happens that some people see their hair turn white as early as their twenties, while others still have naturally coloured hair into their fifties.
From birth, melanocytes, located at the base of the epidermis, synthesise melanin, the pigment that controls hair colour. The melanin is then transferred to the keratinocytes via melanosomes. Keratinocytes are the cells that surround the dermal papilla of the hair follicle. The distribution of melanin in the keratinocytes determines the colour of the hair. However, the activity and number of melanocytes decrease gradually over time. Therefore, the amount of melanin synthesised becomes weaker, which causes the hair to turn white.
Furthermore, the process of hair growth involves the synthesis of hydrogen peroxide as a by-product. This reactive species, capable of triggering a chain reaction leading to the degradation of melanin, is normally naturally degraded by an enzyme called catalase. However, over time, the amount of catalase in the cells decreases, which promotes the degradation of melanin by hydrogen peroxide.