Tanning

 

 

 

Melanin composition is a variable mixture of lighter alkali soluble, sulphur containing pheomelanin and darker insoluble eumelanin; skin types that tan well have more eumelanin. Exposure to UVA induces a rapid but transitory grayish color known as immediate pigment darkening (IPD) which probably results from the photo-oxidation of existing melanins and the redistribution of melanocytic melanosomes from a perinuclear position into the peripheral dendrites. IPD progresses into persistent pigment darkening (PPD), which may remain stable for up to 2 hours post exposure. The biologic functions of IPD and PPD are not known.

Delayed tanning, also known as melanogenesis, is primarily a UVB response that results from increased activity and numbers of melanocytes. This becomes visible 3 to 4 days after UVR exposure and is maximal from 10 days to 3 to 4 weeks depending on complexion and UVR dose. Melanocyte tyrosinase activity also increases, melanocyte dendrites elongate and branch, and melanosomes increase in number and size. UVA tanning has distinct effects that are wavelength dependent. Thus, UVAI produces an increased melanin density localized to the basal cell layer, whereas UVAII increases the synthesis and transfer of melanized melanosomes to the keratinocytes throughout the epidermis, similar to the changes seen with UVB.