The keratinocytes in the stratum basale and stratum spinosum are unique in being not only the primary source of vitamin D for the body, but in possessing the enzymatic machinery to metabolize vitamin D to its active metabolite 1,25(OH)2D(Bikle 2011)
Previtamin D3 is synthesized from 7-DHC in the skin after exposure to UVB between wavelengths of 270 and 300 nm in keratinocytes(2017b). Once previtamin D3 is formed in the skin, it undergoes a temperature-dependent thermal isomerization that takes at least 3 days to complete. The vitamin D-binding protein preferentially translocates the thermal product, vitamin D3, into the circulation(Holick et al. 1980).
Bikle 2011. Vitamin D metabolism and function in the skin. Molecular and Cellular Endocrinology 347(1–2), pp. 80–89. doi: 10.1016/j.mce.2011.05.017.
Holick et al. 1980. Photosynthesis of previtamin D3 in human skin and the physiologic consequences. Science 210(4466), pp. 203–205. doi: 10.1126/science.6251551.
2017b. Vitamin D and Skin Health | Linus Pauling Institute | Oregon State University. Available at: http://lpi.oregonstate.edu/mic/health-disease/skin-health/vitamin-D#reference11.