Pigmented hair follicles Gray hair follicles Unpigmented hair follicles A Melanocytes in the bulge region (stem cell niche) +++ ++ + B Pigment dilution in hair shaft 0 +(+) +++ C Differentiated melanocytes below Auber’s line (stem cell niche?) 0 ++ + D Rounding of hair bulb melanocytes 0 + +++ E Dendricity of hair bulb melanocytes +++ +(+) (+) F TUNEL+ melanocytes 0 + 0 G Oxidative stress in pigmentary unit + + 0 H Hair shaft elongation in culture + Not determined ++ I Selective melanocyte susceptibility to exogenous oxidative stress +++ Not determined Not determined

Pigmented melanocytes detectable in intact isolated hair follicles with the help of an inverted microscope (A, D, E), or melanocytes labeled with NKI-beteb (B–E) and/or TUNEL (F) or 8OHdG (G) were analyzed. Arbitrary units were assigned to the following criteria: melanocytes in the bulge region >4 = +++, 2–4 = ++, 1 = +; pigment dilution in hair shaft: no dilution = 0 to no pigment = +++; melanocytes below Auber’s line: + = 1–2, ++ = 3–4, +++>4; rounding of melanocytes: 0 = multidendritic cells, +++ = fully rounded cells; dendricity of hair bulb melanocytes: +++ = multidendritic to 0 = non-dendritic. A total of 90 hair follicles derived from 10 different donors was included into the analysis. Presence of TUNEL+ or 8OhdG+ melanocytes in the pigmentary unit of graying individuals (full-thickness scalp-skin biopsies of 20 different donors) was summarized as follows: + = present, 0 = not present. Hair shaft elongation in cultured hair follicles of the same donors (20 different donors, each donating 9 hair follicles per group) was summarized as follows: + = present, ++ = significantly larger. Susceptibility of hair bulb constituting cells in pigmented hair follicles to exogenous oxidative stress (hydroquinone) was summarized as follows: +++ = selective susceptibility of hair bulb melanocytes.