Characterization of the melanocyte lineage in patchwork hair follicles

Mice homozygous for the patchwork (pwk) recessive mutation are salt and pepper: their coat contains a mixture of unpigmented and fully pigmented hairs, but no partially pigmented hairs1. In the matrix of pwk/pwk unpigmented hair follicles, there are no mature melanocyte. This phenotype could be due to: (i) absence of the melanocyte stem cell (MSC) population, (ii) impaired differentiation of MSC into transit amplifying cells (TA cells), (iii) inhibition of TA cells proliferation and/or migration, or (iv) death of melanoblasts within the proliferative compartment. To discriminate between these hypotheses, we used transgenic mice expressing reporter genes for the melanocyte lineage, namely Dct-lacZ and Pax3GFP 2. We validated the Pax3GFP reporter strain by comparing its labelling of melanocyte lineage to Dct-lacZ’s. In pwk/pwk hair follicles, whatever the hair color, MSC and TA cells are present though in reduced number in the bulge and in the transitory portion respectively. In pwk/pwk unpigmented hair follicles, some TA cells reach the bulb region where they remain undifferentiated. By contrast, in the bulb of pwk/pwk pigmented hair follicles, TA cells are more numerous and differentiate into functional melanocytes. To test whether the impaired differentiation in the unpigmented hair follicles is related to the lower number of melanoblasts, we dissected pwk/pwk; Dct-lacZ/Dct-lacZ or pwk/pwk; Pax3GFP/+ and control hair follicles and counted cells of the melanocyte lineage within single hair follicles. Which process (survival, proliferation and/or migration) is deficient in the transit amplifying population is under investigation