SURVIVAL OF PATCHWORK MELANOBLASTS IS DEPENDENT UPON THEIR NUMBER IN THE HAIR FOLLICLE AT THE END OF EMBRYOGENESIS

The recessive patchwork (pwk) mutation in mice is associated with a un ique hair follicle phenotype. Mice homozygous for patchwork exhibit a variegated coat containing a mixture of white and fully pigmented hair s, but no partially pigmented hairs. We have investigated the etiology of this mutation. We report here that the white hairs result from the lack of melanocytes in the follicle. As indicated by the coat color p attern of patchwork tt albino chimeras, the target cell for the patchw ork mutation is the melanocyte and/or its precursor. Examination of th ese chimeras also suggested that patchwork does not act in a cell-auto nomous manner. The colonization of the skin by melanoblasts in patchwo rk embryos was studied using a lacZ transgene. Melanoblasts die by apo ptosis in hair follicles from homozygous pwk/pwk fetuses starting at e mbryonic day 18.5, indicating that patchwork acts from this stage. The combination of pwk and Kit(W-ei), a mutation responsible for a reduce d number of melanoblasts in the hair follicle, suggested that pwk gene product is necessary for low numbers of melanoblasts to survive and d ifferentiate in the hair follicle from embryonic day 18.5 onward. We c onclude that the pigmented hairs on the coat of pwk/pwk mice may be at tributed to a community effect among melanoblasts in the hair follicle at the end of embryogenesis. (C) 1998 Academic Press.