STEEL AND C-KIT IN THE DEVELOPMENT OF AVIAN MELANOCYTES - A STUDY OF NORMALLY PIGMENTED BIRDS AND OF THE HYPERPIGMENTED MUTANT SILKY FOWL

We describe here the expression of c-kit and Steel (Sl) genes during t he development of melanocytes in normally pigmented strains of chick a nd quail compared to unpigmented (White Leghorn) and hyperpigmented (S ilky Fowl) strains of chickens. By using the quail/chick chimera syste m, we found that the neural crest cells, which migrate dorso-laterally in the subectodermal mesenchyme to give rise to the melanocytes, expr ess c-Kit as early as E4, that is about 2 days after they have left th e neural primordium. The Sl gene is expressed from E4 onward in the ep idermis but not at all in the dermis at any developmental stage. As fe ather buds develop, Sl mRNA becomes restricted to the apical region of the feather filaments. During formation of the barbs and barbules of the down feather, production of the Steel factor is restricted to the external epidermal cells of the barbules. The cell bodies of the c-kit -positive melanocytes are then located in the internal border of the e pidermal ridges and extend their processes toward the source of the St eel factor. We propose that the spatial restriction of Sl gene activit y at that stage accounts for the morphology of the melanocytes and the ir vectorial secretion of melanin to the external barbule cells. As a whole, these results show that during skin development c-kit positive cells are present in the Steel factor-producing areas at the time when melanoblasts proliferate and differentiate. Interestingly, in the mou se, previous studies showed that the Sl gene is activated in the dermi s where melanoblasts undergo most of their expansion (Nishikawa et al. [1991] EMBO J. 10:2111-2118). In the unpigmented and hyperpigmented m utants that we studied, expression of the Sl message, as judged quanti tatively in Northern blots (for the SF embryos) or spatially by in sit u hybridization, is similar to that observed in normal birds. In SF em bryos the c-kit expressing melanoblasts migrate initially in the dorso -lateral migration pathway as in normal birds. However their number in creases considerably in the dermis from E5 onward. From E7, they invad e mesodermally derived organs that do not express the Sl gene. This su ggests that another, still unknown, factor(s) is responsible for the s urvival, the proliferation, and the extensive spreading of melanocytic cells within the mesoderm of this mutant. (C) 1995 Wiley-Liss, Inc.