Abstract
At the time of hatching, pigment is restricted to black spots on a white background in the Ancona in comparison to the fully pigmented pattern of the Brown Leghorn, which serves as a standard of reference for feather coloration. During successive molts in the Ancona, the pigmented areas are extended until every feather follicle is producing pigment in the adult. Retarded migration of pigment cells is the simplest hypothesis that accounts for the development of the transitory piebald spotted pattern of the Ancona. That hypothesis was tested in the present investigation. Migration of melanoblasts in Ancona and Brown Leghorn embryos was traced at critical stages from stage 20 to time of hatching by transplanting blocks of tissue from different levels between the neural crest and the tip of the wing bud to White Leghorn host embryos. Results indicated that melanoblasts began to invade the wing buds of Brown Leghorn embryos at stage 21 and that all wing buds had received them by stage 23. Melanoblasts rarely invaded the distal 0.5 mm of the wing bud until after stage 32 in the Brown Leghorn. However, by stage 36 even the most distal primary follicle had received melanoblasts. Results for the Ancona were not as consistent. Although melanoblasts began entering the wing bud in stage 22, some of the wing buds still had not received them even by stage 24. Pigment cells apparently had not advanced beyond primary follicle no. 2 by stage 36 in the Ancona. Differentiated pigment granules and pigment deposits were observed in the skin overlying the mesencephalon of Ancona embryos at stages 20 and 23, respectively. There was no evidence of pigment granules or deposits in the same region of Brown Leghorn embryos at stage 23. Precocious differentiation and retarded proliferation of pigment cells appear to be correlated in the Ancona. At the time of hatching, the down and skin of the wing of the Ancona are pigmented in two spots, one having its center located on the dorsoposterior side near secondary follicle no. 8 and the other located just proximal to the base of digit II. In 8 cases where more than one graft was taken from individual Ancona wing buds, the results indicated that very few primordial melanoblasts migrated into the wing buds, possibly only one to the center of each spot. As a result of this study the following proposals are made. Each pigmented spot in the wing of the Ancona chick represents the settling point of a single primordial melanoblast from which pigment cells are proliferated. The same two settling points or centers may exist in fully pigmented as well as piebald spotted breeds. Early differentiation of melanoblasts may be the cause of slow proliferation and retarded migration of pigment cells from the centers in the Ancona, in comparison to normal differentiation, proliferation, and migration in the Brown Leghorn.
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