Wide coverage of the body surface by melanocyte-mediated skin pigmentation.

Skin pigmentation is a powerful defense against ultraviolet irradiation. Particularly in humans, the body surface needs to be widely covered by protective pigmentation, and melanocytes, a major lineage of neural crest derivatives, have evolved several maneuvers to transfer melanin pigment to the skin. Recent studies with embryonic melanocytes of chickens and mice have revealed sequential events mediated by melanocytes to maximize the skin coverage by pigmentation. These processes include the migration of melanocyte precursors in the embryo, the microscopic uniform spacing of individual melanocytes, and melanosome transfer from melanocytes to keratinocytes. In particular, in vivo/ex vivo live-imaging techniques of melanosome transfer and a quantitative method to evaluate the distribution patterns of melanocytes have greatly advanced our understanding of how a limited number of cells can implement a maximal coverage of the large surface area of a developing body.