Figure 2.

Schematic showing the key model features. We assume two types of pigment cells, represented here by the light and dark circles. (Top) When the underlying morphogen field is uniform, both cell types are spread uniformly throughout the two-dimensional tissue layer. The dark cell types dominate, resulting in dark pigmentation, although they are not necessarily the more numerous. (Middle) The formation of a spatial pattern in the morphogen (shown by underlying pattern) provides the information for organization of the white cell types. These cells move along gradients in the directions indicated by the arrows, displacing the black cells. The resulting cell density pattern, shown at Bottom, consists of a series of white stripes separated by the black cells. Because cell movement is relatively slow with respect to changes in the underlying field, the development of the stripes is a gradual process. As the tissue grows, the total number of pigment cells increases, yet the density remains constant. New stripes are formed through a combination of chemotactic movement and proliferation of chromatophores.