A bird's patterned plumage is 'painted' by cell interactions.
Cheng-Ming Chuong at the University of Southern California in Los Angeles and his colleagues found that precursors of pigment-producing cells are positioned in a ring around the base of feather follicles. These precursors divide and develop into pigment-producing cells, which are sent into the feather shaft that emerges from the follicle as feathers grow. Variations in the timing of cell development and positioning of the progenitor cells create distinct designs. Stripes are painted when pigmented cells form in synchronized pulses, and spotted feathers (pictured) result from signals that switch pigment synthesis on and off in adjacent, differentiating cells. These subtle modulations allow complex feather patterns to arise during birds' lives and to evolve over time.
Science http://dx.doi.org/10.1126/science.1230374 (2013)