Cheng-Ming Chuong's fascination with feathers began some 20 years ago. Performing a neurological experiment in chickens, he saw that cells associated with feathers might have multiple roles. “It was a defining moment,” says Chuong, a developmental biologist at the University of Southern California in Los Angeles. He has since assembled an interdisciplinary team to investigate the evolutionary origin, molecular signalling, complex patterning and tissue engineering of feathers.

With interest in stem cells increasing in recent years, Chuong became curious about how feathers are regenerated in adult birds. He decided to try to track down the location of stem cells in feathers. Together with graduate student Zhicao Yue, he began the search. The pair thought that their task should be successful because the location of stem cells in hair —the mammalian equivalent of feathers — had already been identified. But hair stem cells were found in a bulging region in the upper wall of the follicle and there is no such structure in a feather.

The search began in chicken feathers and used typical techniques for labelling and identifying potential stem cells. But the researchers soon realized that they had to adjust their approach because “feathers and hairs have totally different cell dynamics”, Chuong explains.

Once they had found suitable candidates, the researchers had to work out how to prove that they were indeed stem cells and able to generate multiple cell types. In mammals, this is usually done by transplanting transgenically marked cells into mice and tracking their fate. The team came up with a similar trick, eventually transplanting transgenic quail stem cells into chickens. This helped the group to track and map the location of the feather stem cells.

On page 1026 of this issue, Chuong and his team reveal the results of their hunt. They conclude that feather stem cells appear in a ring on the internal wall of the vase-like follicle. As new cells are generated, they move upwards away from the feather's tip. When birds moult, the stem-cell ring remains in the follicle to produce future generations of feathers. This process also helps to explain why the ends of quills taper.

But there was still one outstanding question: how do the stem cells give rise to different feather shapes? There are two basic types of symmetry in feathers. Flight feathers are bilaterally symmetrical, with the central quill dividing two mirror images, whereas downy feathers are radially symmetrical — looking a bit like dandelion seeds. “How these two kinds of symmetry are constructed and evolved is a mystery,” Chuong says. “One wonders about the enormous molecular codes required for these diverse designs.”

To their surprise, Chuong and his team discovered that in flight feathers, the ring of stem cells ‘tilts’ towards the side where the quill arises, but in radially symmetrical feathers the ring is horizontal. It seems that adjusting the angle of the ring's tilt gives rise to different types of feather. “Nature has a simple solution for making complex feather forms,” Chuong says.

Chuong plans to continue his characterization of feather stem cells. He thinks that some of the tricks used in feather regeneration could be used to help regrow human tissues.