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Feather patterns reveal stem cell secrets

Spots v stripes A discovery that reveals how bird feathers get their patterns also has implications for regenerative medicine, say scientists.

Feathers not only keep birds warm and protect them from the elements, they are also used for flight, communication and reproductive success.

Even the most common bird species exhibit great diversity in both plumage pattern and colour.

But the mechanisms driving this diversity have long puzzled scientists.

Now, an international team of researchers led by Professor Chuong Cheng Ming, a developmental biologist from the University of Southern California say they've pinpointed the mechanisms behind the amazing display of spots and stripes.

Their research, published today in Science, reveals that complex patterns are not encoded directly in DNA, but result from the combination of several simple processes regulated by physical, chemical and genetic factors.

To get to the bottom of why birds have different patterned plumage, the team used stem cell technology to identify the specific location of melanocyte (pigment) stem cells in feathers of several different bird types.

They discovered melanocyte cells lie under the skin at the base of the cylinder-shaped feather follicles.

"Feathers grow from the distal end (the tip) to the proximal end (the base). When activated (such as following a bird's seasonal moult), these so-called melanocyte progenitors produce daughter cells that travel up the feather's shaft, colouring the feather barbs," says Chuong.

Over time, the colour of the feathers changes as the stem cells that produce the colouration on newly developing parts of the feather are either switched on or turned off at different times in the feather formation process.

"In addition, the pigment stem cells are held in a three-dimensional ring-shaped configuration, which means that a new feather regenerates as an epithelial cylinder," Chuong says.

"As the feather develops, cells in specific regions die, allowing the feather to split open like the pages of a book."

Interactions of genes with physical or chemical factors that take place in the stem cell's microenvironment at this time can influence which pigmentation patterns gradually form on this 'canvas'.

To explore pattern formation in more detail, the researchers studied the feathers of barred or spotted chicken breeds such as silver laced wyandottes, which have a white spot and black 'laced' edge.

They discovered the same pigment cells are present throughout the feather barbs, whatever their colour. Whether or not a colour appears is controlled by proximity to a melanin inhibiting protein called agouti, which is expressed by the pulp in the central shaft of the feather.

Hormones also play a role in regulating stem cells to form distinct feather patterns, says Chuong.

"As a chick grows into a rooster or hen, the feather patterns change remarkably depending on its age and sex. During regeneration, feathers remake themselves into different colours and shapes, but the feathers are all produced from the same follicles using the same stem cells.

Feathers as a Rosetta Stone

Finding the mechanisms that control feather regeneration provides insight into how stem cells are guided to differentiate into different types of tissue, a process called 'morphogenesis', says Chuong.

In stem cell biology, this process remains unsolved. Scientists can make bone cells, for example, but can not yet guide them to form shapes.

"What this research shows us is that while stem cells can become many things, it is the multiple factors interacting in their micro-environment that guides them to produce different organised tissue patterns," says Chuong.

"In this way, feathers have become a Rosetta Stone that will help reveal the fundamental principles of morphogenesis."