Birds of a Feather
Scientists may be able to repair or regenerate tissues and organs by analyzing
the evolution of chicken feathers.
Scientists from the Reck School of Medicine of USC for the first time have shown experimentally the steps in the origin and development of feathers, using the techniques of molecular biology. Their findings have implications for the study of the embryological development of various tissues and organs-from hairs to lung tissue to mammary glands-and are already shedding light on the controversy over the evolution of dinosaur scales into avian feathers.
"The feather is one of the best research models for understanding the
basic molecular pathways used by all epithelial cells," says Reck School
pathology professor Cheng-Ming Chuong, M.D., Ph.D. "Scientists agree
that whether you're looking at a human mammary gland or a chicken feather,
epithelial cells use the same underlying logic to form an organ. But unlike
a gland, a feather lays everything out there for you."
The question of what makes a feather a feather has become rather heated in
the recent past, with the discovery in China in the 1990s of fossilized dinosaurs
with branching epithelial appendages on their skin.
"Some say these things are feathers, some say they're protofeathers,
others say they're not feathers at all," Chuong says. "Everybody
wants to know which one is the real feather."
And they also want to know how it came to be. Over the years, paleontologists
trying to trace the evolutionary connection between dinosaurs and birds have
looked at the ways in which a reptilian scale might turn into an avian feather.
Most adult feathers have a backbone, called a rachis, off of which the feather's
barbs branch. Each individual barb then branches into the feather's smallest
unit, the barbule.
In a paper published in the scientific journal, Nature, Chuong and his colleagues
demonstrated how barbs and rachides are formed in a modem chicken. They also
showed that the evolution from scale to feather most likely followed a path
in which the individual barbs form first and then fuse to produce a rachis-rather
than a rachis forming first and then being sculpted into barbs and barbules.
To reach their conclusions, Chuong, along with MingkeYu, Ph.D., a postdoctoral
fellow and first author on the paper, Ping Wu, Ph.D., and Randall Widelitz,
Ph.D., developed a novel way to genetically manipulate different genes during
feather formation.
They plucked feathers from chickens, and then prompted the chicken to regenerate
those feathers under controlled conditions, raising and lowering the expression
levels of a number of different genes. Three genes in particular-noggin, bone
morphogenetic protein 4 (BMP4) and the whimsically named sonic hedgehog (Shhs)-were
found to result in new feathers that were rife with abnormal organization
in their rachides and barbs.
When Chuong's team increased the expression of noggin, they found that the
rachis began to split into several small, thin rachides, and the barbs increased
in number. When the group increased the expression of BMP4, they found that
the feather's rachis became gigantic and its barbs merged and were reduced
in numbers. Finally, when they suppressed Shh, they found a residual webby
membrane between the normally separated barbs.
"These results suggest that the barbs form first and later fuse to form
a rachis," Chuong says. They also make pinning down the moment at which
dinosaur scales become chicken feathers "unrealistic," says Chsuong.
"It took 50 million years for nature to transform a scale into a flight
machine. There were many intermediate stages."
These findings have medical applications as well. "With this study, we
learned more about how nature guides epithelial stem cells to form different
organs;' Chuong notes. "For example, BMP, Shh and noggin are used in
different ways in making lungs, limbs and spinal cords. By analyzing these
models, scientists may be able to fully understand nature's 'grammar; and
learn to use it in repairing or regenerating tissues and organs."
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