Chickens with two beaks may bring scientists closer to sorting out the molecular signals that lend faces their distinctive features. The new work, described in the 20 December issue of Nature, may eventually shed light on facial deformities in people, enhancing prevention and treatment.
A chick embryo's face develops largely thanks to cells from an embryonic structure known as the neural tube. A few days after fertilization, so-called neural crest cells migrate to the soon-to-be head, where they help form tiny buds of tissue that eventually develop into facial bones. In previous work, scientists have shifted these primordial neural crest cells around the embryo's brain, causing beaklike bulges to grow out of the bird's neck. Those experiments have led many experts to theorize that neural crest cells carry built-in blueprints of the face. But because the transplanted beaks weren't fully formed, developmental biologist Joy Richman of the University of British Columbia in Vancouver, Canada, wasn't satisfied with that explanation.
Richman's team focused first on bone morphogenetic proteins (BMPs)--a type of transforming growth factor that helps bones grow and may determine which parts of the face neural crest cells become. Hoping to clarify how BMPs guide neural crest cells to form the face, the team coated tiny beads with a protein called noggin, a BMP-blocker, then implanted those beads into the faces of chicken embryos. The result: Bone grew abnormally in some but not all cases, indicating that there were more factors at work.
Next, the team added both noggin and retinoic acid, a derivative of vitamin A, based on previous work showing that too much vitamin A causes facial deformities in mice and chickens. Results were dramatic: A new, fully-formed beak, made of cartilage, grew where a bony cheek should have been in almost every case. Adding vitamin A alone did not induce extra beaks, nor did adding sonic hedgehog, a protein influenced by retinoic acid. This suggests that the combination of BMPs and retinoic acid is the critical signal that reprograms neural crest cells after they migrate--and ultimately shape the face, Richman says.
The new work adds critical information to a hot field, says developmental biologist Jill Helms of the University of California, San Francisco. "The real wow factor here is that they could transform one primordial structure into another," Helms says. Understanding the mechanisms behind this transformation, she says, "might be our first step toward thinking about repairing defects."
Joy Richman's home page
Facial embryology illustrations
Background on human facial development