Humans and other animals may appear to be symmetrical on the outside, but symmetry is only skin deep. Many body organs, such as the stomach, the heart and the liver, are tipped to the right or left side. So how does the developing embryo distinguish left from right?
Salk scientists have now discovered that the foundations for the basic left-right body plan are laid by a microscopic 'pump' on the outer surface of the embryo's underside that wafts chemical messengers over to the left side of the body. This sets up a chemical concentration gradient that tells stem cells how and where to develop. The remarkable findings, including movie footage of the 'pump,' are published in the May 20th edition of the journal Cell.
Juan Carlos Izpisúa Belmonte and his colleagues studied the ventral node, a small patch of specialized cells on the outer surface of the underside ('ventral' side) of early embryos in many animals. Each cell in the ventral node has a single, rapidly rotating thread (cilium) projecting from the cell surface. Belmonte and colleagues at the University of Tokyo in Japan had previously demonstrated that the ventral node and its rotating cilia influence the left-right body plan, but until now no-one knew the mechanisms involved.
In the current study, Belmonte's team compared the ventral node in embryos of mice, rabbits and fish, and discovered the same mechanism in all these animals: the rapid, clockwise rotation of the whip-like cilia was actively moving fluid from the right side to the left side of the developing embryo.
The Salk scientists were intrigued by the finding because the forest of rotating cilia were more likely to create a whirlpool than a river. "The unidirectional flow produced by the rotation-like movement of the cilia required a specific mechanism because a simple circling movement would have, logically, just produced a vortex," said Belmonte.
Over the next three years, using a combination of mathematical modeling, high-speed video recording, and electron microscopy, Belmonte and his Salk colleagues Marta Ibañes and Diego Rasskin-Gutman worked to solve this puzzle jointly with their collaborators at the Parc Cientific de Barcelona in Spain and at the University of Tokyo in Japan. The scientists discovered that the cilia generate a current because they are tipped over at a 40-degree angle, rather like a twirling parasol over the shoulder of a Southern Belle. Their live (in vivo) observations feature in the current Cell article.
The cilia, which twirl at 10 cycles per second, were too fast for conventional video recording and so the scientists captured their complex movement on a specially adapted high-speed video moving at 500 frames per second.
"The net result is that the clockwise rotation of the cilia is converted into a right-to-left current over the embryo," said Ibañes. "The leftward current then acts as an amplifier, translating the ciliary dynamics into a large effect that covers a wide region of the embryo."
The Salk team suspected that the purpose of the cilia 'pump' was to concentrate messenger proteins on the left side of the embryo. This would, in effect, set up a chemical gradient that would tell developing cells whether they were on the right or left side of the body. Using proteins labelled with fluorescent tags, the researchers demonstrated that the cilia pump was capable of transporting proteins of a similar size to known chemical messengers. It took about 4 seconds for the test proteins to be whisked across the pinhead-sized primitive embryo.
Belmonte said that the next step is identifying the chemical messengers involved in the right-left flow and decoding the messages that they carry. "This study not only gives us a fascinating insight into why we are asymmetric; it may also be at the root of how the human body plan is built," he said.
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.