625: Generating right-left asymmetries
We're only sorta bilaterally symmetric: superficially, our left and right halves are very similar, but dig down a little deeper, and all kinds of interesting differences appear. Our hearts are larger on the left than the right, our appendix is on the right side, even our brains have significant differences, with the speech centers typically on the left side. That there is asymmetry isn't entirely surprising—if you've got this long coil of guts with a little appendix near one end, it's got to flop to one side or the other—but what has puzzled scientists for a long time is how things so consistently flop over in the same direction in individual after individual. There has to be some deep-seated mechanism that biases developmental events to favor one direction over the other. We know many of the genes involved in asymmetry, but what is the first step that skews development to make consistent asymmetrical choices?
In mammals, we're getting close to the answer. And it looks to be beautifully elegant—it's a simple trick to convert an anterior-posterior difference into a left-right one.
Nonaka et al. have examined the node of the mouse embryo. At the time of gastrulation, the mouse embryo (and the human embryo as well) is essentially a flat, two-layered sheet, with a groove in the middle called the primitive streak, and a dimple at the anterior end called Henson's node.
Zooming in on that area of the node in C, you can see the tops of the epithelial cells looking vaguely like cobblestones, with white strings scattered around. These are ciliated cells, and the strings the whip-like cilia that would be swinging around in a clockwise rotation if the cells were alive. You can watch a QuickTime movie of node cilia to see it in action.
When you watch those cilia spin around, here's the subtle but important thing to look for: their paths don't form perfect circles, which would indicate that they are pointing straight up at you, but are instead deformed to varying degrees, showing that they are tilted at an angle…and they are all tilted in the same direction, towards the posterior end of the embryo.
That tilt is what generates the left-right asymmetry. The tilt isn't asymmetric—all of the cilia are aimed just a little bit posteriorly, rather than straight up—but because the cilia are also rotating in a clockwise direction, it generates unequal lateral forces.Why does the same mechanism operate in all mammals? Common descent.