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. Looking down on the sheet of cells, we’d see something like this:
This particular diagram is a little cluttered because it’s got all those curved arrows all over it; they’re there because what’s going on at this stage of development is a great deal of cell movement. Cells are moving on the surface layers towards the primitive streak (thick solid lines) and then diving down at the streak into the deeper layers, and migrating from there to various deep places in the embryo (thick dashed lines). This is what gastrulation is about, tucking cells from the surface into a middle layer, creating a three-layered embryo.
This is a cross section through the embryo above, clarifying what’s going on. The blue cells are the superficial epiblast (roughly, the ectoderm), yellow is the hypoblast (endoderm), and the cells that have migrated through the streak are colored red, and are going to form mesoderm.
There’s more than just simple movement going on, of course. The cells are also interacting with their neighbors and sending chemical signals back and forth; these inductive signals instruct cells in what tissues they should differentiate into. The node is a particularly important place for these signals, and cells going through there will become axial mesoderm—central structures like the notochord that will define the midline, and also will specify the formation of essential tissues like the nervous system.
So the node is important as a critical structure for setting up axial structures. It’s also important for generating early asymmetries.
To the right is a scanning EM of a mouse node. Don’t get too confused; the figure labeled B rotated so left is on top and bottom on the right, but the area we care about has an arrow pointing to it. 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.
Because of that tilt, the left-to-right stroke drags along the surface of the cell, and is less efficient at generating fluid flow. The right-to-left stroke, on the other hand, swings up and away from the cell and into the medium, and effectively pushes fluid to the left. It’s a very elegant way to create handedness using an existing anterior-posterior axis to amplify the chirality of the rotating cilia into a directional shift.
The authors cleverly made a mechanical model of cilia with rotating wires in a viscous silicone medium (another QuickTime movie here), with glitter used as a marker that could be seen streaming specifically to one side. That’s how this would work in a real animal: start with a symmetrical embryo which is secreting diffusible molecules from a central point like the node, and then the rotation of the cilia sends the bulk of those molecules streaming to just the left side. Those molecules then induce biases in the pattern of differentiation of one side over the other, leading to all our familiar internal asymmetries.
It should also be obvious why a rare genetic disease, Primary Ciliary Diskinesia, which causes defects in ciliary motors that prevent them from rotating, is also associated with a condition called situs inversus (not all cases of situs inversus are caused by PCD, however), in which humans have their internal asymmetries reversed—heart larger on the right, appendix on the left, etc. If the rotors aren’t spinning properly, they don’t generate the sideways force, so the molecules are randomly spread to both sides…and in those cases it’s just luck which determines which way the asymmetries will form.
Nonaka S, Yoshiba S, Watanabe D, Ikeuchi S, Goto T, et al. (2005) De novo formation of left–right asymmetry by posterior tilt of nodal cilia. PLoS Biol 3(8): e268.