Symmetry is omnipresent in the living world. Credit: S. Priyadarshini
The living world, from radially symmetrical jellyfish to bilaterally symmetrical butterflies, is a study in symmetry. In humans, several organs are bilaterally symmetrical such as kidneys, eyes and ears. Our symmetrical musculoskeletal system is necessary to perform mechanical functions such as walking.
But the origin of the musculoskeletal system in vertebrate animals is asymmetrical, a study of zebrafish embryos has now shown1. This asymmetry corrects itself over time.
Sundar R. Naganathan, a postdoctoral fellow at EPFL, Switzerland and his team used zebrafish as a model because its embryos are transparent. This allowed researchers to simultaneously image the left and right sides of a developing embryo.
Musculoskeletal systems in vertebrates develop from soft embryonic tissues called somites, the number varying from species to species. A zebrafish embryo has 32 somites, for instance, while a chicken embryo has 55. Somites don’t form at the same time along the body axis of an embryo, but at fixed intervals which, like the somite numbers, are specific to a species.
The textbook view, says Naganathan, is that somites form on the left and the right side of the embryo in a symmetrical manner in size, shape and position.
But Naganathan and colleagues observed that in a single pair of somites, one may be bigger than the other. “Sometimes, both somites are the same size but misaligned between the left and right side,” says Naganathan. The researchers found that in zebrafish embryos, about 30–40% of the somites are asymmetrical at first, but the asymmetries are usually resolved within an hour of formation. The error correction was a result of surface tension of somites and not due to any interaction between the left and right somites, the researchers found.
Naganathan’s team perturbed different molecules known to be involved in surface tension, and showed that as surface tension is perturbed, so is the error correction mechanism. The forces generated by surface tension are very important for the error correction mechanism, he explains.
“It is becoming clear that genetic programmes on their own cannot make precise patterns, but require additional feedback such as the physical mechanism shown here,” says Sean Megason, a biologist at Harvard Medical School, who was not part of the experiment.
The findings could be used to improve personalised medicine. Naganathan says that developing reproducible ‘organoids’ – tissues cultured in the laboratory to test the effects and dosage of drugs – that are of the same shape and size has been a huge challenge for researchers. “If you want to make a reproducible precise tissue, you need to consider the role of tissue mechanics such as surface tension.”
