Article about the physical mechanics behind the unusual contraction of the fruit fly’s ventral nerve cord during embryonic development. Written for MBI.

Based on: Condensation of the Drosophila nerve cord is oscillatory and depends on coordinated mechanical interactions; Read here too: www.mbi.nus.edu.sg/science-features/a-nervous-contraction/

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When we think about an embryo in development, we often picture cells multiplying, tissues growing and organs expanding. Development is often seen as being synonymous with growth, but that is not always the case. In some instances, development can be reductive. Take for example how the webbings between the digits of a foetus’ hands disintegrate to produce free-moving fingers. Or how the final few segments of our spinal cord (known as the vestigial tail) fuse and reduce to make the tailbone.

Another prominent instance of such development in nature lies in the nervous system of the fruit fly (Drosophila melanogaster). More specifically, their ventral nerve cord (VNC) — a structure functionally analogous to our spinal cord — was found to shrink into its final shape during embryonic development. Visually, the fly’s VNC is a long and ladder-like structure while also possessing a soft and bulbous texture. Biologically, that is due to having repeated segments of ganglia, which are dense clusters of interlinked nerve cells (or neurons). A connected pair of ganglia makes up a single segment of the VNC and each segment is linked to one another, just like how the multiple rungs are linked together via the sides of the ladder.

3D visualisation of a section of the VNC

The genetics underpinning the development of the VNC is extremely well-studied, but the mechanical factors driving this phenomenon of down-scaling was yet to be uncovered. In 2018, MBI hosted Prof. Enrique Martin-Blanco from the Molecular Biology Institute of Barcelona, Spain, as a Visiting Professor with expertise in Drosophila development. Collaborating with Assoc. Prof. Timothy Saunders from MBI and his knowledge of live-imaging development processes and quantitative biology, they put together a team to explore the mechanical inner-workings of a contracting VNC. For a start, they needed to first establish some fundamental parameters surrounding the VNC and its condensation — such as how the VNC contracts, what its material properties are, what forces are at work, then how all these elements interplay.

Using live-imaging to visualise and measure the changing length and velocity of the VNC, it was unveiled that the contraction did not take place in one sitting, but in three phases with periods of rest sandwiched in between. The VNC is compressed from both ends towards a stationary, intermediary point between the thorax and the abdomen, like an accordion. However, these compressions do not occur simultaneously, but rather in an oscillatory or “see-saw” fashion. One end of the VNC first compacts itself a little towards the fixed point, followed by compaction on other end, then proceeding from the first end again, and so forth, creating a wave-like effect. Such extensive, repetitive action fostered the possibility that some sort of large-scale force and coordination must be involved.

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Live-imaging of VNC contraction

Having quantified the contraction throughout its morphogenesis, the team brought on board Prof. Jose Munoz from Universitat Politècnica de Catalunya, Barcelona, Spain, an expert in modelling and characterising tissues. Together, they investigated the material and mechanical properties of the VNC; Properties such as viscoelasticity and stiffness influence its contractility. Simply put, viscoelasticity refers to how much a material permanently changes shape or recovers its original form when a force is applied. And depending on the rate in which the material is deformed, the stiffness changes as well.