During early development, embryos coordinate the growth of different tissues to ensure that they reach the correct proportions. A new paper in Development shows that tissue scaling occurs in the tail of the post-gastrulation zebrafish embryo. The study suggests that this scaling is underpinned by multi-tissue tectonics, a mechanism whereby the deformation of one growing tissue can impact the dynamics of a neighbouring tissue. To learn more about the story behind the paper, we caught up with first author Dillan Saunders and corresponding author Benjamin Steventon, an Assistant Professor at the University of Cambridge, UK.
Benjamin Steventon (left) and Dillan Saunders (right)
Benjamin, what questions are your lab trying to answer?
BS: We are really interested in how embryos can produce a well-patterned body plan in a way that is robust and resilient to external changes. This might involve thinking about making experimental perturbations and seeing how the embryo regulates itself to still generate well-patterned tissues, as in Dillan's work. Or it might be thinking in an evolutionary context about how changes in reproductive strategy (leading to changes in yolk size, the evolution of extra-embryonic tissues and alternations in nutrient provision) can be accommodated in early development to generate a conserved body plan. We also then apply what we learn from the answers to the fundamental questions to engineering different collective cell behaviours from embryonic organoids.
Dillan, how did you come to work in the lab and what drives your research today?
DS: Shortly after being accepted onto the Wellcome Trust PhD program in Developmental Mechanisms at the University of Cambridge, I read a review article that Ben wrote on the nature of the early embryonic organizer (Martinez Arias and Steventon, 2018) and it pretty much blew my mind in the way they really drilled down into the nuances and implications of classical embryological principles. Around the same time, I approached Ben at a British Society for Developmental Biology conference and he introduced me to the work they were doing on neuromesodermal progenitors, and I became fascinated by this elusive cell state. After that, I was quite certain I wanted to work with Ben, and his lab, and doing a rotation with them as part of my PhD programme only confirmed that for me. Thankfully, he was on board with me joining the lab!
Tell us about the background of the field that inspired your work
DS: A long-standing phenomenon of early development is the ability of the embryo to form a correctly proportioned body after considerable cell loss. We were curious as to whether any of this regulative ability was retained in the tailbud in the final stages of body axis formation: the generation of the tail.
Neuromesodermal progenitors have been extensively investigated in the mouse embryo by Val Wilson and her lab (Tzouanacou et al., 2009; Wymeersch et al., 2016). They have demonstrated that these cells persist in the caudal region of the embryo and give rise to progeny that contribute to both neural and mesodermal tissue. Work from Ben Martin and David Kimelman has shown that the transduction of Wnt signalling can bias caudal progenitors to either neural or mesodermal contribution in the zebrafish (Martin and Kimelman, 2012). However, work from our lab found that, due to low division levels in the tailbud, progenitors contributing to both tissues cannot be identified during tail elongation. This idea built on Ben's previous work, which demonstrated that during zebrafish posterior body formation, volumetric growth occurs in the spinal cord and notochord rather than the tailbud. Subsequently, there have been several papers both from our lab and others, such as Tim Saunders' (no relation) group, which looks at the role of tissue morphogenesis in shaping and elongating the body axis. So, on one hand we have a tailbud which contains progenitors that can be switched between neural and mesodermal states, and on the other hand we have a growing body of evidence that tail elongation can be tuned by multi-tissue interactions. Either of these mechanisms, or a combination of both, could facilitate proportional regulation of tail elongation.
A long-standing phenomenon of early development is the ability of the embryo to form a correctly proportioned body after considerable cell loss
Can you give us the key results of the paper in a paragraph?
DS: We used multi-photon ablation to reduce progenitor number in the dorsal part of the tailbud, which is normally fated to form spinal cord. We found that ablation resulted in decreased tail elongation and that targeting up to 250 cells resulted in a proportional reduction in spinal cord and paraxial mesoderm length. We found no evidence for changes to division levels, gene expression patterns or global cell flow within the tailbud. Furthermore, ablation of a similar number of laterally located progenitors, normally fated to paraxial mesoderm, does not lead to tail reduction. These results demonstrate that neuromesodermal competent cells are not driving proportional regulation in the tail. Finally, we show that direct ablation of spinal cord cells leads to a decrease in the length of paraxial mesoderm, thus pointing to the elongation of the spinal cord as a driver of tail elongation and a regulator of tissue proportions.
When doing the research, did you have any particular result or eureka moment that has stuck with you?
DS: I'm not sure there was one particular eureka moment, rather it was a long process of becoming more and more certain that my interpretation of the results probably wasn't totally wrong. I think the main conceptual change for me was thinking about how, regardless of everything else, the cells I was ablating from the tailbud were the raw material of the spinal cord. This shifted my focus from the tailbud itself to the tissues of the tail. This change in my thinking was coupled with my experimental finding that removal of progenitors fated to contribute to paraxial mesoderm does not have an effect on tail elongation (unlike those fated to spinal cord) and therefore emphasising the importance of spinal cord elongation.
Zebrafish embryo trunk and tail at the end of somitogenesis (30 h post-fertilisation). The sample is stained with DAPI (magenta) and Phalloidin647 (green). Multiple overlapping confocal images (20× magnification) were taken along the body length and stitched together using the Pairwise Stitching plug-in in FIJI.
Zebrafish embryo trunk and tail at the end of somitogenesis (30 h post-fertilisation). The sample is stained with DAPI (magenta) and Phalloidin647 (green). Multiple overlapping confocal images (20× magnification) were taken along the body length and stitched together using the Pairwise Stitching plug-in in FIJI.
And what about the flipside: any moments of frustration or despair?
DS: Many! Though of course, that's part of doing any project. A major part of the middle of the work was investigating whether ablation has any effect on the behaviour of progenitors in the tailbud, but every experiment I did, the result was that there was no change to their behaviour. In retrospect, this is an interesting part of the paper but at the time it felt that nothing I tried was moving the work forward and I was very despondent about the whole project. Ultimately, I ended up going back to the basics, really mulling over what we already knew from existing literature and what I was sure of from my own experiments; this eventually led to a shift in my thinking about the project, away from the tailbud and towards what the tissues were doing in the tail itself.
I ended up going back to the basics, really mulling over what we already knew from existing literature and what I was sure of from my own experiments
Why did you choose to submit this paper to Development?
BS: We are really happy to publish this in Development, as it follows from a series of Development papers that together provide a picture of posterior body elongation in zebrafish. Steventon et al. (2016) first mapped the volumetric growth contributions of different tissues, Attardi et al. (2018) lineage traced neuromesodermal competent cells at single cell resolution, and McLaren and Steventon (2021) investigated the role of the notochord in coordinating different aspects of elongation. Here, we look at proportional regulation directly and uncover an additional key role for spinal cord elongation.
Dillan, what is next for you after this paper?
DS: I'm staying in the lab for a bit to finish off various things and I'm currently applying to postdoctoral fellowships to join another lab next year.
My current interests have moved away from tail formation and neuromesodermal progenitors to questions of how early embryonic development is altered (or not) by changes to maternal input, such as egg size, in different species. I've long been fascinated by evo-devo and I'm excited to get more immersed in the field. However, I'm keen to retain the approach to research that I've learnt from Ben and others in the lab, by investigating developmental mechanisms across multiple scales in a quantitative embryological manner.
Benjamin, where will this story take your lab next?
BS: Carlos, who also contributed to this work, is doing some super exciting work looking at how the notochord might be actively sensing elongation rates and adjusting its growth dynamics accordingly. We are really excited about this and have recently established a fruitful collaboration with Osvaldo Chara and Alberto Ceccarelli at the University of Nottingham, UK to model this.
Finally, let's move outside the lab – what do you like to do in your spare time?
DS: For me it's going to the pub and the cinema, and learning about non-science things – I'm currently listening to a podcast on the history of esotericism.
BS: I mainly fend off attacks from my 4-year-old twin daughters. Other than that, I like to brew beer and cook, and I'm (slowly) learning to play the drums.
Department of Genetics, University of Cambridge, Cambridge, UK, CB2 3EH.
E-mail: [email protected]
