Background

Embryos do not form as a collection of isolated parts. During development, tissues grow, bend, move, and reshape their surroundings at the same time. A central question is therefore not only how each tissue changes shape, but how neighboring tissues coordinate these changes.

In this preprint, Weißenbruch and colleagues focus on two major events in vertebrate head development: closure of the neural tube, which gives rise to the brain and the spinal cord, and migration of neural crest cells, a highly migratory and plastic embryonic population that contributes to many head structures and the peripheral nervous system¹. These processes have often been studied separately. Yet in the head, they unfold side by side. Building on previous work from the Mayor lab on collective neural crest migration, tissue stiffness and durotaxis ^2–4, the authors asked whether neural tube closure and neural crest migration actively help each other.

Using live Xenopus embryos, tissue sections, explants, and human induced neural crest cells, the authors turned a simple observation, two tissues developing next to each other, into a mechanistic model in which each tissue helps create the conditions the other needs.

Key findings

Neural crest cells build a fibronectin-rich interface

The authors first looked at the boundary between the neural crest and the neural plate during Xenopus head neurulation. At this boundary, they focused on fibronectin, an extracellular matrix protein that forms part of the material surrounding cells and can influence how they attach, move, and organize. Using embryo sections stained for neural plate, neural crest, and fibronectin markers, they showed that fibronectin progressively accumulates at this interface as the neural tube closes.

They then asked where this fibronectin comes from. Using RNA detection and targeted perturbations, they determined that it is not primarily produced by neural crest or neural plate cells, but largely by the underlying cephalic mesoderm. Neural crest cells then remodel this mesoderm-derived matrix as they migrate, repositioning fibronectin at the boundary with the neural plate.

This is a beautiful first step in the story: neural crest cells are not simply moving through a pre-existing environment. As they migrate, they reshape the matrix and create a new boundary between themselves and the neural plate.

Neural crest cells reposition fibronectin to help the neural tube close

The authors next focused on MMP14, a cell-surface enzyme expressed by neural crest cells that can cleave matrix proteins. When MMP14 is depleted, fibronectin no longer accumulated properly at the neural crest-neural plate interface, and the neural tube failed to close normally.

Importantly, neural crest cells lacking MMP14 could still migrate on fibronectin in culture. This suggests that MMP14 is not simply required for neural crest cell motility. Instead, it is required for them to remodel fibronectin in the three-dimensional embryo. Live imaging of labeled fibronectin supports this idea, showing neural crest cells splitting and moving fibronectin as they migrate.

The fibronectin rescue experiment was especially important. When MMP14 was depleted, adding fibronectin back did not restore neural crest migration, but it did improve neural tube closure. This separates the two steps: neural crest cells require MMP14 to cleave and redistribute fibronectin, and this redistributed fibronectin then supports closure of the neighboring neural tube. In other words, neural crest cells help the neural tube close by rebuilding the matrix interface between the two tissues.

Fibronectin guides neural plate cell behavior

The next question was how this fibronectin interface helps the neural tube close. Using live imaging and high-resolution electron microscopy, the authors showed that neural plate cells failed to change shape and move properly toward the midline when MMP14-dependent fibronectin remodeling was blocked. In control embryos, these cells elongated and organized. In MMP14-depleted embryos, many remain rounded and less polarized.

The authors then tested the role of fibronectin more directly using neural plate explants grown on different matrix proteins. On fibronectin, cells spread, elongated, and intercalated. On laminin, they remained more compact. This experiment makes the conclusion very clear: fibronectin is not just a scaffold around the tissue. It can guide the cell behaviors needed for folding.

Neural tube closure primes neural crest migration

The feedback also works in the opposite direction. To determine whether neural tube closure influences neural crest migration, the authors blocked closure by perturbing Shroom3, a protein required for the cell-shape changes that allow the neural plate to fold. Neural crest migration was strongly reduced.

Grafting experiments showed that this is not simply a defect inside neural crest cells. Normal neural crest cells failed to migrate well in embryos where neural tube closure was blocked, whereas neural crest cells from Shroom3-perturbed embryos could migrate in a normal host. The surrounding tissue environment is therefore crucial.

The authors then performed a series of striking mechanical experiments. They showed that neural crest cells become compressed as the neural tube closes, using deformable oil droplets and a sensor that reports nuclear pressure. They also manipulated tension in the superficial ectoderm: reducing tension shortened neural crest migration, while applying pulling forces to embryos with defective neural tube closure partially restored neural crest migration. This shows that tissue mechanics are not just correlated with migration. They actively contribute to it.

Finally, the authors connected compression to differential gene expression. They showed that in human induced neural crest cells, compression increased genes involved in migration and matrix remodeling, including MMP14. What I find very elegant is that the authors showed that if neural crest explants are compressed outside the embryo, they could migrate when grafted into embryos with defective neural tube closure.

This closes the loop:  neural crest migration remodels the matrix required for neural tube closure, and neural tube closure mechanically activates neural crest migration.

What I like about this preprint

As a developmental biologist, I am always drawn to studies that ask how tissues coordinate their morphogenesis in vivo. We often describe development through cell fate decisions, gene programs, or lineage trajectories, but this preprint reminds us that these trajectories do not unfold in a fixed environment. The environment itself is built and reshaped by the tissues as they develop.

What I find particularly beautiful here is that this idea becomes visible in a living embryo. Neural crest cells do not simply migrate through a pre-existing path. As they move, they remodel their extracellular matrix and build a new interface with the neural plate. This new interface then feeds back on the neural tissue and helps it fold. In other words, movement, tissue architecture, and local environment emerge together.

Questions for the authors

1. Your work primarily focuses on cephalic neural crest cells; could the model you describe be extended along the body axis?

2. What pathway connects mechanical compression to MMP14 expression?

3. In your discussion, you suggest that reciprocal tissue interactions may be important for organoid and tissue-engineering approaches. Could similar tissue dialogues help explain why some morphogenetic events are difficult to reproduce in simplified organoid systems?

References

1. Theveneau E, Mayor R. Neural crest delamination and migration: From epithelium-to-mesenchyme transition to collective cell migration. Dev Biol. Academic Press Inc. 2012;366(1):34-54. doi:10.1016/j.ydbio.2011.12.041
2. Shellard A, Mayor R. Collective durotaxis along a self-generated stiffness gradient in vivo. Nature. 2021;600(7890):690-694. doi:10.1038/s41586-021-04210-x
3. Barriga EH, Franze K, Charras G, Mayor R. Tissue stiffening coordinates morphogenesis by triggering collective cell migration in vivo. Nature. 2018;554(7693):523-527. doi:10.1038/nature25742
4.  Theveneau E, Mayor R. Collective cell migration of the cephalic neural crest: The art of integrating information. Genesis. 2011;49(4):164-176. doi:10.1002/dvg.20700

Tags: biomechanics, frog, neural crest, neurulation

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