Focus on zebrafish
For their experiment, the researchers observed embryos of zebrafish (Danio rerio), which are one of the most important model organisms in developmental biology. As vertebrates, they are more closely related to humans in evolutionary terms than, for instance, fruit flies, which do not produce keratin. The fact that zebrafish embryos are also transparent and develop outside the womb makes them ideal for scientific scrutiny.
The study focuses on a process that biologists refer to as epiboly. “Initially, the embryo exists only as a ball of cells resting on a massive source of nutrients, known as the yolk, just as in an egg. In the course of epiboly, this cell layer spreads out until it eventually encloses the entire yolk,” explains Naik. “During this process – as the Heisenberg group had previously demonstrated – the cells hardly divide at all. The tissue doesn't grow because the number of cells increases, but it is stretched and pulled over the nutrient vesicle under great mechanical tension.”
The entire process is driven by a thin layer of tissue in which the embryonic cells fuse – the nuclei and cytoplasm of different cells share the same space. Generating enormous pulling force, this layer drags the thin membrane over the surface of the yolk. The necessary structural proteins also emerge just in time: “The keratins are formed in the cell layers of the zebrafish embryo at the very moment the tissue begins to move,” notes Naik. “Following the activation of the respective gene, various types of keratin are produced extremely rapidly.”
Keratin genes switched off by “gene scissors”
Until now, the role played by these keratins in the embryonic tissue movements of epiboly has not been fully understood. With the help of the “gene scissors” CRISPR-Cas9, Naik switched off the genes responsible for keratin production – exactly which genes these are has only been known for a few years – and no keratin network could form in the embryonic tissues.
As a result, epiboly proceeded significantly more slowly. “This seems counterintuitive. One would think that the expansion process proceeds more easily and quickly without the rigid keratins,” says Naik. “But the opposite is true. Whilst the cells become more supple and fluid, measurements showed that the viscosity of the tissue was reduced. It was softer and more easily deformed, but harder to pull.”
The search for the reason behind this contradictory result revealed that the cells in healthy embryos aligned themselves in the pulling direction. “When you pull on a rubber band, you see stretch marks along the direction of the pull. It is similar with tissue,” explains Naik. “The cells align their longest axis in the direction of the pull. But without keratin, this realignment is completely lost. The cells can no longer transmit forces to one another.”
No keratin, no force transmission
In a separate experiment, the researchers investigated the significance of the lack of force transmission at cellular level. “We removed the cells in the center of a cell cluster and then observed how tissue movements changed – a scenario that, while on a much smaller scale, is similar to epiboly,” Naik explains. “We saw that in the presence of keratin, even tissue farther away reacted to the loss of cells. In its absence, however, only the cells at the very edge of the wound moved.”
These findings on force transmission between cells could prove relevant for the development of future medical applications. Defective keratin formation is, for instance, a hallmark of the severe genetic disorder that leads to so-called “butterfly skin.” Keratin also plays a central role in regulating wound healing. In this context, further research could lead to new therapeutics that accelerate the formation of a new skin layer. One thing has become very clear: keratin is much more than a passive “shaper” of tissue. It forms a sensitive network for interaction and communication between cells, the significance of which is far from being exhaustively understood.
About the researchers
Suyash Naik studied at the Indian Institute of Science Education and Research in the city of Pune in the state of Maharashtra in the west of India. Naik was a PhD student in the Heisenberg Group at the Institute of Science and Technology Austria (ISTA) in Klosterneuburg and he is now an imaging analyst at ISTA’s Imaging & Optics Facility.
Carl-Philipp Heisenberg had a position at the Department of Anatomy and Developmental Biology at University College London and at the Max Planck Institute of Molecular Cell Biology and Genetics in Germany before being appointed professor at ISTA in 2010. His research group at ISTA focuses on morphogenesis in embryonic development. Set to run from 2024 to 2027, the project “Keratins in epithelial tissue spreading” is funded by the Austrian Science Fund FWF.
Publication
Keratins coordinate tissue spreading by balancing spreading forces with tissue material properties, in: Nature Communications 17, 2026
