When a fish becomes fluid

December 17, 2018, Institute of Science and Technology Austria

Zebrafish aren't just surrounded by liquid, but turn liquid—in part—during their development. As the zebrafish embryo develops from a ball of cells to a fully-formed fish, a region of the embryo switches its phase from viscous to liquid in a process known as fluidity transition. Such fluidity transition has long been speculated to exist in living matter, but is described for the first time to occur in a living organism in a study published today in Nature Cell Biology. The study was carried out by the group of Carl-Philipp Heisenberg at the Institute of Science and Technology Austria, with first author and Postdoc Nicoletta Petridou, together with the groups of Guillaume Salbreux at The Francis Crick Institute and Edouard Hannezo, also at IST Austria.

Zebrafish are particularly suited for studying animal development as the embryo is transparent and develops outside the mother. At the very beginning of zebrafish development, a tissue layer, the so-called blastoderm, spreads over the yolk. The blastoderm changes shape to form a dome, hence this process is known as 'doming.' In the study, Petridou et al. investigated the mechanical forces at play during this shape change. By applying pressure to embryonic tissue through a pipette and measuring how fast it deforms, the researchers could infer how viscous or fluid the tissue is: tissue that deforms slowly is more viscous/less fluid than tissue that deforms quickly. Repeating the experiment at several time points and regions in the developing embryo, the researchers found that during doming, the tissue suddenly fluidizes at a very specific time and tissue region. "Such a fluidity transition was predicted to happen by theory and models, but here we show for the first time that it happens in a real, living organism," says first author Nicoletta Petridou.

Lost in division

Why and how does zebrafish tissue become liquid? In "normal" viscous tissue, the cells are in close contact with each other. The authors found that the fluidity transition happens because cells keep on dividing during development. During division, the cells become round and detach from their neighbors. The more the cells divide, the more connections are lost between them, until they eventually lose so many contacts that the tissue turns liquid. "This is a mechanical and not biochemical change," explains Petridou, "The embryo is programmed to divide, it cannot escape it."

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