And, it turns out, they must:
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, and together with the group of Guillaume Salbreux at The Francis Crick Institute and Edouard Hannezo, also at IST Austria…
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.”“When a fish becomes fluid” at Institute of Science and Technology Austria
Funny how this kind of thing happens and chaos does not usually ensue. “The embryo is programmed to divide, it cannot escape it.” Pure randomness, right?
Abstract: Tissue morphogenesis is driven by mechanical forces that elicit changes in cell size, shape and motion. The extent by which forces deform tissues critically depends on the rheological properties of the recipient tissue. Yet, whether and how dynamic changes in tissue rheology affect tissue morphogenesis and how they are regulated within the developing organism remain unclear. Here, we show that blastoderm spreading at the onset of zebrafish morphogenesis relies on a rapid, pronounced and spatially patterned tissue fluidization. Blastoderm fluidization is temporally controlled by mitotic cell rounding-dependent cell–cell contact disassembly during the last rounds of cell cleavages. Moreover, fluidization is spatially restricted to the central blastoderm by local activation of non-canonical Wnt signalling within the blastoderm margin, increasing cell cohesion and thereby counteracting the effect of mitotic rounding on contact disassembly. Overall, our results identify a fluidity transition mediated by loss of cell cohesion as a critical regulator of embryo morphogenesis. (paywall) More. – ‘Fluidization-mediated tissue spreading by mitotic cell rounding and non-canonical Wnt signalling’, Nicoletta I. Petridou, Silvia Grigolon, Guillaume Salbreux, Edouard Hannezo, and Carl-Philipp Heisenberg. DOI: 10.1038/s41556-018-0247-4
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See also: Remarkable vid of a mouse embryo developing
Breaking: A “junk DNA” jumping gene is critical for embryo cell development
Embryonic Development Reveals Staggering Complexity