The fluid fish: first phase transition found in biology

For the first time, researchers have documented a living organism passing through a liquid phase during its development. 

In a study in the journal Nature Cell Biology, a group of scientists led by Carl-Philipp Heisenberg of the Institute of Science and Technology Austria show that cells within embryonic zebrafish (Danio rerio) turn temporarily into a liquid as the embryo grows. 

“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 lead author Nicoletta Petridou.

The yolk of zebrafish eggs is covered with a blastoderm, a thin layer of tissue. As the egg develops, the blastoderm forms a dome. By testing the tissue throughout the development of the embryo, the team finds that during “doming,” the tissue at its centre suddenly becomes a fluid. 

The researchers explain that the fluidity occurs when cells divide rapidly. Cells are normally connected to their neighbours, but during this phase of massive change, they effectively become free-floating. {%recommended 5626%}

In the study, Heisenberg and his team determine fluidity by testing the rate of tissue deformation under pressure. More viscous tissue deforms less quickly than liquid. 

“This is a mechanical and not biochemical change,” Petridou explains. “The embryo is programmed to divide, it cannot escape it.”

Obviously, the entire organism cannot become liquid, though. The fluidity occurs only in the centre and only for a short time. The researchers found that “the non-canonical Wnt signaling pathway” – a type of cell communication involving proteins – prohibits the cells at the edges of the embryo from also liquidising. 

“Non-canonical Wnt signaling keeps cells connected and allows the embryo margins to bypass fluidisation,” Petridou says. 

“We think that the default of the tissue is to become fluid, but the signaling keeps specific areas from turning fluid.” 

The researchers say they might have found the first known instance of phase transition – a concept in physics that describes a change in the structure of matter, as when water turns from a frozen solid into a liquid and then into a vapour – in organisms. 

“We called the phenomenon observed in zebrafish ‘fluidity transition’ as we are not certain that it is, in fact, a phase transition in the true sense of physics,” Petridou says. 

“However, we are working further to define whether this is a phase transition. Phase transitions can happen in molecular networks, but we don’t yet know if they can happen in a tissue or in an embryo.”