Nav: Home

How the cytoplasm separates from the yolk

May 09, 2019

The segregation of yolk from the surrounding cytoplasm in the very early fish embryo is a key process for the development of the fish larva. To identify its underlying mechanisms, biologists at the Institute of Science and Technology Austria (IST Austria) teamed up with their colleagues from theoretical physics. The discovery: Actin dynamics in the bulk of the cell drive phase segregation in zebrafish oocytes.

A single-cell fish egg evolves into a multi-cell embryo in less than two hours after fertilization. Within these two hours, the cytoplasm, which will later form the animal body, must separate completely from the yolk, which the larva is going to feed on. Previously, cell biologists had proposed that local expansion of the cell surface at one pole of the egg mediates this segregation. However, direct evidence supporting this model was lacking.

Joined forces: lab experiments and physical theory

To understand the physical basis of this segregation process, Shayan Shamipour, PhD student in the research group of developmental biologist Carl-Philipp Heisenberg, teamed up with the research group of theoretical physicist Edouard Hannezo. Based on the combined expertise of these two groups, the authors, also including a third professor of IST Austria, Björn Hof, reveal that the forces exerted at the cell surface are dispensable for this process--as opposed to previous models. Instead, they discovered that combined pulling and pushing forces within the embryo facilitate the segregation of cytoplasm from the yolk granules. Importantly, the theory developed to describe this process can be applied to any segregation due to the forces exerted from an active fluid and could thus also be used to examine potential similar processes in mammalian/human embryos.

Size matters

But how are these concerted pulling and pushing movements generated? In the bulk of the cell, far away from the cell surface, filaments of actin and myosin--proteins also involved in muscle cell contraction--form a dense mesh. Polymerization and contraction of this mesh trigger actin flows towards the animal pole of the egg, the hemisphere that is going to differentiate into the later embryo. Via passive frictional forces, these actin flows drag along cytoplasm. The bigger yolk granules, in contrast, are not dragged along by actin since their friction with actin is much lower. Instead, they are actively pushed, or rather squeezed, towards the opposite vegetal pole of the egg by comet-like actin structures--particular actin structures whose function had not been reported in developmental processes before. The combination of these pulling and pushing forces ensures a robust segregation of the cytoplasm and yolk granules within the developing embryo.

Bringing darkness into the light

By examining deeper parts of the cell more closely, the multi-disciplinary team has revealed that animal pole expansion at the cell surface, as previously proposed, is not essential for the yolk-cytoplasm segregation. "The actin structures at the cell surface appear very bright and are therefore quite easy to study. Maybe that's why scientists have so far simply missed to look more deeply into the much darker bulk area, which makes up most of the cell," says Shayan Shamipour, lead author of the study. Refined image processing allowed the IST Austria researchers to take a closer look at the developments in fish eggs during the moments right after fertilization. But, as Shamipour adds, another key to success was something else: "To catch the very first moments of egg development, we had to be really fast: Whenever one of our fish had started to release its eggs into the water, I would press start on my stop watch and my colleagues would see me sprint from the fish facility to the microscopy room to observe and record the process."

Curiosity-driven teamwork at its best

According to the cell biologist with a background in physics, Shamipour had been suspicious of the prevailing surface-based explanation for a while: "The embryo follows a big goal: It has to divide from one into thousands of cells in a very short amount of time. It was thus evident that the proposed surface mechanism alone could not accomplish this segregation and that the embryo would have to come up with some other mechanisms to accelerate the process." It is this curiosity-driven attitude of the young scientist paired with the interdisciplinary research culture of the Heisenberg and Hannezo groups--a mode of scientific work IST Austria particularly fosters--that enabled Shamipour to identify and analyze central cell processes that could be relevant in many other settings and organisms.
-end-
This project has received funding from the European Union (European Research Council Advanced Grant) and from the Austrian Science Fund (FWF).

IST Austria

The Institute of Science and Technology (IST Austria) is a PhD-granting research institution located in Klosterneuburg, 18 km from the center of Vienna, Austria. Inaugurated in 2009, the Institute is dedicated to basic research in the natural and mathematical sciences. IST Austria employs professors on a tenure-track system, postdoctoral fellows, and doctoral students. While dedicated to the principle of curiosity-driven research, the Institute owns the rights to all scientific discoveries and is committed to promote their use. The first president of IST Austria is Thomas A. Henzinger, a leading computer scientist and former professor at the University of California in Berkeley, USA, and the EPFL in Lausanne, Switzerland. The graduate school of IST Austria offers fully-funded PhD positions to highly qualified candidates with a bachelor's or master's degree in biology, neuroscience, mathematics, computer science, physics, and related areas. http://www.ist.ac.at

Animal welfare

Understanding cell biological processes is only possible by studying real cells, in this case of zebrafish. No other methods can serve as alternatives. The animals were raised, kept and treated according to the strict regulations of Austrian law.

Institute of Science and Technology Austria

Related Fish Articles:

Ten million tonnes of fish wasted every year despite declining fish stocks
Industrial fishing fleets dump nearly 10 million tonnes of good fish back into the ocean every year, according to Sea Around Us research.
Distant fish relatives share looks
James Cook University scientists have found evidence that even distantly related Australian fish species have evolved to look and act like each other, which confirms a central tenet of evolutionary theory.
A fish of all flavors
Japanese researchers achieve atomic resolution images of taste receptors in fish.
Fish step up to lead when predators are near
Researchers from the University of Bristol have discovered that some fish within a shoal take on the responsibilities of leader when they are under threat from predators.
Fish evolve by playing it safe
New research supports the creation of more marine reserves in the world's oceans because, the authors say, fish can evolve to be more cautious and stay away from fishing nets.
More Fish News and Fish Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Teaching For Better Humans
More than test scores or good grades — what do kids need to prepare them for the future? This hour, guest host Manoush Zomorodi and TED speakers explore how to help children grow into better humans, in and out of the classroom. Guests include educators Olympia Della Flora and Liz Kleinrock, psychologist Thomas Curran, and writer Jacqueline Woodson.
Now Playing: Science for the People

#535 Superior
Apologies for the delay getting this week's episode out! A technical glitch slowed us down, but all is once again well. This week, we look at the often troubling intertwining of science and race: its long history, its ability to persist even during periods of disrepute, and the current forms it takes as it resurfaces, leveraging the internet and nationalism to buoy itself. We speak with Angela Saini, independent journalist and author of the new book "Superior: The Return of Race Science", about where race science went and how it's coming back.