Cooperation of cell biologists and physicists at IST Austria unravels physical basis of yolk-cytoplasm segregation in early fish embryo | Study published in Cell
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.
Illustration of the segregation process: The octopus represents the actin organizing center of the zebrafish oocyte. It pulls the cytoplasmic pockets up, while it pushes the bigger yolk granules down
IST Austria/Justine Renno
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.
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.
This project has received funding from the European Union (European Research Council Advanced Grant) and from the Austrian Science Fund (FWF).
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. www.ist.ac.at
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.
Carl-Philipp Heisenberg, email@example.com,
and Edouard Hannezo, firstname.lastname@example.org
Shayan Shamipour, Roland Karos, Shi-Lei Xue, Björn Hof, Edouard Hannezo & Carl-Philipp Heisenberg. 2019. Bulk actin dynamics drive phase segregation in zebrafish oocytes. Cell. DOI: 10.1016/j.cell.2019.04.030
https://ist.ac.at/en/research/life-sciences/heisenberg-group/ Heisenberg group
Dr. Elisabeth Guggenberger | idw - Informationsdienst Wissenschaft
Machine learning microscope adapts lighting to improve diagnosis
20.11.2019 | Duke University
The neocortex is critical for learning and memory
20.11.2019 | Max-Planck-Institut für Hirnforschung
Conventional light microscopes cannot distinguish structures when they are separated by a distance smaller than, roughly, the wavelength of light. Superresolution microscopy, developed since the 1980s, lifts this limitation, using fluorescent moieties. Scientists at the Max Planck Institute for Polymer Research have now discovered that graphene nano-molecules can be used to improve this microscopy technique. These graphene nano-molecules offer a number of substantial advantages over the materials previously used, making superresolution microscopy even more versatile.
Microscopy is an important investigation method, in physics, biology, medicine, and many other sciences. However, it has one disadvantage: its resolution is...
Nanooptical traps are a promising building block for quantum technologies. Austrian and German scientists have now removed an important obstacle to their practical use. They were able to show that a special form of mechanical vibration heats trapped particles in a very short time and knocks them out of the trap.
By controlling individual atoms, quantum properties can be investigated and made usable for technological applications. For about ten years, physicists have...
An international team of scientists, including three researchers from New Jersey Institute of Technology (NJIT), has shed new light on one of the central mysteries of solar physics: how energy from the Sun is transferred to the star's upper atmosphere, heating it to 1 million degrees Fahrenheit and higher in some regions, temperatures that are vastly hotter than the Sun's surface.
With new images from NJIT's Big Bear Solar Observatory (BBSO), the researchers have revealed in groundbreaking, granular detail what appears to be a likely...
The Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in Dresden has succeeded in using Selective Electron Beam Melting (SEBM) to...
Carbon nanotubes (CNTs) are valuable for a wide variety of applications. Made of graphene sheets rolled into tubes 10,000 times smaller than a human hair, CNTs have an exceptional strength-to-mass ratio and excellent thermal and electrical properties. These features make them ideal for a range of applications, including supercapacitors, interconnects, adhesives, particle trapping and structural color.
New research reveals even more potential for CNTs: as a coating, they can both repel and hold water in place, a useful property for applications like printing,...
15.11.2019 | Event News
15.11.2019 | Event News
05.11.2019 | Event News
20.11.2019 | Life Sciences
20.11.2019 | Physics and Astronomy
20.11.2019 | Health and Medicine