Scientists are investigating how frictional forces drive the development of marine organisms

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Formation of the contraction pole. Microscopic time-lapse of cell shape changes in ascidian oocytes after fertilization: from unfertilized oocyte (first image from left) to initiation of contraction pole (2nd and 3rd images from left) and formation of contraction pole (4th image on the left). Credit: Caballero-Mancebo et al./Nature Physics

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Formation of the contraction pole. Microscopic time-lapse of cell shape changes in ascidian oocytes after fertilization: from unfertilized oocyte (first image from left) to initiation of contraction pole (2nd and 3rd images from left) and formation of contraction pole (4th image on the left). Credit: Caballero-Mancebo et al./Nature Physics

As the potter works with the spinning wheel, the friction between their hands and the soft clay helps them mold it into all kinds of shapes and creations. In a striking parallel, oocytes (immature eggs) are used by friction in various compartments within themselves to undergo developmental changes after conception. A study by the Heisenberg Group of the Institute of Science and Technology Austria (ISTA), published in Natural physicsnow describes how this works.

The sea is full of fascinating life forms. From algae and colorful fish to sea snails and sea spray, a completely different world is revealed underwater. In particular, sea squirts or ascidians are very unusual: after a free-moving larval stage, the larva settles, attaches itself to hard surfaces such as rocks or corals, and develops tubes (siphons), their defining characteristic.

Although they look like rubber blobs as adults, they are humans’ closest invertebrate relatives. Especially in the larval stages, sea squirts are surprisingly similar to us. Ascidians are therefore often used as model organisms to study the early embryonic development of vertebrates to which humans belong.

“While ascidians exhibit the main developmental and morphological characteristics of vertebrates, they also have the cellular and genomic simplicity typical of invertebrates,” explains Carl-Philipp Heisenberg, professor at the Institute of Science and Technology Austria (ISTA). “The ascidian larva in particular is an ideal model for understanding early vertebrate development.”

Recent work by his research group now provides new insights into their development. The findings suggest that when ascidian oocytes are fertilized, frictional forces play a crucial role in reshaping and reorganizing their interior, heralding the next steps in their developmental cascade.


A live look inside the eggs. The researchers labeled the actin protein on the actomyosin cortex (left, green staining) and myoplasm (right, blue staining) to visualize their movement after the oocyte was fertilized. As the actomyosin cortex moves across the bottom of the oocyte, it mechanically interacts with the myoplasm, causing it to bend. The locks eventually become the contraction pole. Credit: Caballero-Mancebo et al./Nature Physics

Decoding oocyte transformation

Oocytes are female germ cells involved in reproduction. After successful fertilization by male sperm, animal oocytes usually undergo cytoplasmic reorganization, changing their cellular content and components. This process establishes the blueprint for the subsequent development of the embryo. In ascidians, for example, this displacement results in the formation of a bell-shaped protuberance—a small protuberance or nose shape—known as the contraction pole (CP), where essential materials that facilitate embryo maturation are collected. However, the underlying mechanism driving this process is unknown.

Researchers set out to decipher this mystery. For this endeavor, Heisenberg’s group imported adult ascidians from Roscoff Naval Station in France. Almost all seabirds are hermaphrodites in that they produce both male and female germ cells.

“In the lab, we keep them in saltwater tanks in a species-appropriate way to obtain eggs and sperm to study their early embryonic development,” says Silvia Caballero-Mancebo, first author of this study and a former Ph.D. student in Heisenberg’s laboratory.


Formation of the contraction pole. Microscopic time course of cell shape changes in ascidian oocytes after fertilization: from unfertilized oocyte to pole contraction initiation to pole contraction formation to pole absorption. Credit: Caballero-Mancebo et al./Nature Physics

The scientists analyzed fertilized ascidian eggs under a microscope and realized that very reproducible changes in cell shape followed, leading to the formation of the contraction pole. The researchers’ first study focused on the actomyosin (cell) cortex, a dynamic structure found beneath the cell membrane in animal cells. Composed of actin filaments and motor proteins, it normally acts as a motor for changes in cell shape.

“We found that when cells are fertilized, the increased tension in the actomyosin cortex causes it to contract, causing it to move (flow), which leads to the initial changes in cell shape,” continues Caballero-Mancebo. Actomyosin fluxes, however, stop during the extension of the contractile pole, suggesting that there are additional players responsible for the bumps.


Sylvia Cabayero-Mancebo. The ISTA graduate finds great joy in unraveling the puzzles of nature and turning them into narratives. Credit: Nadine Poncioni/ISTA

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Sylvia Cabayero-Mancebo. The ISTA graduate finds great joy in unraveling the puzzles of nature and turning them into narratives. Credit: Nadine Poncioni/ISTA

Frictional forces influence cell remodeling

The scientists took a closer look at other cellular components that might play a role in expanding the contraction pole. In doing so, they encounter the myoplasm, a layer composed of intracellular organelles and molecules (related forms found in many vertebrate and invertebrate eggs) located at the bottom of the ascidian egg cell.

“This specific layer behaves like a stretchy solid – it changes shape along with the oocyte during fertilization,” explains Caballero-Mancebo.

During the flow of the actomyosin cortex, the myoplasm folds and forms many buckles due to the frictional forces established between the two components. When actomyosin movement stops, frictional forces also disappear.

“This arrest ultimately leads to an extension of the contractile pole as the multiple myoplasmic buckles become a well-defined bell-shaped protrusion,” adds Caballero-Mancebo.

The study provides new insight into how mechanical forces determine cell and organism shape. It shows that frictional forces are key to shaping and forming a developing organism. However, scientists are only at the beginning of understanding the specific role of friction in embryonic development.

Heisenberg adds: “Myoplasm is also very intriguing because it is involved in other ascidian embryonic processes. Exploring its unusual material properties and understanding how they play a role in shaping sea spray will be very interesting.”

More info:
Frictional forces determine cytoplasmic reorganization and shape changes in ascidian oocytes at fertilization, Natural physics (2024). DOI: 10.1038/s41567-023-02302-1

Log information:
Natural physics

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