Unveiling the Power of Friction In A Marine Organism

The colorful Polycarpa aurata sits in a bed of white bryozoans (Triphyllozoon inornatum).
The colorful Polycarpa aurata sits in a bed of white bryozoans (Triphyllozoon inornatum).
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Researchers at ISTA’s Heisenberg group have shed light on a remarkable phenomenon: how sea squirt oocytes utilize internal friction to jumpstart development after fertilization, a finding published in Nature Physics.

In a groundbreaking study published in Nature Physics, scientists from the Heisenberg group at the Institute of Science and Technology Austria (ISTA) have delved into the fascinating world of sea squirts, uncovering how friction plays a pivotal role in propelling their developmental journey.

Sea squirts, or  ascidians, are marine invertebrates with a unique life cycle. Their journey begins as tiny, immature egg cells known as oocytes. After conception, these oocytes undergo a remarkable transformation, a process intricately orchestrated by the forces of friction within their internal compartments. This revelation opens a window into the complex interplay of forces at the molecular level, shedding light on how organisms harness physical forces to sculpt their own development.

The study offers a glimpse into the intricate mechanisms at play within living organisms, highlighting the importance of understanding the role of friction in shaping life itself. By unraveling the mysteries of sea squirt development, scientists not only gain insights into the fundamental processes of life but also pave the way for understanding vetebrate development.

“While ascidians exhibit the basic developmental and morphological features 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). “Especially the ascidian larva is an ideal model for understanding early vertebrate development.”

So how did they scientists figure this out? Oocytes, the female germ cells crucial for reproduction, undergo cytoplasmic reorganization following successful fertilization by male sperm. This rearrangement modifies their cellular contents and components, laying the foundation for the embryo’s future development. In certain animals like ascidians, this restructuring gives rise to a distinct feature called the contraction pole (CP), resembling a small bump or nose shape, where vital materials congregate to support embryo maturation. Despite its significance, the mechanism behind this phenomenon has remained elusive.

The scientists analysed fertilised oocytes, focussing on actomyosin (cell) cortex, which is a dynamic structure found beneath the cell membrane in animal cells. It is composed of actin filaments and motor proteins and acts as a driver for the changing of shapes in cells.

“We uncovered that when cells are fertilized, increased tension in the actomyosin cortex causes it to contract, leading to its movement (flow), resulting in the initial changes of the cell’s shape,” Caballero-Mancebo continues.

What they noticed however was that the actomyosin flows stopped during the expansion of the contraction pole, suggesting that something else was responsible for the bump.

When they took a closer look they came across the myoplasm. This is a layer composed of intracellular organelles and molecules (similar structures are found in many vertebrate and invertebrate eggs), that are situated in the lower region of the ascidian egg cell.

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

This myoplasm fold into several buckles due to friction between the two components. Once the actomyosin movements ceases the friction between the two also disappears.

“This cessation eventually leads to the expansion of the contraction pole as the multiple myoplasm buckles resolve into the well-defined bell-like-shaped bump,” Caballero-Mancebo adds.

This study provides a fascinating and new insights into how mechanical forces, such as friction, also impact cell and organism shapes

Heisenberg adds: “The myoplasm is also very intriguing, as it is involved in other embryonic processes of ascidians as well. Exploring its unusual material properties and grasping how they play a role in shaping sea squirts, will be highly interesting.”

Reference: “Friction forces determine cytoplasmic reorganization and shape changes of ascidian oocytes upon fertilization” by Silvia Caballero-Mancebo, Rushikesh Shinde, Madison Bolger-Munro, Matilda Peruzzo, Gregory Szep, Irene Steccari, David Labrousse-Arias, Vanessa Zheden, Jack Merrin, Andrew Callan-Jones, Raphaël Voituriez and Carl-Philipp Heisenberg, 9 January 2024, Nature Physics.
DOI: 10.1038/s41567-023-02302-1


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