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Friction is the force that initiates life in sea squirts

In the vast depths of the ocean, scientists have made an amazing discovery in a seemingly unassuming creature: the sea squirt. 

Researchers from the Institute of Science and Technology Austria have uncovered how sea squirt oocytes (immature egg cells) utilize friction within their interior compartments to trigger developmental changes after conception. This revelation opens a new chapter in our understanding of the forces that shape life.

The unusual life of sea squirts

Sea squirts, or ascidians, are peculiar marine organisms. After spending their early life as free-moving larvae, they eventually settle down, attaching to solid structures such as rocks or corals. 

As they mature, sea squirts develop distinctive tubes known as siphons, transforming into what appear to be rubbery blobs. 

Model organisms

Sea squirts have a close evolutionary relationship with humans as they are our most closely related invertebrate relatives. The larval stages of sea squirts, in particular, bear striking similarities to human development.

Due to these similarities, ascidians have become model organisms for studying early vertebrate embryonic development. 

“While ascidians exhibit the basic developmental and morphological features of vertebrates, they also have the cellular and genomic simplicity typical of invertebrates,” said Professor Carl-Philipp Heisenberg. “Especially the ascidian larva is an ideal model for understanding early vertebrate development.”

Oocyte transformation

The team’s research focuses on the transformation of ascidian oocytes, the female germ cells, post-fertilization. Typically, animal oocytes undergo significant cytoplasmic reorganization after fertilization, which sets the stage for the embryo’s development. 

In ascidians, this rearrangement leads to the formation of a contraction pole (CP), a bell-like protrusion crucial for the embryo’s maturation.

The mystery of cell shape change

In their quest to understand the underlying mechanisms of this process, the team observed reproducible changes in cell shape leading to the CP formation. 

Initial investigations focused on the actomyosin cortex, a dynamic structure beneath the cell membrane. The experts discovered that upon fertilization, increased tension in this cortex leads to its contraction and subsequent cell shape changes.

“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,” explained study first author Silvia Caballero-Mancebo. 

However, the team noted that the actomyosin flows stopped during the expansion of the contraction pole, suggesting that there are additional factors responsible for the bump.

The role of myoplasm 

Further investigations led to the discovery of the role of myoplasm, a stretchy solid-like layer in the lower region of the egg cell. 

During the actomyosin cortex flow, the myoplasm folds and forms many buckles due to friction forces between these two components. As the actomyosin movement halts, these friction forces dissipate.

“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,” explained Caballero-Mancebo.

Mechanical forces in development

The study not only sheds light on the specific role of friction in embryonic development but also emphasizes the importance of mechanical forces in shaping organisms. 

“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,” said Professor Heisenberg.

The study is published in the journal Nature Physics.

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