How sea squirts use friction to change and develop after fertilization

A potter shapes clay on a spinning wheel, and the friction between the potter's hands and the soft clay helps create different shapes and designs. Just like that, egg cells of sea squirts, organisms that live in marine environments, use friction inside their compartments to change and develop after they are fertilized.

A study from the Heisenberg group at the Institute of Science and Technology Austria (ISTA) explained how this process occurs. What are sea squirts? Sea squirts are part of the broader group of invertebrates, and they exhibit a unique life cycle. During their early stages of development, sea squirts have oocytes, which are the precursor cells to eggs.

These oocytes undergo a process called oogenesis, where they mature and eventually become viable eggs ready for fertilization. Once fertilized, these eggs can develop into larvae and, eventually, into adult sea squirts. They start off as tiny creatures that can swim around freely. But then, as they grow up, they settle down and stick themselves to things like rocks or corals.

That's when they start to look a bit like rubbery blobs and develop special tubes, kind of like straws, that are their most noticeable feature. Sea squirts are frequently chosen as model organisms for scientific studies because they are most closely related invertebrate relatives to humans, especially at larval stages.

“While ascidians exhibit the basic developmental and morphological features of vertebrates, they also have the cellular and genomic simplicity typical of invertebrates,” said Carl Philipp Heisenberg, Professor at the ISTA. “Especially the ascidian larva is an ideal model for understanding early vertebrate development.” What did the researchers study? The researchers studied tiny sea squirt eggs (ascidian oocytes) under a microscope.

They found that after the eggs were fertilized, changes happened inside them, leading to the creation of a special structure called the contraction pole, which faciltates the sea squirt's baby to mature. The scientists focused on a part inside the cells called the actomyosin cortex. After further observation, they discovered that when the cells were fertilized, the actomyosin cortex tightened, causing the cell to change shape.

However, when the contraction pole expanded, the flow of actomyosin stopped, suggesting other factors were involved. They then looked at another part called the myoplasm, found at the bottom of the egg cells. This part acts like a stretchy solid and changes shape during fertilization. Friction forces between the actomyosin cortex and the myoplasm, especially during the tightening process, led to the myoplasm folding and creating bumps.

When the actomyosin movement stopped, the bumps turned into the contraction pole. This study helps us understand how mechanical forces, like friction, just like in pottery, play a role in shaping cells and organisms during the early stages of development. The researchers say there's still more to discover, especially about the myoplasm and how it influences sea squirt babies growing inside their eggs.

The , carried out by researchers from ISTA, Université de Paris Cité, CNRS, King’s College London, and Sorbonne Université was published in ..