Scientists have discovered a surprising new mechanism that explains how very large embryonic cells divide. This finding challenges the long-held view that cells of all kinds use a complete tightening ring to split into two. Instead, researchers found that giant cells use a “mechanical ratchet” process powered by changing physical properties inside the cell and support from internal fibers.

Traditionally, textbooks say cells divide by forming a ring made of the protein actin at their midpoint. This ring tightens like a drawstring and pinches the cell in half. However, this model does not work well for huge cells with large yolk sacs, such as those seen in many egg-laying animals.

🔬 Zebrafish Reveal Hidden Mechanics

Researchers at the Cluster of Excellence Physics of Life (PoL) at Technische Universität Dresden studied zebrafish embryos to understand this phenomenon. Zebrafish embryos have very large yolk-rich cells during early development, making them ideal models for this kind of research.

Using precise lasers to cut the actin ring in these cells, scientists observed that the band continued to move inward even after it was severed. This indicated that the division process did not rely on a fully closed ring.

🧠 Cytoskeleton and Stiffness Play Key Roles

The team discovered that microtubules, another part of the cell’s internal framework, provide crucial support. When microtubules were disrupted chemically or physically, the actin band collapsed. This outcome showed that microtubules help stabilise the division apparatus.

In addition, scientists found that the internal stiffness of the cell’s material — the cytoplasm — changes during different phases of the cell cycle. During one phase, the cytoplasm becomes stiff and supports the actin band. In the next phase, it becomes more fluid, allowing movement. These alternating shifts help push the division forward step by step.

⚙️ A Mechanical Ratchet Rather Than a Simple Ring

Instead of dividing in one continuous motion, these giant cells split over several cycles of stiffness and fluidity. Each cycle advances the division just a bit further. Because the cytoplasm stiffens again after each step, the progress does not reverse. This stop-and-go process acts like a mechanical ratchet, gradually guiding the cell toward full division.

Researchers say this mechanism helps explain how very large embryonic cells manage to divide efficiently, despite their size. The new ratchet model may apply to a wide range of organisms, especially egg-laying species with large yolks where traditional division methods are impractical.

📖 Study Challenges Textbook Models

The discovery challenges classic textbook explanations of cell division. Scientists once believed that a continuous, fully formed contractile ring was the central driver of cytokinesis in all organisms. However, this research shows that physical properties and structural dynamics inside the cell can produce division without a complete ring.

The findings were published in the journal Nature by a research team led by scientists at the Cluster of Excellence Physics of Life. They propose that this mechanism offers a new framework for understanding how embryonic cells divide when conventional models fall short.