Recent research has unveiled a groundbreaking understanding of how cells divide, particularly in large embryonic organisms such as sharks, platypuses, birds, and reptiles. Traditionally, the process of cell division has been explained by the formation of an actin ring at the cell's center, which constricts to separate the cell into two. However, this model falls short for larger cells, where a significant yolk sac hinders the complete closure of the actin ring, leaving scientists puzzled about the division mechanism.
Alison Kickuth, a PhD graduate from the Brugués group at the Cluster of Excellence Physics of Life (PoL), led a study that addresses this challenge. The research, published in Nature, utilized zebrafish embryos, known for their rapid development and large yolk-rich cells, to explore how these cells successfully divide.
Microtubules Provide Essential Stability
In their experiments, the researchers employed lasers to sever the actin band in zebrafish embryos. Surprisingly, the actin band continued its inward movement even after being cut, indicating that it was stabilized along its length rather than solely at the ends. They also observed that microtubules, critical components of the cytoskeleton, expanded and adjusted when the actin band was disrupted, suggesting their role in maintaining the band's stability during contraction.
To further investigate, the team disrupted microtubules through chemical means and physical interference. In both scenarios, the actin band collapsed without the support of microtubules, highlighting their crucial role in providing mechanical support during the division process.
Dynamic Changes in Cytoplasmic Stiffness
As cells transition through their life cycle, the cytoskeleton reorganizes significantly. During the mitotic phase (M-phase), when DNA divides, large microtubule structures known as asters spread throughout the cytoplasm. These asters appear to help define where the actin band will form, marking the future division site. The researchers discovered that the cytoplasm becomes stiffer during interphase, creating a supportive environment for the actin band, while becoming more fluid during M-phase, facilitating the inward movement of the band.
A Unique Mechanical Ratchet Mechanism
One of the intriguing findings was how the actin band maintains its integrity despite the cytoplasm's fluidity in M-phase. The researchers observed that while the band does experience instability during contraction, it is "rescued" by the quick succession of embryonic cell cycles. As the cell enters interphase again, the cytoplasm stiffens, stabilizing the band and allowing it to continue contracting in the subsequent fluid phase. This process operates like a 'mechanical ratchet', enabling division in a stepwise manner rather than relying on a fully closed contractile ring.
Jan Brugués, the study's corresponding author, emphasized that this new understanding fundamentally changes our perception of cytokinesis, particularly for large embryonic cells that divide rapidly. This research not only enhances our grasp of cellular processes but also opens new avenues for studying early development across various species.