What if culture cells divide asymmetrically?
At the heart of biology, cell division is a fundamental process in the development or regeneration of cells throughout an organism's life. It is based on the assembly of a mitotic spindle, made up of microtubules, responsible for distributing chromosomes equally between two new daughter cells. During this process, centrosomes (green and pink structures in the figure below) play an important role in microtubule assembly and, consequently, in the control of the mitotic spindle. For some years now, scientists have realised that these centrosomes differ in age according to the number of cell divisions they have undergone, but the impact of this difference on the symmetry of the mitotic spindle remains poorly understood.
Cultured cells do not divide completely symmetrically
In a recent study published in the Journal of Cell Biology, researchers from Prof Patrick Meraldi's laboratory studied human cells in culture that were previously known to divide symmetrically. They discovered that the difference in age between the centrosomes creates a subtle asymmetry in the mitotic spindle, which in turn results in a slight difference in size between the two daughter cells, as can be seen in the diagram below. By exploring the mechanisms involved in more detail, they demonstrated that this asymmetry is based on the modification of a protein present on the oldest centrosome.
The age difference between the centrosomes (green and pink structures) causes a subtle asymmetry in the microtubule network (left), resulting in a slight asymmetry in the size of the two daughter cells resulting from cell division (right). © UNIGE – Laboratory of Prof Patrick Meraldi
Asymmetric, but so what ?
These results are particularly surprising because the cells studied by the scientists, two cell lines widely used in medical research, had previously been known to divide symmetrically. The discovery of this subtle asymmetry offers a new understanding of the mechanisms controlling cell division and sheds light on how the cell can modulate the asymmetry of its division, a process that is particularly important during stem cell division.