35.15: Determining the Plane of Cell Division

Positioning the cell division plane is a critical step during development and cell differentiation, particularly during mitosis when the plane is essential for determining the size of the two daughter cells. The cell division plane is perpendicular to the plane of chromosome segregation, but different types of organisms have different cell division mechanisms to suit their morphology and function.

Animal cells

In animal cells, the cleavage furrow forms along the plane of cell division starting from the cell cortex, the region below the plasma membrane. The mitotic spindle plays a decisive role in determining the cell division plane; therefore, a change in the axis of the mitotic spindle can alter the division plane. This strategy is used by some eukaryotic organisms, such as Caenorhabditis elegans, to switch from symmetric to asymmetric cell division. Symmetric cell division produces two identical daughter cells and is often used to grow tissues containing many of the same cell type. In contrast, asymmetric division creates cellular diversity as it generates two non-identical daughter cells. For example, neuroepithelial cells initially increase their numbers by undergoing symmetric divisions. After which, they use asymmetric division to produce daughter cells similar to the parent cell as well as differentiating neurons.

Yeast

In budding yeast, such as Saccharomyces cerevisiae, the plane of cell division is decided in the G1 phase by septin proteins. Cytokinesis is achieved by forming a cleavage furrow at the neck region of the budding yeast cells; however, the mitotic apparatus does not play any role in determining the plane of cell division. In contrast, in fission yeast, such as Schizosaccharomyces pombe, the position of the interphase nucleus decides the location of the plane of cell division in the G2 phase. During interphase, the Mid1 protein is mainly localized in the nucleus; however, during mitosis, Mid1 is exported from the nucleus to the cell cortex, where it recruits actomyosin ring components to the medial region of the cell. This eventually results in the mother cell dividing into two similar-sized daughter cells.

Plants

In most plant cells, the plane of cell division is decided before the cell commits to mitosis. A preprophase band is formed along the equator of the parent cell, which marks the plane of cell division. The preprophase band is a ring-like structure composed of microtubules and F-actin. It gives rise to a microtubule bundle called the phragmoplast, which provides a structural scaffold for cell plate expansion and positioning.

A dividing cell accurately determining its plane of cell division is essential for the successful separation of chromosomes and the distribution of cytoplasmic contents to its daughter cells.

In animals, the division plane is located at the cell equator in between the segregated chromosomes, and its location is determined during anaphase. Microtubules that form the mitotic spindle interact with the cell cortex and play a decisive role in specifying the cleavage furrow.

Three different models are used to explain the determination of the cell division plane – the astral stimulation model, the polar relaxation model and the central spindle stimulation model.

In the astral stimulation model, radial arrays of astral microtubules interact with the equatorial cortex and induce the formation of the cleavage furrow.

In the polar relaxation model, astral microtubules send inhibitory signals to the cell poles and thereby suppress the formation of the cleavage furrow at both poles. This leads to cortical contraction at the cell equator.

Once the anaphase starts, the interpolar microtubules present between the segregated chromosomes rearrange into the overlapping antiparallel microtubules, also known as the central spindle. In the central spindle stimulation model, the central spindle stimulates the cleavage furrow formation at the equatorial cortex. 

The general consensus is that all three processes act together to determine the plane of cell division. This multi pronged approach can make the system respond to fluctuations in protein concentration as well as cell size and thereby increases the accuracy of cell division.

The contribution of each method depends upon the cell type and size. For example, in C. elegans embryos, a large array of astral microtubules is present and thus may play a more active role in the determination of the cell division plane.