7.15
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 esse…
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.
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Q1: What determines where the cell division plane forms in animal cells?
In animal cells, the cell division plane is determined during anaphase and is located at the cell equator between segregated chromosomes. Microtubules forming the mitotic spindle interact with the cell cortex to specify the cleavage furrow location. Three models explain this process: astral stimulation, polar relaxation, and central spindle stimulation, which work together to ensure accurate plane positioning.
Q2: How do astral microtubules influence cleavage furrow formation?
In the astral stimulation model, radial arrays of astral microtubules interact with the equatorial cortex and directly induce cleavage furrow formation. This mechanism is particularly prominent in large cells like C. elegans embryos, where extensive astral microtubule arrays play an active role in determining the cell division plane accurately.
Q3: What is the central spindle and what role does it play in cell division?
Once anaphase begins, interpolar microtubules between segregated chromosomes rearrange into overlapping antiparallel microtubules called the central spindle. In the central spindle stimulation model, this structure stimulates cleavage furrow formation at the equatorial cortex, contributing to accurate positioning of the cell division plane.
Q4: How do plant cells determine their division plane differently from animal cells?
Plant cells decide their division plane before committing to mitosis by forming a preprophase band along the parent cell equator. This ring-like structure composed of microtubules and F-actin marks the division plane and gives rise to the phragmoplast, which provides structural scaffolding for cell plate expansion and positioning.
Q5: What is the difference between symmetric and asymmetric cell division?
Symmetric cell division produces two identical daughter cells, often used to grow tissues with many of the same cell type. Asymmetric division generates two non-identical daughter cells, creating cellular diversity. For example, neuroepithelial cells use symmetric division to increase numbers, then switch to asymmetric division to produce both parent-like cells and differentiating neurons.
Q6: How do septin proteins control cell division in budding yeast?
In budding yeast like Saccharomyces cerevisiae, septin proteins decide the cell division plane during the G1 phase. Unlike animal cells, the mitotic apparatus plays no role in determining the division plane. Instead, cytokinesis occurs by forming a cleavage furrow at the neck region of budding yeast cells.
Q7: Why do cells use multiple mechanisms to determine the division plane?
Using three coordinated mechanisms—astral stimulation, polar relaxation, and central spindle stimulation—allows the cell division system to respond to fluctuations in protein concentration, cell size, and other variables. This multi-pronged approach increases the accuracy and reliability of cell division across different cell types and developmental contexts.