Cytokinesis, as we briefly introduced in our mitosis in Mesenchymal Stem Cells post, refers to the obtention of two separated daughter cells after mitosis. The process of separation can coincide in time with anaphase and/or telophase, the two final steps in mitosis.
Human Mesenchymal Stem Cells from bone marrow (kindly provided by PromoCell GmbH) were cultured in low-serum cell growth medium and observed with the 3D Cell Explorer. Nanolive imaging allows for long-term (up to weeks) live cell imaging. Its low power laser is ideal for the visualization of mesenchymal stem cells, which are particularly sensitive to stresses such as phototoxicity due to the various excitation lights used in fluorescent imaging approaches.
Cytokinesis has classically been segmented based on time and morphology in four subprocesses. In the footage, the progression of cytokinesis is observed.
The cytoskeleton, membrane systems, and cell cycle engine work in a coordinated way and in harmony with numerous proteins throughout the process of cytokinesis, . Even if the general mechanism is preserved, there are subtle variations among the different cell types in response to specific biological requirements (e.g.: cytokinesis in blastomeres vs. cytokinesis in stem cells).
Anaphase and cytokinesis can happen at the same time and are orchestrated by key cytoskeletal structures called microtubule spindles. While anaphase relies on spindles to bring sister chromatids from the metaphase plate to the opposite ends of the cell, cytokinesis relies on central spindle to guide equatorial constriction,. The position of the central spindles also helps to keep the two populations of sister chromatids correctly segregated, which prevents chromosome loss .
This footage permits the characterization of microtubule spindles as their high protein content has a detectable refractive index. According to literature, it is very likely that the central spindles could be found in the signaled position in Figure 1.
Figure 1. Microtubular organization. Microtubular spindles characterization and central spindle predicted localization.
Cleavage plane specification and contractile ring assembly
Cleavage takes place perpendicular to the axis of chromosome segregation. The mechanism underlying this cleavage plane specification is still unresolved . Actin and myosin II are two cytoskeletal proteins essential in the formation of the contractile ring that will grow inward until the separation in two daughter cells , .
In the footage, we can predict the cleavage plane specification axis by drawing the axis of chromosome segregation (Figure 2, top). The effects of ring contraction are also visible (Figure 2, bottom). As we just described, the contraction results from the combination of force generation from the cytoskeletal structures helped by an increase of plasma membrane surface area, coming from membrane storage in vesicles and plasma membrane recycling –.
Figure 2. Cleavage plane specification (top) and contractile ring assembly effects (bottom)
The final and optional step in cytokinesis is completion. Two independent daughter cells are obtained as a result of completion. Even if it usually takes place, there are some exceptions. In specific cell types such as in embryotic blastomeres, the cells remain connected via intercellular bridges instead of undergoing a final scission . Nevertheless, completion is important as a mechanism to prevent aneuploid cell accumulation .
After ring contraction, the two daughter cells are connected by the previously mentioned midbody structure (Figure 3, top). This bridge-shaped structure is formed with tightly bundled, antiparallel microtubules deriving from the central spindle and it gets narrower as cytokinesis progresses. The high content of proteins in the spindles and their refractive index make them detectable under the 3D Cell Explorer. Removal of the cytoskeletal components of the midbody and membrane fission leads to abscission of the two daughter cells (Figure 3, bottom) , . Centrosomes seem to also play a role in completion, as they had been localized in the intercellular bridge late in cytokinesis .
Figure 3. Completion. Top: Midbody structure. Bottom: Daughter cells after abscission.
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