Micrograph showing condensed chromosomes in blue and the mitotic spindle in green during prometaphase of mitosis

In cell biology, the spindle apparatus(also called spindle fibers) is the structure that separate the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells. Depending on the type of cell division, it is also referred to as the mitotic spindle during mitosis and the meiotic spindle during meiosis.

Shape and components

The cellular spindle apparatus includes the spindle microtubules, associated proteins, and any centrosomes or asters present at the spindle poles.^[1] The spindle apparatus is vaguely ellipsoid in shape and tapers at the ends but spreads out in the middle. In the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells. Acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during gametogenesis in animals.^[2] In fungi, spindles form between spindle pole bodies embedded in the nuclear envelope. Most plants lack centrosomes or spindle pole bodies and instead spindle microtubules are nucleated on the nuclear envelope.^[3]

Assembly, chromosome attachment and function during cell division

During spindle assembly in prometaphase, some of the spindle's microtubules attach to the kinetochores that assemble on the centromere portion of the chromosomes. After kinetochore "capture" of the spindle fibers, the chromosomes are pulled into alignment along the spindle midzone to form the metaphase spindle. Since the center of the spindle specifies the plane along which the cell will divide during cytokinesis, this ensures that each daughter cell will receive one of each chromatid. Spindle formation is complete during metaphase when:

• the chromosomes have been aligned in the metaphase plate of the spindle
• non-kinetochore ("polar") microtubules from opposite spindle poles overlap, and
• the aster microtubules are in contact with the plasma membrane.^[1]

Once all the chromosomes are aligned with sister chromatids pointing to opposite ends of the spindle, the cell enters anaphase when proteins holding the sister chromatids together are inactivated.^[1] This allows the chromatids to separate into full-fledged chromosomes that start moving toward their respective poles. This movement is mediated by motor proteins on the kinetochores that "walk" the chromosome along the microtubule towards the nearest pole. Similarly, motor proteins attach to non-kinetochore microtubules and walk them away from each other, thus elongating the spindle and pushing apart the spindle poles.^[1]

Regulation of spindle assembly

The mitotic kinase aurora A is required for proper spindle assembly and separation. Lamin B is a key component of the spindle matrix helping microtubule assembly, and the mitotic spindle will not form without it.^[4]

Polo-like kinase, also known as PLK, especially PLK1 has important roles in the spidle maintenance by regulating the microtubulin dynamics^[5].

Mitotic spindle assembly checkpoint

The completion of spindle formation is a crucial transition point in the cell cycle called the spindle assembly checkpoint. If some chromosomes are not properly attached to the mitotic spindle by the time of this checkpoint, the onset of anaphase will be delayed.^[6] Failure of this spindle assembly checkpoint can result in aneuploidy and may be involved in aging and the formation of cancer.^[7]

References

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