Spindle Fibres

Microtubules are proteinaceous filaments formed by the polymerisation of the tubulin protein. They are hollow, rigid, and unbranched tubular structures present in the cytoplasm of an eukaryotic cell, that might be freely distributed or remain arranged in bundles in some specialised structures (centriole, cilia, flagella, etc.). Spindle fibres are the aggregates of microtubules that help the chromosomes move to the opposite poles during both mitotic and meiotic cell divisions.

Spindle fibres have their main role in dividing the genetic material of a cell. They are absolutely imperative in the equal division of the chromosomes between two daughter cells during nuclear division: mitosis or meiosis. At the time of mitosis, these fibres are called the mitotic spindles. However, at the time of meiosis, the spindle fibres are said to be the meiotic spindle.

Formation of Spindle Fibres in Animal Cells

In an animal cell, the single membrane-bound organelle centrosome, is present. The centrosome consists of two cylindrical bodies called centrioles.
Centrioles (cylindrical structures made of microtubule triplets) act as a part of the centrosome.
Centrosomes function as the main microtubule-organising centre (MTOC) from which spindle fibres are nucleated.
The microtubules extend from centrioles in every direction possible, forming a spindle. Some of them attach the poles to the kinetochores of the chromosomes.
During metaphase, these tubules pull and align the chromosomes at the equatorial region, forming the metaphasic plate and the chromosomes separate and move to opposite sides during anaphase.
The spindle apparatus helps the chromosomes from the mother cell to move in opposite directions, and ensures the even distribution of chromosomes between the daughter cells.
The spindle apparatus is comprised of spindle fibres, motor proteins, chromosomes, and microtubule arrays called asters. Thus, the spindle fibres are one of the important components.

Formation of Spindle Fibres in Plant Cells

Plant cells entirely lack centrosomes and centrioles.
MTOCs (Microtubular Organising Centres) are structures in plant cells that are similar to the centrosome and remain associated with the phragmoplasts. Spindle fibres originate from the MTOCs.
Plants utilise a variation of ‘spindle self-organisation’ that utilises the early polarity of plant cells and accentuates the role of kinetochores in stabilising the spindle midzone in prometaphase.
The spindle fibres, after being formed, attach to the centromeres of the chromosomes, and the sister chromatids move to opposite poles of the cell during anaphase.

Structure of Spindle Fibres

Spindle Apparatus is a structure with a multifaceted composition, but its core component, the spindle fibres, are typically microtubules.
Microtubules are elongated, cylindrical structures composed of protein tubulin units.
Microtubules are made of two distinct tubulin subunits: α-tubulin and β-tubulin.
The α- and β-tubulin heterodimers are arranged head-to-tail to form protofilaments.
Thirteen protofilaments align side by side to make the hollow cylindrical microtubule. Both α- and β-subunits alternate along the length; there is no fixed internal/external positioning.
Microtubules are dynamic and continuously undergo elongation and contraction via the addition and removal of tubulin subunits.

Functions of Spindle Fibres

Spindle fibres ensure that chromosomes are separated and distributed correctly to the daughter cells.
Spindle fibres attach to chromosomes at the kinetochores. Once attached, the fibres exert force, pulling the chromosomes apart towards the opposite poles.
Helps in the correct segregation of the chromosomes so that each daughter cell inherits a complete set of chromosomes. This event is crucial for the proper growth and function of the cell.
In the absence of spindle fibres, chromosomes would fail to segregate properly, leading to aneuploidy or cell death. Persistent segregation errors may contribute to chromosomal instability, a hallmark of cancer.