Spindle Fibre Formation and their organisation

                                                                

                                                                             Spindle Fibre 

What is the Spindle Apparatus - Spindle apparatus also called spindle fiber is the structure that's separate the chromosomes into daughter cells during cell division. Depending upon the type of cell division , it is also referred to as mitotic spindle during mitosis and meiotic spindle during meiosis. 

Shape and Formation -  The cellular spindle apparatus includes the spindle microtubules, associated proteins, and any aster present at the spindle poles. The spindle apparatus is vaguely ellipsoid in shape and tapers at the end but spread out in the middle portion also known as midzone. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosome or aster in most of the animals cells. 

Assembly, Chromosome Attachment, and Function During Cell Divison -  During spindle assembly in prometaphase, some of the spindle's microtubules attach to kinetochores that assemble on the centromere portion of the chromosomes. After kinetochore capture of the spindle fiber, the chromosomes are pulled into alignment along the spindle midzone to form the metaphase plate.

Since the center of the spindle specifies the plane along which the cell will divide during cytokinesis, this ensure that each daughter cells will receive one of each chromatid. Spindle fiber formation is complete during metaphase when- 

• The chromosomes have been aligned in the metaphase plate of the spindle.
  
  
• Non- kinetochore microtubules from opposite spindle poles overlap.
  
  
• The aster microtubules are in contact with the plasma membrane. 

Once all the chromosomes are aligned with the sister chromatid pointing the opposite ends of the spindle , the cell enter anaphase when protein holding the sister chromatid together is inactivated. This allow the chromatid to separate into full-fledged chromosomes that start moving toward their respective poles. This movement is mediated by the motor proteins along the kinetochores that walk the chromosomes along the microtubules toward the nearest poles.

Similarly, motor proteins attach to non-kinetochores microtubules and walk them away from each other, thus elongating the spindle and pushing apart the spindle poles.

Regulation Of Spindle Assembly - The mitotic kinase Aurora-A is required for proper spindle assembly and separation. LAmbin -B is a key component of spindle matrix helping microtubules assembly and the mitotic spindle will not form without it.

Polo-like kinase, also known as PLK especially PLK1 has important role in spindle maintenance by regulating microtubulin dynamics.