Spindle Fibers: Definition and Function

Spindle Fibers Mitosis
This is a fluorescence micrograph of a cell during metaphase of mitosis. At metaphase, the chromosomes (green) line up along the center of the cell, and the spindle fibers (purple) grow from their poles to the centromeres (yellow), at the center of each chromosome.

DR PAUL ANDREWS, UNIVERSITY OF DUNDEE/Science Photo Library/Getty Images

Spindle fibers are aggregates of microtubules that move chromosomes during cell division. Microtubules are protein filaments that resemble hollow rods. They are found in eukaryotic cells and are a component of the cytoskeleton, cilia, and flagella. Spindle fibers are a part of the spindle apparatus, which moves chromosomes during mitosis and meiosis to ensure that each daughter cell gets the correct number of chromosomes. The spindle apparatus consists of spindle fibers, motor proteins, chromosomes, and, in some cells, structures called asters. In animal cells, spindle fibers are produced from cylindrical microtubules called centrioles. Centrioles form asters and asters generate spindle fibers during the cell cycle. Centrioles are located in a region of the cell known as the centrosome.

Spindle Fibers and Chromosome Movement

Spindle fiber and cell movement is the result of interactions between microtubules and motor proteins. Motor proteins are specialized proteins, powered by ATP, that actively move microtubules. Some motor proteins, such as dyneins and kinesins, move along microtubules as the fibers either lengthen or shorten. It is the disassembly and reassembly of microtubules that produces the movement needed for cell division to occur. This includes chromosome movement as well as cytokinesis (the division of the cytoplasm at the end of mitosis or meiosis).

Spindle fibers move chromosomes during cell division by attaching to chromosome arms and chromosome centromeres. A centromere is a specific region of a chromosome where duplicated chromosomes are joined. The identical, joined copies of a single chromosome are known as sister chromatids. The centromere is also where specialized protein complexes called kinetochores are found. Kinetochores generate kinetochore fibers, which attach sister chromatids to spindle fibers. Kinetochore fibers and spindle polar fibers work together to manipulate and separate chromosomes during mitosis and meiosis. Spindle fibers that don't contact chromosomes during cell division extend from one cell pole to the other. These fibers overlap and push cell poles away from one another in preparation for cytokinesis.

Spindle Fibers in Mitosis

During prophase of mitosis, spindle fibers form at opposite poles of the cell. In animal cells, the mitotic spindle initially appears as asters, which surround each centriole pair. The cell becomes elongated as spindle fibers extend from each cell pole. Sister chromatids attach to spindle fibers at their kinetochores.

During metaphase, spindle fibers called polar fibers extend from the cell poles toward the midpoint of the cell, which is known as the metaphase plate. Chromosomes are held at the metaphase plate by the force of the spindle fibers pushing on the centromeres of the chromosomes.

In anaphase, spindle fibers shorten and pull sister chromatids toward the spindle poles. Sister chromatids separate and move toward opposite cell poles. Spindle fibers not connected to chromatids lengthen and elongate the cell.

In telophase, the spindle fibers disperse as the chromosomes are separated and become housed within distinct new nuclei.

At the end of mitosis and cytokinesis, two daughter cells are formed, each with the correct number of chromosomes. In human cells, there are 23 pairs of chromosomes, for a total of 46. Spindle fibers function similarly in meiosis, where four daughter cells are formed instead of two.