Asters

Asters are radial microtubule arrays found in animal cells. These star-shaped structures form around each pair of centrioles during mitosis. Asters help to manipulate chromosomes during cell division to ensure that each daughter cell has the appropriate complement of chromosomes. They consist of astral microtubules that are generated from cylindrical microtubules called centrioles. Centrioles are found within the centrosome, an organelle located near the cell nucleus that forms the spindle poles.

Asters and Cell Division

Asters are vital to the processes of mitosis and meiosis. They are a component of the spindle apparatus, which also includes spindle fibers, motor proteins, and chromosomes. Asters help to organize and position the spindle apparatus during cell division. They also determine the site of the cleavage furrow that splits the dividing cell in half during cytokinesis. During the cell cycle, asters form around the centriole pairs located at each cell pole. Microtubules called polar fibers are generated from each centrosome, which lengthen and elongate the cell. Other spindle fibers attach to and move chromosomes during cell division.

Asters in Mitosis

• Asters initially appear in prophase. They form around each centriole pair. Asters organize spindle fibers that extend from the cell poles (polar fibers) and fibers that attach to chromosomes at their kinetochores.
• Spindle fibers move chromosomes to the center of the cell during metaphase. Chromosomes are kept in place at the metaphase plate by the equal forces of the spindle fibers pushing on the centromeres of the chromosomes. Polar fibers extending from the poles interlock like the fingers of folded hands.
• Duplicated chromosomes (sister chromatids) separate and are pulled toward opposite ends of the cell during anaphase. This separation is accomplished as spindle fibers shorten, pulling attached chromatids along with them.
• In telophase, spindle fibers break down and separated chromosomes are enveloped within their own nuclear envelope.
• The final step of cell division is cytokinesis. Cytokinesis involves the division of the cytoplasm, which separates the dividing cell into two new daughter cells. In animal cells, a contractile ring of microfilaments forms a cleavage furrow that pinches the cell in two. The position of the cleavage furrow is determined by the asters.

How Asters Induce Cleavage Furrow Formation

Asters induce cleavage furrow formation due to interactions with the cell cortex. The cell cortex is found directly beneath the plasma membrane and consists of actin filaments and associated proteins. During the course of cell division, asters growing from centrioles extend their microtublules toward one another. Microtubules from nearby asters interconnect, which helps to limit expansion and cell size. Some aster microtubules continue to extend until contact is made with the cortex. It is this contact with the cortex that induces the formation of a cleavage furrow. Asters help to position cleavage furrows so that cytoplasmic division results in two evenly divided cells. The cell cortex is responsible for producing the contractile ring that constricts the cell and "pinches" it into two cells. Cleavage furrow formation and cytokinesis are essential for proper development of cells, tissues, and for proper development of an organism as a whole. Improper cleavage furrow formation in cytokinesis can produce cells with abnormal chromosome numbers, which can lead to the development of cancer cells or birth defects.

_Sources:

• _{Lodish, Harvey. “Microtubule Dynamics and Motor Proteins during Mitosis.” Molecular Cell Biology. 4th edition., U.S. National Library of Medicine, 1 Jan. 1970, www.ncbi.nlm.nih.gov/books/NBK21537/.}
• _{Mitchison, T.J. et al. “Growth, Interaction and Positioning of Microtubule Asters in Extremely Large Vertebrate Embryo Cells.” Cytoskeleton (Hoboken, N.J.) 69.10 (2012): 738–750. PMC. www.ncbi.nlm.nih.gov/pmc/articles/PMC3690567/.}