Asters represent a structural arrangement within a dividing animal cell, appearing during mitosis and meiosis. These structures are defined as a star-like array of microtubules that radiate outward from a central point, which is the cell’s main microtubule-organizing center. The formation of these radial arrays is a distinct feature of cell division in most animal species. Asters are important for accurately positioning the cellular division apparatus, ensuring that the genetic material is divided equally between the two resulting daughter cells.
The Anatomy of an Aster
The aster’s structure is centered around the Microtubule Organizing Center (MTOC), typically the centrosome in animal cells. The centrosome, duplicated before cell division, is composed of a pair of cylindrical centrioles surrounded by the pericentriolar material (PCM). This amorphous mass of proteins is the site where the microtubules are nucleated and begin to grow.
Microtubules are hollow, rigid tubes made of the protein tubulin and form the radiating “rays” of the aster. These are called astral microtubules to distinguish them from other microtubules of the spindle apparatus. They are dynamically unstable, rapidly growing and shrinking, which allows them to explore the inner boundaries of the cell. The outward growth of these astral microtubules from the centrosome establishes the spindle pole, from which the main division machinery will operate.
Functional Role in Cell Division
The primary function of asters is to organize and correctly position the mitotic spindle apparatus within the cell’s cytoplasm. As asters form at the two poles of the dividing cell, the astral microtubules extend toward the cell’s inner surface, known as the cell cortex. This interaction provides the necessary anchoring and orientation for the entire spindle.
Motor proteins, such as dynein, are situated on the cell cortex and attach to the growing astral microtubules. Because these proteins are tethered to the cell membrane, their attempt to move toward the centrosome generates a pulling force. This force helps draw the two spindle poles apart and accurately center the spindle within the cell. Through this anchoring and force generation, asters ensure that the chromosomes align correctly and that the plane of cell division is established symmetrically. The astral microtubules also play a role in signaling the location of the cleavage furrow, the indentation that ultimately pinches the cell into two new cells during the final stage of division, called cytokinesis.
How Asters Differ Across Life Forms
Asters are a feature of cell division in most animal cells, but their presence is not universal across all eukaryotic life forms. The type of cell division that includes asters is termed astral mitosis, which is characteristic of metazoans. Many other eukaryotes, including higher plants and most fungi, undergo anastral mitosis.
In anastral mitosis, cells lack a centrosome with centrioles, meaning the star-shaped array of microtubules does not form. These organisms still form a functional mitotic spindle, but the microtubules are nucleated by other Microtubule Organizing Centers. These MTOCs are often located around the nuclear envelope or from dispersed sites within the cytoplasm. This difference highlights the evolutionary diversity in how organisms achieve the goal of accurate chromosome separation. Even without the anchoring provided by asters, these cells can organize their spindles and successfully divide their genetic material.
Importance in Cellular Health
The successful completion of cell division relies on the accurate anchoring of the mitotic spindle by astral microtubules. This anchoring is necessary for ensuring that sister chromatids are pulled apart equally into the two daughter cells. When asters malfunction or the spindle is improperly anchored, the resulting division can be asymmetric or misaligned.
These errors can lead to aneuploidy, where the daughter cells receive an incorrect number of chromosomes. Aneuploidy is a form of genomic instability associated with the development and progression of various diseases. For instance, many cancer cells exhibit high levels of aneuploidy, often due to failures in the mitotic process stemming from defects in the astral microtubule apparatus.