An aster is a star-shaped cellular arrangement composed of a centrosome and its radiating microtubules. This structure forms within animal cells during the initial phases of mitosis. Its formation marks an early step in preparing the cell for the precise distribution of genetic material, providing foundational organization as the cell prepares for replication.
Anatomy of an Aster
The core of an aster is the centrosome, the primary microtubule-organizing center within the cell. Extending outward from this central body are numerous long, fiber-like structures known as astral microtubules. These microtubules fan out radially, giving the structure its characteristic star-like appearance.
Astral microtubules are a specific subset emerging from the centrosome. Unlike other microtubules in cell division, these do not directly attach to chromosomes. Their formation involves nucleation, where new microtubules polymerize from the centrosome, growing outwards into the surrounding cytoplasm.
Role in Cell Division
During cell division, asters play a role in the precise arrangement of the mitotic spindle, an apparatus responsible for chromosome segregation. Astral microtubules extend towards the cell cortex, the inner surface of the cell’s outer membrane. This connection anchors the spindle poles securely within the cell.
Attachment of astral microtubules to the cell cortex helps position the mitotic spindle accurately. Motor proteins, specifically dyneins, are involved, pulling on these microtubules to adjust the centrosomes’ location. This pulling action contributes to the proper orientation of the spindle and influences where the cell will ultimately divide.
Accurate spindle positioning ensures each new daughter cell receives an identical and complete set of chromosomes. This placement also helps determine the cleavage furrow’s location, the indentation that forms as the cell prepares to split. The aster’s collective actions support the fidelity of cell division.
Presence in Animal Cells vs. Plant Cells
Asters are a prominent feature in animal cell division. They form around the centrosomes as these cells divide. This presence is a distinguishing characteristic of animal cell mitosis.
In contrast, asters do not form during mitosis in higher plants and fungi. These organisms employ different mechanisms for organizing their mitotic spindles. Instead of concentrated centrosomes, plant cells utilize multiple smaller microtubule-organizing centers scattered throughout the cytoplasm to assemble their spindle apparatus.