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Eukaryotic cells, that is, those that make up protists , fungi, plants and animals, present a scaffolding similar to a skeleton, the so-called cytoskeleton (etymologically, “cellular skeleton”). This cytoskeleton maintains the shape and internal organization of the organelles, allows various movements, and mediates the transit of structures and substances at the intracellular level. One of the components of the cytoskeleton are microtubules., which are tubular structures made up of proteins called alpha and beta tubulins. Among other functions, microtubules participate in cell division by facilitating the movement of chromosomes, which in turn are structures composed of deoxyribonucleic acid, the molecule that carries genetic material.
Many types of eukaryotic cells have a specialized array of microtubules called centrioles, which are found in the region of the cytoplasm close to the nuclear envelope known as the centrosome. In dividing cells, the centrioles appear surrounded by a group of short filaments arranged in a star shape: the asters.
The function of asters during cell division
Before entering cell division, during a stage called interphase, cells duplicate their genetic material, their organelles, and structures such as their centrosome (along with the centrioles it contains). Towards the end of interphase, the duplicated centrosome splits, leaving two centrosomes, each with a pair of centrioles.
Once interphase is over, cells begin their cell division by entering prophase, a stage during which the microtubules reorganize to form a structure called the mitotic spindle. Spindle formation is preceded by the appearance of asters: each aster migrates to opposite positions within the cell, thus establishing the poles from which the spindle will form.
The already formed mitotic spindle is composed of three types of fibers: the asters, which surround the centrioles and whose ends radiate in all directions; the kinetochore microtubules, which are attached at one end to the kinetochores of each duplicated chromosome; and polar or interpolar microtubules, which grow without finding a kinetochore to attach to.
At the end of prophase and the beginning of the next stage, metaphase, the microtubules of the asters are much more numerous and shorter than in interphase, and they do not establish contact with the surrounding pair of centrioles.
In the next stage, anaphase, the spindle lengthens due to the action of proteins that form bridges between polar microtubules, pulling them towards the pole from which they came. Other types of proteins bind the microtubules of the aster to the membrane or proteins of the underlying cell (ie, one of the cells that will remain after the original dividing cell breaks away); This contributes to the displacement of the centrioles and asters, to the elongation of the cell and to the cell poles becoming more spherical before the separation of the daughter cells.
Precisely, the separation of the daughter cells or cytokinesis is produced by the strangulation of the cytoplasm. Here, the role of spindle microtubules is not very clear, taking into account experiments in which they have been removed after metaphase in hedgehog cells, in which cytokinesis occurs normally and the aster disappears in the spindle. telophase, stage after anaphase and before the separation of the cytoplasm.
The question about the role of asters in cytokinesis is not the only one pending to be resolved. Among other issues, it remains to determine the mechanism that allows the radius of each aster microtubule not to change as it expands, to identify the mechanism for the aster to separate from the centrosome, and to establish how its growth is inhibited. All these questions require the study of new molecular, biochemical and biophysical mechanisms.
Sources
Alfredo de Jesús Rodríguez-Gómez, Sara Frias-Vázquez. Mitosis and its regulation . Pediatric Act of Mexico. 35(1): 55-86, 2014.
Paniagua, R., Nistal, M., Sesma, P., Álvarez-Uría, M., Fraile, B., Anadón, R., Sáez, F. Cellular Biology . 3rd edition. McGraw Hill Inter-American., Madrid, 2007.
TJ Mitchison, M. Wühr, P Nguyen, K. Ishihara, A. Groen, and CM Field. Growth, interaction and positioning of microtubule asters in extremely large vertebrate embryo cells . Cytoskeleton (Hoboken) . 69(10): 738–750, 2012. doi:10.1002/cm.21050.
