Centrosome

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The centrosome is the main microtubule organizing center (MTOC) of the cell as well as a regulator of cell-cycle progression. It was discovered in 1888 by Theodor Boveri and was described as the 'especial organ of cell division.' Although the centrosome has a key role in efficient cell division, it has been recently shown that it is not necessary [1].

Centrosomes are composed of two orthogonally arranged centrioles surrounded by an amorphous mass of pericentriolar material (PCM). The PCM tethers proteins responsible for microtubule nucleation and anchoring, γ-tubulin, pericentrin and ninein. Each centriole is comprised of nine triplet microtubule blades in a pinwheel structure as well as centrin, cenexin and tektin [1].

Centrosomes are associated with the nuclear membrane during interphase of the cell cycle. In mitosis the nuclear membrane breaks down and the centrosome nucleated mictrotubules can interact with the chromosomes to build the mitotic spindle. The centrosome nucleates and organizes microtubules, playing a role in mitosis. The mother centriole, the one that was inherited from the mother cell, also has a central role in making cilia and flagella [1].

Higher eukaryotic cells possess a centrosome. Yeast cells have a spindle pole body (SPB) which is equivalent to metazoan centrosomes. The spindle pole body differs from the centrosome in many ways, the major difference is the lack of centrioles. Typical angiosperm plant cells do not have centrosomes or anything analogous to them in size, function or organization, but have a number of noncentrosomal MTOCs that lack centrioles. One plant is used as a model organisim for the study of the centrosome - the algae chlamydomonas.

The centrosome self replicates once and only once per cell cycle so that each daughter cell can inherit one centrosome. Aberrant numbers of centrosomes in a cell have been associated with cancer. The centrosome replicates during the S phase of the cell cycle. During the prophase of mitosis, the centrosomes migrate to opposite poles of the cell. Mitotic spindle forms between the two centrosomes. Upon division, each daughter cell receives one centrosome.

Interestingly, centrosomes are not required for the progression of mitosis. When the centrosomes are irradiated by laser, mitosis proceeds normally with a morphologically normal spindle. The microtubules associate into clusters without the presence of a MTOC. Many cells can completely undergo interphase without centrosomes [1].

Figure 1: Role of the centrosome in cell cycle progression. [2]

Although centrosomes are not required for mitosis or survival of the cell, they are required for survival of the organism. Acentrosomal cells lack radial arrays of astral microtubules. They are also defective in spindle positioning and in ability to establish a central localization site in cytokinesis. The function of centrosome in this context is hypothesized to ensure the fidelity of cell division as it is not necessary but greatly increases the efficacy. Cells that have lost their centrosomes arrest in the following cell cycle, because many regulators of the cell cycle associate with the centrosome [1] (Figure 1).

When the nematode C. elegans egg is fertilized the sperm delivers a pair of centrioles. These centrioles will form the centrosomes which will direct the first cell division of the zygote and this will determine its polarity. It is not yet clear whether the role of the centrosome in polarity determination is microtubule dependent or independent.

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