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6.9.2.7. Centrosome

Centrosome

The centrosome or cytocenter is a nonmembranous organelle, present in all animal cells  that can divide (they are not present, for example in human red blood cells or in neurons, which cannot divide).

The centrosome is located close to the nucleus, often surrounded by the Golgi apparatus.

Plant cells do not have centrioles.

Structure and composition of the centrosome

The centrosome is made up of:

  • Two centrioles or diplosome. The structure of the centrioles is identical to that of the basal corpuscles of the cilia. They occupy the center of the centrosome. They are oriented perpendicular to each other. Each centriole is cylinder-shaped, with walls formed by nine triplets of microtubules, without any central microtubule, forming the so-called 9 + 0 structure.

The microtubules that make up triplets are called:

    • Microtubule A: the innermost microtubule, circular in section (13 protofilaments), and closest to the axis of the cylinder.
    • Microtubule B: located between microtubules A and C. Its section is shaped like a half moon, and it shares 3 protofilaments with microtubule A.
    • Microtubule C: the outermost. It also has a crescent-shaped section and shares 3 protofilaments with microtubule B.

The triplets are linked together by a bridge formed by a protein called nexin.

When the cell divides, the centrioles double, and each diplosome goes to a daughter cell.

In cell division, each of the centrioles gives rise, by duplication, to its partner. In this way, two diplosomes are obtained, one for each daughter cell.

  • The pericentriolar material or centrosphere surrounds the diplosome. It is an optically dense material.

The microtubule organizing center (COMT) is the set of centrioles and pericentriolar material.

  • The aster, fibers made up of microtubules that grow and are organized radially around the centrosome. Aster microtubules give rise to achromatic spindle microtubules in cell division.

The centrosome or cytocenter is called the set of diplosome, centrosphere and aster.

All microtubule structures such as cilia, flagella and the achromatic spindle come from the centriole. Although the cell does not have centrioles, the pericentriolar material and aster fibers will appear.

Centrosome function

The function of the centrosome is to organize the microtubules. But to perform this function, it seems that it can be performed only with the pericentriolar material, since the achromatic spindle is also formed in plant cells and these lack centrioles. They only differ in that in plant cells the fibers of the achromatic spindle start from a diffuse zone (anastral mitoses), while in the rest of cells, the fibers start from a specific point.

All structures formed by microtubules are derived from the centrosome, such as:

Fundamental ideas about the cytocenter or centrosome

The centrosome or  cytocenter

  • It is a nonmembranous organelle, characteristic of animal cells, so it is not found in plant cells.
  • Structure of the centrosome:
    • Two centrioles or diplosome.
    • Centrosphere or pericentriolar material, which surrounds the centrosome.
    • Aster.
  • Centrosome function:
    • Organization of microtubules.
    • All structures formed by microtubules are derived from the centrosome:
      • Cilia and flagella.
      • Achromatic spindle.
      • Cytoskeleton.