Centrosomes and Centrioles

The Centrosome

• located in the cytoplasm attached to the outside of the nucleus.
• It consists of two centrioles — oriented at right angles to each other — embedded in a mass of amorphous material.
• It is duplicated during S phase of the cell cycle.
• Just before mitosis, the two centrosomes move apart until they are on opposite sides of the nucleus.
• As mitosis proceeds, microtubules grow out from each centrosome with their plus ends growing toward the metaphase plate. These clusters of microtubules are called spindle fibers.

The photo (courtesy of Tim Mitchison) shows microtubules growing in vitro from an isolated centrosome. The centrosome was supplied with a mixture of alpha and beta tubulin monomers. These spontaneously assembled into microtubules only in the presence of centrosomes.

Spindle fibers have three destinations:

• Some attach to one kinetochore of a dyad with those growing from the opposite centrosome binding to the other kinetochore of that dyad.
• Some bind to the arms of the chromosomes.
• Still others continue growing from the two centrosomes until they extend between each other in a region of overlap.

All three groups of spindle fibers participate in

• the assembly of the chromosomes at the metaphase plate at metaphase. Proposed mechanism (the diagram shows only 1 and 2):
  1. Microtubules attached to opposite sides of the dyad shrink or grow until they are of equal length.
  2. Microtubules motors attached to the kinetochores move them
     • toward the minus end of shrinking microtubules (a dynein);
     • toward the plus end of lengthening microtubules (a kinesin).
  3. The chromosome arms use a different kinesin to move to the metaphase plate.
• the separation of the chromosomes at anaphase.
  • The sister kinetochores separate and, carrying their attached chromatid,
  • move along the microtubules powered by minus-end motors, dyneins, while the microtubules themselves shorten (probably at both ends).
  • The overlapping spindle fibers move past each other (pushing the poles farther apart) powered by plus-end motors, the "bipolar" kinesins.
  • In this way the sister chromatids end up at opposite poles.

Other Functions of Centrosomes

• Formation of the network of microtubules that participate in making the cytoskeleton.
• Signaling that it is o.k. to proceed to cytokinesis. Destruction of both centrosomes with a laser beam prevents cytokinesis even if mitosis has been completed normally.
• Signaling that it is o.k. for the daughter cells to begin another round of the cell cycle; specifically to duplicate their chromosomes in the next S phase. Destruction of one centrosome with a laser beam still permits cytokinesis but the daughter cells fail to enter a new S phase.
• Segregating signaling molecules (e.g., mRNAs) so that they pass into only one of the two daughter cells produced by mitosis. In this way, the two daughter cells can enter different pathways of differentiation even though they contain identical genomes. [Link to further discussion.]
• Organizing the primary cilium.
• In at least some developing neurons, the position of the centrosome establishes the point at which the axon will grow out.

Centrosomes and Cancer

Cancer cells often have more than the normal number (1 or 2 depending on the stage of the cell cycle) of centrosomes . They also are aneuploid (have abnormal numbers of chromosomes), and considering the role of centrosomes in chromosome movement, it is tempting to think that the two phenomena are related.

Mutations in the tumor suppressor gene p53 seem to predispose the cell to excess replication of the centrosomes.

Chromosome movement in mitosis also involves polymerization and depolymerization of the microtubules. Taxol, a drug found in the bark of the Pacific yew, prevents depolymerization of the microtubules of the spindle fiber. This, in turn, stops chromosome movement, and thus prevents the completion of mitosis. Taxol is being used with some success as an anticancer drug.

Centrioles

Centrioles are built from a cylindrical array of 9 microtubules, each of which has attached to it 2 partial microtubules.

The photo (courtesy of E. deHarven) is an electron micrograph showing a cross section of a centriole with its array of nine triplets of microtubules. The magnification is approximately 305,000.

When a cell enters the cell cycle and passes through S phase, each centriole is duplicated. A "daughter" centriole grows out of the side of each parent centriole. Thus centriole replication — like DNA replication (which is occurring at the same time) — is semiconservative.

Once formed, most of the functions of the centrosomes can be accomplished without centrioles. However,

• Centrioles appear to be needed to organize the centrosome in which they are embedded.
• The primary cilium forms from the older — "mother" — centriole.
• Microtubules grow out only from the "mother".
• Sperm cells contain a pair of centrioles; eggs have none. The sperm's centrioles are absolutely essential for forming a centrosome which will form a spindle enabling the first division of the zygote to take place.
• When stem cells divide, one daughter cell remains a stem cell; the other goes on to differentiate. [Discussion] In two animal systems that have been examined (mouse glial cells and Drosophila male germline cells), the cell that receives the old ("mother") centriole remains a stem cell while the one that receives what had been the original "daughter" centriole goes on to differentiate. (You can read about these findings in Wang, X., et. al., Nature, 15 October 2009.)
• Centrioles are also needed to make cilia and flagella.

Cilia and Flagella

• locomotion for the cells (e.g., sperm) or
• move fluid past the cells (e.g., ciliated epithelial cells that line our air passages and move a film of mucus towards the throat).

• many short ones, we call them cilia or
• only one or a few long ones, we call them flagella.

• a cylindrical array of 9 evenly-spaced microtubules, each with a partial microtubule attached to it. This gives the structure a "figure 8" appearance when view in cross section.
• 2 single microtubules run up through the center of the bundle, completing the so-called "9+2" pattern.
• The entire assembly is sheathed in a membrane that is simply an extension of the plasma membrane.

This electron micrograph (courtesy of Peter Satir) shows the 9+2 pattern of microtubules in a single cilium seen in cross section.

Motion of cilia and flagella is created by the microtubules sliding past one another — Link. This requires:

• motor molecules of dynein, which link adjacent microtubules together, and
• the energy of ATP.

Each cilium or flagellum grows out from, and remains attached to, a basal body embedded in the cytoplasm. Basal bodies are identical to centrioles and are, in fact, produced by them.

The Primary Cilium

Motile, "9+2", cilia are found only on certain cells in the vertebrate body, e.g., the epithelia lining the airways.

But almost every cell in vertebrates has — or had — a single primary cilium. The primary cilium grows out of the older of the two centrioles that the cell inherited following mitosis.

The primary cilium does not beat because it lacks the central pair of microtubules; that is, it is "9+0".

Some examples:

Mechanoreceptors

Chemoreceptors

Photoreceptors

The outer segment of the rods in the vertebrate retina is also derived from a primary cilium. [View]