|Centrioles and Centrosomes
The centrosome is the microtubule-organising centre (MTOC) of the animal cell. This means that it plays
a central role in triggering and directing the growth of protein rods called microtubules, which are part of the
cytoskeleton. Microtubules have a number of functions. They provide mechanical support for cell appendages,
such as undulipodia. They also act rather like 'monorails' along which organelles can be transported from one
part of teh cell to another. Phylogenetic studies suggest that the last common ancestor of all eukaryotes had a
centrosome, but that was lost later on by plant cells and fungi, but retained in cells whose lineages have cilia or
flagella. The centrosome consists of one pair of centrioles and the surrounding periocentriolar matrix
(PCM). Each centriole is about 250 nm in diameter and 700 nm long (1 nm = 1 nanometre = 1 billionth or 10^-9
of a metre). The diagram above shows a 3D model of a centriole in end-view. Below is the centriole viewed at
an angle (left) and from side-view (right).
Each centriole is itself made up of microtubules, actaully a cyinder of 9 microtubule triplets (shown in blue). Each
microtubule is a hollow cylinder, about 25 nm in diameter (with a central bore about 15 nm in diameter). Copies of
a linker protein connect adjacent triplets together (shown in red).
Each microtubule is itself a hollow cylinder
whose wall consists of 13 protofilaments of
a protein called tubulin, as shown on the left
and right. In the central, however, each triplet
consists of one complete microtubule joined to
two incomplete microtubules, containing 10
protofilaments each (exact numbers of
protofilaments seem to vary).
For a detailed and advanced article on the
role of microtubules in cell crawling click here.
The pair of centrioles in a cell are arranged with each centriole perpendicular to the other. The purpose of this
arrangement is not clear, however the oldest of the two centrioles, the mother centriole, can incorporate into a
cilium of flagellum as the basal body (see undulipodia) and does so, for example, in mammalian cells where it
incorporates into the primary cilium as the basal body. Plants and fungi have lost their flagella/cilia at all stages of
their life-cycle and so lost the animal-type MTOC, but they still have MTOCs which lack the pair of centrioles
which therefore appear to function primarily in basal body formation. With the mother centriole inserted into the
base of a cilium, presumably the younger daughter centriole is perpendicular to hold it out of the way.
[Note: by organelles I mean a 'little organ' or structural part of the cell which is specialised to perform specific
functions, irrespective of whether or not the organelle is membrane-bound. Some scientists in recent times have
adopted the term 'organelle' to refer only to membrane-bound structures, but this was not the earlier meaning of
'organelle' which I adopt. Centrosomes have no membranes.]
There is normally only one centrosome (one pair of centrioles) per animal cell, at least at the start of the cell
cycle. The centriole, however, duplicates once per cell cycle (i.e. once per cell duplication) so that when a cell
divides into two, both daughters receive one centrosome.
To see the role of the centriole in organising the mitotic spindle in cell division see the article on the cell cycle.
To be continued...
Article last updated: 8/3/2014.
Above: a simplified diagram of the centrosome. The mother and daughter centriole are
at right-angles to one-another. A central region, the periocentriolar matrix (which
sometimes appears to be centred on the mother centriole) is surrounded by radiating
microtubules. The bases of the two centrioles are actually joined by filamentous proteins
which cross the periocentriolar matrix from one centriole to the other (these have been
omitted as their precise arrangement seems unclear and may be variable). Many of the
protein components of these complex organelles have been omitted as their
arrangements and functions remain poorly understood.