Biology Reference
In-Depth Information
20.1 Centrosome Structure and Major Functions
The centrosome is a complex organelle with a crucial role in the organization of
microtubule (MT) cytoskeleton, which in turn organizes many aspects of cell
architecture including polarity and ciliogenesis. Typically, a centrosome is com-
posed of a pair of centrioles surrounded by electron dense fibrillar pericentriolar
material (PCM) (Fig.
20.1
). In fact, the centrosome is organized by centrioles.
The centrosome controls crucial cellular processes such as cell division,
polarity, motility, sensing the environment and communication with neighbor
cells. These cellular centrosomal functions allow coordination of embryo devel-
opment starting from oogenesis and ending with tissue differentiation. The cen-
trosome is the major MT-organizing center (MTOC) in animal cells, even if in
certain cell types, other MTOCs (secondary or diffused) may take control over MT
organization. During the interphase, centrosomes nucleate MTs. The plus (+) ends
of MTs are directed toward the cell periphery, while their minus (-) ends remain
close to the MTOC. The centrosome controls the formation of the organized array
of long cytoplasmic MTs radiating toward cell periphery from the discrete ''cell
center'' determined by the position of the centrosome, which is usually located in
the vicinity of the nucleus or in a discrete nuclear pocket. This function of the
centrosome is crucial for establishment and maintenance of cell polarity (recently
reviewed by Bornens
2012
). Centriole duplication takes place in the S phase, thus,
in the G2 phase of the cell cycle each cell possesses already four mature centrioles.
At the G2/M transition, centrosomes separate to build future division spindle poles
and increase their MT nucleating activity. During mitosis the centrosomes actively
participate in the formation of the bipolar spindle via nucleation of MTs, very
dynamic, and much shorter than in the interphase. Each of duplicated and sepa-
rated centrosomes forms a spindle pole. MTs originating from centrosomes are
captured by chromosomes, and more precisely by the specialized structures called
kinetochores, to complete the assembly of the spindle.
The size and number of centrioles, the key constituent of a centrosome, is
precisely controlled in the cell, while the volume of PCM change during the cell
cycle and varies in different cell types changing the size of the centrosome. Within
the centrosome, both centrioles are connected through their proximal ends during
G1 and S phase. The two centrioles are functionally and structurally unequal. Due
to the semi-conservative mode of centriole duplication (each centriole gives rise to
a new centriole, then the pair splits and give rise to a new centrosome), one of
them is always more matured than the other (Kochanski and Borisy
1990
).
Figure
20.2
illustrates the centrosome cycle, including centriole duplication and
maturation within the centrosome in relation to the nuclear cell cycle phases G1, S,
G2, and M. The centriole more advanced in maturation is called the mother
centriole (centriole ''père'' in French, or ''father'', since the centriole is masculine
in French) and the younger one: the daughter centriole (centriole ''fils'' in French,
or ''son'') (see also Fig.
20.1
). At the structural level the mother centriole pos-
sesses appendages at the distal end. The daughter centriole is slightly shorter. The
