Biology Reference
In-Depth Information
20.1.1 Role of Pericentriolar Material (PCM) vs. Centrioles
Centrioles, through their capability to duplicate once per cell cycle, assure also the
duplication of centrosomes. Similar to DNA replication the centrosome duplica-
tion must occur once and only once per cell cycle to keep their correct number.
However, in contrast to DNA replication, centrosome duplication can be discon-
nected from cell cycle progression, despite that, physiologically, both remain
tightly correlated in time (Gard et al.
1990
; Hinchcliffe et al.
1999
). The PCM, on
the other hand, bears the functional and regulatory machinery for the nucleation of
MTs. Centrosomes nucleate four kinds of MTs, the interphasic, mitotic, centriolar,
and those of cilia. Not all cells nucleating MTs contain centrosomes. In verte-
brates, including Xenopus laevis, fully grown female germ cells (oocytes) do not
have centriole, but they possess functional MTOCs formed exclusively by the
PCM foci. Because the centrosome is structurally defined as an organelle com-
posed of centrioles and PCM, it precludes that oocytes have no centrosomes.
However, there is no doubt that oocytes do possess MTOCs. Molecular studies of
the composition of oocyte MTOC have confirmed that it corresponds to the cen-
trosomal PCM. Thus, in oocytes the PCM alone is apparently sufficient to fulfill
the major functions of centrosomes, namely MT nucleation and organization.
As mentioned above, centrosomes are self-replicating organelles and centriole
duplication plays a critical role in this process. In somatic cells the restriction of
centrosome duplication to only once per cell cycle is of the highest importance for
their physiology (Tsou and Stearns
2006
). An incorrect number of centrosomes
predisposes cells to chromosome missegregation and induces aneuploidy eventu-
ally leading to carcinogenesis (Nigg
2007
).
It is still unknown how in the cells lacking centrosome sensu stricto, in which
MTOCs (PCM foci) nucleate MT, the information about the structure of centro-
some is transmitted from a mother cell to a daughter and how the progeny of such
cells are able to develop centrioles and centrosomes during development. The best
example is a mouse embryo in which cell divisions during at least five initial
cleavages occur without the centrioles (Gueth-Hallonet et al.
1993
). How are these
MTOCs re-substituted after each division remains absolutely unknown? Do they
retain the structural memory of the centriole structure revealing itself when cen-
trioles form de novo? We mention here the mouse model to illustrate the general
lack of knowledge on the nature of the heredity of centrosomal structures.
Although it is now accepted that the centrosome does not contain any DNA, it
was suggested that the centrosome may contain specific RNAs. These RNAs were
identified in centrosomes isolated from the clam Spisula solidissima oocytes
(Alliegro et al.
2006
; Alliegro and Alliegro
2008
) and they also seem to be present
in Xenopus laevis and human centrosomes (Blower et al.
2007
; Alliegro
2008
).
Thus, besides the proteins, which form an extremely well-organized structure, the
centrosome may contain RNA. The presence of ''centrosomal RNAs'' is still a
very controversial area. It stimulates, however, a discussion on whether such
RNAs can be involved in centriole/centrosome duplication. Two major questions
