Biology Reference
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Fig. 2.1 Schematic
representation of the
centrosome. A pair of barrel-
shaped centrioles formed by
nine triplet microtubules
(diplosome) is surrounded by
the PCM where c-tubulin ring
complexes (cTuRCs) serve as
nucleation sites for
microtubules. Distal and
subdistal appendages can be
observed at the upper end of
the mother centriole.
Reproduced with permission
from J. Lüders
microtubule organizing center (MOCT) of the cell (Fig.
2.1
). The PCM is orga-
nized as a framework that supports microtubular motor proteins like kinesins and
dynein, coiled coil proteins, centrin, pericentrin, speriolin, Cdc20 (spindle
checkpoint protein), and NuMA (Nuclear Mitotic Apparatus protein) among about
100 other proteins (reviewed by Schatten and Sun
2009
). Centrosomes are
involved in numerous cell functions, among them translocation of signal trans-
duction molecules, movement of cell organelles along microtubules and organi-
zation of the cytoskeleton, mitotic spindle, and zygote sperm aster. Microtubules
do not originate from centrioles themselves but from the c-tubulin ring complex, a
collection of annular structures contained in discrete densities of the PCM
(Figs.
2.1
,
2.2
). The c-tubulin ring complex serves as nucleation site for tubulin,
the main component of microtubules, polarized structures with a minus end
anchored to the PCM and a distal plus end where microtubules elongate by
polymerization of a- and b-tubulin heterodimers.
Centrioles and basal bodies are structurally similar and functionally intercon-
vertible. In dividing cells, centrosomes organize the mitotic spindle for chromo-
some alignment, duplication, and partition between daughter cells (Fig.
2.2
).
During generation of cilia and flagella, centrosomes migrate to the cell periphery
where distal centrioles dock to the cell membrane to become basal bodies from
which ciliary and flagellar axonemes originate. When these cells enter mitosis,
basal bodies move back to the cytoplasm and reconstitute centrosomes. This
alternating dual role is essential to understand the functioning of spermatids and
spermatozoa.
Mammalian spermatozoa are the end product of spermiogenesis, a complex
differentiation process in which organelles of round spermatids undergo a series of
