Biology Reference
In-Depth Information
Nevertheless, the absence of centrioles is tolerated only in appropriate
physiological conditions, as confirmed by a series of studies of acentriolar MTOC
and acentriolar spindles in Drosophila melanogaster (for further reading see e.g.
Basto et al.
2006
; Stevens et al.
2007
; Debec et al.
2010
). The discovery that flies
do not need a centriole for cell division confirms that the centriole is a dispensable
element of MTOC. However, the cells lacking centriole cannot generate cilia and
acentrosomal Drosophila eventually die due to cilia deficiencies. Thus, the dom-
inant role of centrioles and centrosomes appears to be linked to their function in
ciliogenesis.
Recent study has shown that some planarians lack centrosomes (Azimzadeh
et al.
2012
). Centrioles and cilia are formed only in their terminally differentiating
muliciliated cells. How planarian cells organize their MT network is, however,
unknown. Notably, the lack of centrosomes does not alter cell division, but seems
to be related to the change in the mode of cleavages during embryo development
from spiral to anarchic-like (ibid.). Curiously, mammalian embryos also lacking
centrosomes have much less stereotypic pattern of cleavages than invertebrates or
amphibians. These two examples suggest that centrosomes may control the pattern
of cleavages during early development more tightly than it was previously thought.
20.2 Centrosome Reduction During Gametogenesis
As stated above, during gametogenesis the centrosomes are reduced in both male
and female gametes. The restoration of a fully functional centrosome occurs
during fertilization. Theodor Boveri already suspected and wrote 100 years ago
that the male gamete ''sperm'' was carrying a ''division centre'' with the ''cyto-
plasm active substance'' whereas the female gamete ''oocyte'' was carrying the
''cytoplasm active substance'' with the ''division centre''. After fertilization the
fusion of the division center and the cytoplasm active substance would give rise to
an active centrosome. It is now known that, in most cases, upon fertilization the
sperm brings the centrosome and the oocyte is the source of the cytoplasm
proteins.
20.2.1 Centrosome Reduction in Spermatozoa
The centrosomes reduction was reported in many species of invertebrates (insects,
mollusks) suggesting that this phenomenon is the rule rather than exception during
spermatogenesis (reviewed by Manandhar et al.
2005
). Among vertebrates, this
process was the most extensively studied in mammalian spermatozoa (Manandhar
et al.
1998
,
2000
), and not much information is available on the centrosome
reduction in spermatozoa in amphibians. Centrioles are present in Xenopus laevis
spermatozoa (Bernardini et al.
1986
). However, the fact that Xenopus sperm heads
