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do not contain c-tubulin demonstrates that centrosomes are also reduced in this
species (Félix et al.
1994
; Stearns and Kirschner
1994
).
20.2.2 Centrosome Reduction in Oocytes
The phenomenon of centrosome reduction during oogenesis has been much better
described for mouse than for Xenopus oocytes. The reason for that is the large size
of fully grown amphibian oocytes. It makes their electron microscopy study by
conventional methods very difficult and thus the direct examination of such tiny
structures as potential centrosomes or PCM foci is almost impossible. The
immunofluorescence localization of selected antigens has been of great help. Early
studies of Xenopus laevis oocytes by immunolocalization of MTs suggested, as
judged by the barrel shape of the meiotic spindles and the general manner of MTs'
organization upon the entry into the first meiotic division, that centrioles are absent
from these oocytes (Huchon et al.
1981
). Later studies using confocal microscoy
confirmed these observations (Gard et al.
1995a
,
b
). The immunolocalization of
centrin and electon microscopy images of very early stages of oogenesis in ovarian
cysts of female froglets show the presence of centrioles (Kloc et al.
2004
, and
Fig.
20.1
in this paper). Thus, in Xenopus laevis the centrioles disappear at the
beginning, or during the long-lasting phase of oocyte growth. Similarly, the
presence of centrioles was also demonstrated in mouse fetal oogonia still orga-
nized in the cytocysts (Kloc et al.
2008
). The centrioles were, however, absent
from spindle poles of fully grown metaphase I and metaphase II mouse oocytes
(Szöllosi et al.
1972
). Thus, both in Xenopus and the mouse the disintegration of
centrioles seems to occur during the oocyte growth phase, but so far the mecha-
nism and precise timing of this phenomenon remain unknown.
20.3 Centrosome Inheritance upon Fertilization
The clear biological relevance of the reduction of centrosomes in the gametes is
unknown. The morphological reduction is probably related to the functional one.
The spermatozoon and the oocyte are fully differentiated cells. The reduction of
centrosomes may be related to a diminution of their MTOC function in gametes.
However, as these functions are of great importance for the somatic cells, fully
active centrosomes must be rebuilt following fertilization.
Upon fertilization the spermatozoon and the oocyte join together and reduce
paternal and maternal centrosomes (Fig.
20.3
). The MTOC activity of the paternal
centrosome is easily detectable due to the rapid formation of so-called sperm aster
of MTs. The sperm aster's major role is to bring the male and female pronuclei
together. This role of the paternal centrosome was the easiest to be visualized
in transparent zygotes of marine invertebrates such as sea urchin of starfish
