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(Sluder et al.
1986
; Zhang et al.
2004
). Immunofluorescence studies confirmed that
the aster is indeed composed of MTs (Schatten et al.
1986
,
1987
). In the sea urchin
Lytechinus pictus zygote, the mechanical separation of male and female pronuclei
(with their accompanying respective cytoplasm including the paternal centrosome
next to the male pronucleus) immediately after fertilization, produces two haploid
half-embryos behaving in two very distinct ways. The male pronucleus-containing
half-embryo forms the sperm aster, centers its pronuclus, and then assembles a
division spindle and undergoes series of regular cleavages. On the other hand, the
female pronucleus-containing half-embryo undergoes nuclear envelope breakdown
and reformation without either spindle formation or cleavages (Kubiak
1991
).
Thus, apparently, the sperm aster formed by the paternal centrosome is the
prerequisite for successful embryo cleavages. The spermatozoon delivers the
paternal centriole to the oocyte during fertilization and it is necessary to support
the centrosome reconstitution and further duplication for the first mitotic division.
We described here the example of transparent and relatively small sea urchin
embryo, especially appropriate for this type of experiments, but the same role of
the paternal centrosome is also attributed to the first cell cycle during Xenopus
laevis development (Fig.
20.3
). This is the best illustrated by the absence of
cleavages in parthenogenetic Xenopus embryos (Klotz et al.
1990
). The parthe-
nogenetic activation of eggs results in cycling embryos undergoing cell cycle
transitions, from interphase to mitosis and back to the interphase, without
cleavages. However, when a single centriole is experimentally introduced into
such a one-cell parthenogenetic embryo, the division spindle forms (Maller et al.
1976
) and cleavages reappear with normal frequency and timing (Tournier et al.
1989
; Klotz et al.
1990
). Moreover, only the centrioles capable to duplicate have
the capacity to support the parthenogenetic development of frog (Tournier et al.
1991a
,
b
). Surprisingly, an electron microscopy study showed that the mature
centrioles injected into Xenopus laevis parthenogenetic embryos were transformed
into the juvenile centrioles showing the capacity of the cytoplasm to transform
these organelles into a novel juvenile form (Nadezhdina et al.
1999
). The absence
of centrioles in Xenopus parthenogenetic embryos prevents successful develop-
ment. Thus, the evolutionary conserved mechanism involved in centrosome
reduction and centriole elimination in the oocytes seems to protect the species
against parthenogenetic mode of reproduction. It may be, therefore, one of the key
mechanisms promoting sexual reproduction in animal kingdom.
Interestingly, the mouse, which, as we already described above, also loses
centrioles during oogenesis (Kloc et al.
2008
; Szöllosi et al.
1972
), the cleavages
of early parthenogenetic embryos proceed normally (Tarkowski et al.
1970
).
However, in this species, during normal fertilization the sperm-delivered centriole
disintegrates in the zygote and the early cleavages occur without centrioles until
their de novo formation in the early blastocyste stage (Gueth-Hallonet et al.
1993
).
This implies that in the mouse, the mature centrosomes are exclusively of maternal
origin. In this context it is not surprising that in contrast to Xenopus, mouse
parthenogenetic and normal embryos fully support cleavages and early steps of
development. However, we have to stress here that in other mammals, including
