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1997 ). Single or double centrioles (diplosomes) were associated with nuclei at
interphase (Figs. 5.21 , 5.22 ), when they were often replicating or occupying
pivotal positions on spindle poles during mitoses (Fig. 5.20 ). Sperm remnants
were sometimes associated with centrioles and were found at most cleavage
stages. Centrioles were also seen in trophoblast, embryoblast, and endoderm cells
in blastocysts. PCM was associated with most centrioles and nucleated spindle
MTs indicating their functionality. We were fortunate to detect three centrioles in
an ICM cell of a blastocyst (Fig. 5.23 ).
5.9 Centriolar Duplication
According to Boveri's classical theory (Boveri 1900 ), the centrosome is a self-
duplicating organelle. The replication of centrioles has been reported in detail in
our previous publications (Sathananthan et al. 1996 ; Sathananthan 1997 ). It occurs
during interphase, beginning at the pronuclear stage prior to syngamy and later at
embryonic interphases. They duplicate only once during each cell cycle and the
daughter centriole arises as a perpendicular outgrowth of the parent (Mazia 1987 ;
Uzbekov and Prigent 2007 ) it is seen to grow progressively and acquires PCM as it
migrates toward the poles of the mitotic spindle. Such immature centrioles have
less PCM and do not nucleate MTs. There is also dense material within each
centriole, as was seen within the sperm centriole. However, we have also observed
a centriole at a spindle pole at syngamy, which appeared as an annular conden-
sation of dense material devoid of MTs. This, called a procentriole, elongates by
accretion of tubulin material to its free end and the pin-wheel arrangement of MT
triplets gradually appears in this dense ring. Classically, in cell division, two
diplosomes (two doublet centrioles) are formed from an original pair of centrioles
and take up positions at opposite poles of the mitotic spindle (Uzbekov and Prigent
2007 ; Alieva and Uzbekov 2008 ). We have observed two centrioles, sometimes
one, at a spindle pole during syngamy and at later cleavage (Figs. 5.21 , 5.22 ) and
in two instances a single centriole at the opposite pole. It is obvious that due to
technical difficulties we are not detecting one of the pair of centrioles at each pole.
We have detected three centrioles in an ICM cell of a blastocyst (Fig. 5.23 ). It is
very likely that centriolar duplication occurs in early embryonic cells that is very
much like that observed in somatic cells, and diplosomes so formed migrate to
opposite poles of the spindle. These centrioles are very likely the ancestors of
those seen in fetal and adult somatic cells and can be demonstrated in embryonic
stem cells, as well. As expected, the behavior of centrosomes during the formation
of the bipolar mitotic figure is similar to that in cell cycles of somatic cells, as
expected (Fig. 5.25 ).
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