Biology Reference
In-Depth Information
neurona, another apicomplexa parasite of veterinary importance. In this aspect
centrocone and possibly kinetochore persistence could be a general aspect of
apicomplexan nuclear organization.
As the IMC associated with the subpellicular microtubules extends, the forming
daughter cell includes first the centrosome and the Golgi complex followed by the
remaining organelles. New sets of apical organelles important for migration and
host cell invasion (rhoptries and micronemes) are formed in both apical ends of the
nascent daughter cells. The mitochondrion is the last organelle to be incorporated
into the forming cell (Nishi et al.
2008
), although just how this works is less clear
as for the other organelles. Each daughter cell continues to mature until the
cytoplasm and all its contents are divided between the two daughter cells. Even-
tually, the inner membrane complex of the mother cell breaks down, and the
plasma membrane is used to form part of the plasma membrane of the daughter
cells. The cleavage to separate the daughter cells begins at the apical pole,
extending through the cell bodies. During this process more plasma membrane is
formed through the fusion of new membranous structures derived from Rab-11-
mediated vesicular traffic (Agop-Nersesian et al.
2009
,
2010
). At the end of this
process, there are two daughter cells and a residual body at the posterior end
connecting the new cells. Within the same host cell, each daughter cell can then
replicate again and again until ultimately their progeny will exit the host cell to
infect neighboring cells (Fig.
19.4
a).
During interphase, the centrosome locates near the apical polar ring. As the
division process continues the centrosome links to the spindle pole plaque or
centrocone, a structure localized at the nuclear envelope and responsible for the
formation of the intranuclear spindle microtubules (Fig.
19.4
c). At the initial steps
of endodyogeny, the centrosome is associated with the Golgi apparatus. The
centrosome protein centrin is located near the Golgi during mitosis (Stedman et al.
2003
). As the Golgi grows, the centrosome remains associated at one side.
Afterwards, the centrosome re-localizes to the posterior pole of the mother cell
nucleus. At this location, the centrosome divides, returning then to the apical part
of the mother nucleus, re-connecting with the Golgi complex, with each centro-
some associating with each inner end of the duplicated Golgi.
Another important role of the centrosome is the division of the apicoplast
(Striepen et al. 2000). Curiously, parasites in which the apicoplast was destroyed
can undergo one full cycle of replication before dying a ''delayed death'' (He et al.
2001
). Thus, like the single mitochondrion and the Golgi, this organelle must be
correctly duplicated and segregated to each forming cell. To assure this, the
apicoplast division is linked to the parasite's mitotic division process by associ-
ation with the dividing centrosomes. As these move away from each other, the
apicoplast elongates. With the budding of the daughter cells, the apicoplast is
pulled into a U shape and divided in two halves with a dynamin-related protein
playing a key role during fission (van Dooren et al. 2009). Each new daughter
plastid stays connected to one centrosome and migrates together with the newly-
formed nucleus into the daughter cell. Thus, the association with the nucleus via
the centrosome is a perfect strategy to assure that each daughter cell receives a
