Biology Reference
In-Depth Information
Fig. 19.1 Examples of microtubular structures in Apicomplexa as revealed by electron microscopy.
a Negative-stained Toxoplasma after membrane extraction showing the conoid (C), subpellicular
microtubules (arrowheads)andtheupper (thick arrow)andlower (thin arrow) polar rings (Modified
after Morrissette et al.
1997
; with permissions of The Company of Biologists). b A thin section of
Plasmodium undergoing nuclear division, showing centriolar plaques (Cp), intranuclear microtubules
(Nm), and electron-dense structures (Ch and open arrows). Arrow points totheparasiteplasma
membrane and the arrowhead to the nuclear envelope (Modified after Aikawa and Beaudoin
1968
;
with permissions of The Rockefeller University Press). c Centrioles of Eimeria with the unusual
''9+1'' arrangement. In the vicinity, extranuclear microtubules are observed (After Dubremetz and
Elsner
1979
; with permissions of the John Wiley and Sons)
In order to successfully infect a host and be able to be transferred from one host
to another, these parasites must proliferate. As obligatory intracellular pathogens,
the apicomplexans duplicate and divide within a host cell. The nuclear divisions in
apicomplexans are achieved by cryptomitosis, where the nuclear membrane con-
tinues to be present during the different phases of the cell division process, with no
chromosomal condensation during karyokinesis (Striepen et al.
2007
). The repli-
cation process occurs in three different manners: endodyogeny, where two
daughter cells are formed within a mother cell; schizogony, in which sequential
nuclear divisions are followed by budding of new parasites; or by endopolygeny,
with sequential DNA replication without nuclear division (Fig.
19.2
). Endody-
ogeny allows the parasite to remain infective even during the division process,
since the mother cell still preserves its apical structures. In schizogony, however,
the apical complex and the cytoskeletal system break down. They are eventually
being formed de novo just before the budding of progeny cells occurs. This process
