Coccidian Development In The Definitive Host

2.3.1 Host-parasite relationship

Coccidian development is limited to the epithelial cells of the small intestine of the cat (the definitive host). In all stages undergoing coccidian development, the parasites are located within a tight-fitting,

FIGURE 2.9 Toxoplasma gondii developing in enterocytes of the cat intestine.

(A) Early developmental stage located in a thick-walled, tight-fitting parasitophorous vacuole (PV). N, nucleus. Bar = 1 | m.

(B) Enlargement showing the laminated structure of the electron-dense membrane (PVM) limiting the parasitophorous vacuole. Note a conical structure protruding into the membrane (arrow). P parasite pellicle. Bar = 0.1 |im.

(C) Tangential section through the membrane of the parasitophorous vacuole illustrating the circular shape of the conical protrusion in to the parasitophorous vacuole membrane (arrows). Bar = 0.1 | m.

(D) Detail in which the membrane of the vacuole can be resolved into three unit membranes (arrowheads). Bar = 0.1 | m.

(E) Mid-stage schizont with a number of nuclei (N), and a centrally located elongated apicoplast (A). Mi, mitochondrion. Bar = 1 |im.

(F) Detail showing the double membranes enclosing the mitochondrion (Mi) and nucleus (N), compared to the multiple membranes enclosing the apicoplast (A). Bar = 0.5 |im.

thick walled parasitophorous vacuole (Figure 2.9A) (Ferguson et al., 1974; Ferguson, 2004). At higher power the wall has a laminated appearance, which in certain areas can be seen to consist of three closely applied unit membranes (Figure 2.9D). In addition there are a number of conical-shaped, dense structures impinging on the luminal surface of the PV (Ferguson, 2004) which, in certain cases, appear to connect the surface of the parasite to the parasitophorous vacuole membrane (Figures 2.9B). In contrast to the host-parasite relationship of the tachyzoite, there is no evidence of formation of the tubular structure within the PV or the congregation of the host-cell mitochondrion or strands of rER around the periphery of the PV (Figure 2.9A). These structural differences correlate with the lack of expression of the majority of dense granule proteins. Of the GRAs 1-8 and NTPase identified in the tachyzoite, only GRA7 and NTPase are expressed by the gut stages (Ferguson et al., 1999a, 1999b; Ferguson, 2004). This laminated, thick-walled PV is similar to that observed for certain Isospora species (Ferguson et al., 1980) to which T. gondii is closely related, but differs from those of the genus Eimeria, which are limited by a single-unit membrane (Ferguson et al., 1976).

2.3.2 Asexual development

During coccidian development only a single asexual process has been observed, which has unique structural features and has been termed endopolygeny (Piekarski et al., 1971). This term had been used previously to describe an abnormal type of development observed for the tachyzoite (Vivier, 1970). However, the abnormal tachyzoite development described did not represent an internal budding process. Therefore, because of the accuracy of the description and its usage over the years, it would appear appropriate to retain the term for the description of the asexual multiplication of the coccidian stages. In studies of coccidian development of both type I and type II strains of T. gondii occurring between 4 and 10 days p.i. only a single process was observed, although there are marked variations in the number of daughters formed. The process involves growth of the parasite and repeated nuclear divisions (Figure 2.9E)

employing an excentric intranuclear spindle, as described during endodyogeny. There is also a marked increase in the size of the mitochondria, which are located predominately around the periphery. In addition it is possible to observe multiple profiles of the apicoplast (limited by four membranes), but these were more centrally located and, from immunocytochemistry, appeared to consist of a single branched structure (Ferguson et al., 2005). These three organelles can be differentiated by their ultrastructural features (Figure 2.9F). The number of nuclear divisions varies between parasites, which has a direct effect on the number of daughters formed. It is the presence of this proliferative phase prior to daughter formation that distinguishes endopolygeny from endodyogeny. It is not clear how the number of nuclear divisions is controlled, but it does not appear to relate to parasite size or a given number of nuclear divisions, since these can vary markedly between parasites (cf Figures 2.10A, 2.10C).

The trigger for the end of the proliferative phase and the initiation of the differentiation phase (daughter formation) is unclear. It is at the end of the proliferative phase that the elongated apicoplast divides simultaneously into a number of fragments equal to the number of nuclei (Ferguson et al., 2005). Daughter formation can occur at any time between the 4- and approximately 20-nuclei stage, and is initiated during or just after the final nuclear division (Figures 2.10A, 2.10B). The first evidence of the initiation of daughter formation is the appearance of a conical structure formed by a number of flattened vesicles each with underlying longitudinally running microtubules and with the conoid in the apex (Figure 2.10B). The initiation of daughter formation is synchronized with all daughters forming at the same time (Figures 2.10A, 2.10C). The mechanism of daughter formation is similar to that observed for the two daughters formed during endodyogeny of the tachyzoite or bradyzoite. Since this occurs at a multi-nucleated stage, numerous daughters are formed - thus the appropriateness of the term endopolygeny. The simultaneous formation of a large number of daughters requires an extremely well-coordinated process to ensure that all daughters receive a full complement

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