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FIGURE 2.5 Tachyzoite endodyogeny.

(A) Early stage of mitosis: the mitotic spindle elongates into a cytoplasmic funnel through the nucleus (arrow), between the centrioles (Ce). G, golgi body; Mi, mitochondrion. Bar = 0.5 pm.

(B) Early stage of daughter zoite genesis where a dense fiber (arrow) extends between the centrosome (Ce, centriole) and the newly formed conoid, (C). The apical part of the inner membrane complex (imc) and subpel-licular microtubules have started developing. Bar = 0.2 pm.

(C) Centrocone (ct) budding off the nuclear envelope (Ne) between a centriole (Ce) and three kinetochores (arrows) in an early stage of daughter zoite formation. R, rhoptry. Bar = 0.2 pm.

the centrosomes, and covered on the nucleoplas-mic side with a layer of multilayered structures corresponding to the kinetochores (Figure 2.5C). What is specific to this stage is that each round of mitosis occurs simultaneously with the development of two daughter individuals.

2.2.5.2 Zoite biogenesis

Soon after the centrosomes separate and centro-cones are formed, the future apical complex of each daughter tachyzoites starts to develop adjacent to each centrosome. The details of this biogenesis have not been studied as thoroughly as in Eimeria spp.(Dubremetz, 1975), but follow the same scheme (Vivier and Petitprez, 1969; Vivier, 1970; Hu et al., 2002b). A very early stage of development shows a bent fiber originating between the pair of centrioles and joining an area where the conoid is being assembled (Figure 2.5B). The inner membrane complex and underlying subpellicular microtubules array appears to form around the conoid and, in a coordinated manner, starts to grow posteriorly (Figures 2.5C, 2.6A). This occurs within the mother-cell cytoplasm rather than in association with the mother-cell plasmalemma

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FIGURE 2.6 Tachyzoite endodyogeny.

(A) Early stage of endodyogeny showing two developing daughters (arrows), with early rhop-tries (R). The Golgi body (G) has divided. Only one nuclear pole (ct) and centriole (ce) is present in the plane of the section. A, apicoplast; D, dense granule; imc, inner membrane complex; Mi, mitochondrion, N, nucleus. Bar = 0.5 pm.

(B) Later stage of endodyogeny where the daughter nuclei (N) have entered the developing zoites. Imc, inner membrane complex. Bar = 1 pm.

that is characteristic of daughter formation in classical schizogony undergone by most Apicomplexa. Early stages are short, flattened cones above the centrocones (Figure 2.6A), which later elongate into the grossly cylindrical shape that will eventually surround the mature organism (Figure 2.6B).

The Golgi apparatus divides concomitantly with spindle formation, with each newly formed Golgi body being found on the upper nuclear envelope, near each centrocone (Figures 2.5A, 2.6A) (Pelletier et al., 2002). The apicoplast elongates and appears to divide during daughter formation, with a portion entering each daughter. Rhoptry precursors are observed at this time as heterogeneous, irregularly shaped vesicles of about 0.3 pm near the Golgi bodies and within the inner membrane complex (Figure 2.6A). As development proceeds, the nucleus become U-shaped and the developing inner complex elongates and engulfs the daughter nuclei (Figure 2.6B), while additional organelles (rhoptry precursors and then micronemes) are formed anterior to the Golgi bodies. The rhoptry contents condense to eventually acquire their mature labyrinthine appearance, while the rhoptry ducts appear and elongate towards the conoid.

As the daughters grow, the inner membrane complex of the mother cell breaks down along with the anterior organelles. The fully formed daughters fill much of the mother-cell cytoplasm, and their inner membrane complex comes in contact with the mother-cell plasmalemma to form the daughter pellicle (Figures 2.7A, 2.7B). This is initiated at the anterior end and results in the daughters remaining connected via a small portion of residual cytoplasm before finally separating. Repeated rounds of division lead to accumulation of tachyzoites within the vacuole, which may adopt a typical rosette appearance when grown in flat cell (such as human foreskin) fibroblasts (Figure 2.8A).

In certain cases, each of the daughters, while remaining attached by their posterior ends, can undergo a new cycle of endodyogeny (Figure 2.8B). There is evidence for the synchronized division of the tachyzoites within a vacuole (Figures 2.8A, 2.8B). In quantitative studies it was observed that

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