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FIGURE 2.17 A diagrammatic representation of the changes observed during the development of the zygote and formation of the sporoblasts. Ce, centriole; ER, rough endoplasmic reticulum; G, Golgi body; L, lipid droplet; MI, mitochondrion; MP, micropore; N, nucleus; NP nuclear pole; NU, nucleolus; OW, oocyst wall; PG, polysaccharide granule; V, vacuole. Reproduced from Ferguson et al., 1979a, with permission.

that probably provide structural strength and increase resistance to external insult (Figure 2.19B).

Within the cytoplasm of the developing sporo-cyst a nucleus was observed at either end of the elongated sporocyst, with the majority of the cytoplasm containing polysaccharide granules and lipid droplets (Figure 2.18A). It was observed that the anlagen of two daughters formed adjacent to the plasmalemma above each nucleus at either end of the sporocyst (Figure 2.18C). The process of daughter formation was similar to that observed during endodyogeny, with the nucleus appearing to divide during the posterior growth of the inner membrane complex of each daughter and two daughters forming at either end of the sporocyst (Figure 2.18C). This inner membrane growth continued until the daughters were fully formed and enclosed a nucleus, apicoplast, and mitochondrion, and the apical organelles (micronemes, rhoptries, and dense granules). This formation of the daughters adjacent to the sporocyst plasmalemma differs from the internal formation associated with endodyogeny or endopolygeny. In this situation, it was observed that the plasmalemma invaginated with the growth of the inner membrane complex to form the sporozoite pellicle, and in this respect is similar to classical schizogony. This resulted in the formation of four daughters (two from each end). A small residual cytoplasmic mass remained within each sporo-cyst. The process of sporulation is represented diagrammatically in Figure 2.20.

2.3.7 Excystation

There have been few ultrastructural studies on the process of excystation (Christie et al., 1978; Ferguson et al., 1979d; Speer et al., 1998). In certain studies the oocyst wall was broken mechanically by grinding, although it has been reported that reasonable excystation can occur without this process (Speer et al., 1998). Excystation is stimulated by incubation in a mixture of trypsin and bile salts (sodium taurocholate). This excystation fluid

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