Figure 221

(A) Scanning electron micrograph of a sporocyst undergoing excystation, showing the separation of the plates of the sporocyst wall (arrows). Bar = 1 |im.

(B) Transmission electron micrograph through an excysting sporocyst, showing separation and infolding of the plates of the sporocyst wall (arrows). The sporozoites (SP) contain a posteriorly located nucleus (N) and numerous micronemes (MN) and polysaccharide granules (PG). C, conoid. Bar = 1 |im.

(C) Early stage in excystation showing inward curling (arrow) of the sporocyst wall at the junction of the plates of the inner layer (I), resulting in a separation of the plates and the intermediate strip (IS). Bar = 100 nm.

(D) More advanced stage of excystation showing separation of the the inner plates and rupture of the outer layer (arrow). Bar = 200 nm.

(E) Example of the continued curling of the plates of the sporocyst wall to form tightly wound, scroll-like structures. Bar = 100 nm.

majority of tissue cysts are observed are striated muscles, including the heart and the central nervous system. However, this can vary between species. For example, the majority of tissue cysts are located in the musculature in pigs (Dubey, 1986), but predominantly in the brains of mice. This again contrasts with the in vitro situation, where almost any cell type can act as a host cell during stage conversion. This again emphasizes the need for caution when extrapolating from in vitro results.

Stage conversion has been examined in detail in mice. At 12-15 days following oral infection, lesions are observed in the brain that consist of numbers of parasites undergoing tachyzoite development admixed with parasites forming early tissue cysts (Figure 2.22A). It is observed that only a small subpopulation of the tachyzoites underwent conversion. The lesions consist of numbers of extracellular tachyzoites plus a few intracellular organisms located in typical tachy-zoite-like PVs (Figures 2.22B, 2.22D). These can often be identified as inflammatory cells, which form part of the lesion. However, it is also possible to identify a number of early tissue cysts (Figures 2.22C, 2.22E). A number of cysts are seen within the lesion, and indeed it is possible to observe two cysts forming within the one host cell. These can be differentiation from tachyzoite-like vacuoles by the distinctive structure of the PV and PMV enclosing parasites, which could be identified at the one- to two-cell stage (Figure 2.22E). The distinctive PVs appear to form at the time of invasion. This is also consistent with the immunocytochemical results using the stage-specific antibodies (SAG1 and ENO2 for the tachy-zoites and BAG1 and ENO1 for bradyzoites). The parasitophorous vacuoles of early tissue cyst are characterized by their tight fit and being limited by a membrane with numerous irregular shallow invaginations (Figure 2.22E). These vacuoles lack the tubular network, but possess a thin layer of amorphous material. In addition, there is no congregating of the host-cell mitochondrion or rER around the vacuole. When examined by immuno-electron microscopy, it is observed that the material beneath the membrane reacts positively to the cyst wall protein recognized by the antibody CC2. It has been reported that a small subpopulation of the tachyzoites from peritoneal exudates contained lucent cytoplasmic granules which were positive with CC2 (Gross et al., 1996).

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