The bradyzoite and tissue cysts

The bradyzoite (brady = slow) is the organism dividing slowly within a tissue cyst (Frenkel 1973) and is a synonym of cystozoite. A tissue cyst is a collection of bradyzoites surrounded by a well-defined host cell membrane. The bradyzoites are slender and measure approximately 7 x 1.5^m (Mehlhorn and Frenkel 1980). The bradyzoites also multiply by endodyogeny. Tissue cysts range from 5 to 60^m in the brain and 100^m in other tissues (Dubey 1993) and contain four to several hundred bradyzoites. Tissue cysts may develop in any tissue but are most prevalent in neural and muscular organs, like eye and brain, skeletal and cardiac muscles. The tissue cyst develops in the host cell cytoplasm and its wall is partly of host origin (Ferguson and Hutchison 1987; Sims et al. 1988). In older cysts, degenerating bradyzoites may occasionally be found (Pavesio et al. 1992).

Bradyzoites differ only slightly from the tachyzoites. They are more slender than tachyzoites and their nucleus is located more to the posterior end compared to that of the tachyzoites. The contents of the rhoptries of bradyzoites are electron dense in older cysts (Ferguson and Hutchison 1987). The prepatent period in cats following infection by bradyzoites is shorter (3-10 days) than following infection with tachyzoites (21 days or more) (Dubey and Frenkel 1976). The transition from tachyzoites to bradyzoites can be observed in vitro, and occur through an intermediate stage which expresses both usually exclusive tachyzoite and bradyzoite antigens (Tomavo et al. 1991; Bohne et al. 1992; Soete et al. 1993). The external mechanisms behind stage conversion from tachyzoites to bradyzoites are not known, but studies have shown that nitric oxide and IFNg may be among other factors involved (Bohne et al. 1994).

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