Brain, toxoplasmosis, tissue cyst, PAS
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Toxoplasma gondii tissue cyst (arrow), PAS stain. Each cyst contains 50-5000 bradyzoites, which replicate following cyst rupture, unless destroyed by cellular immunity (the usual case). Cause of cyst rupture is unknown, but may be related to status of the immune response in the host. Some cysts rupture from time to time in normal hosts, for unknown reasons, but the bradyzoites are quickly destroyed and a small glial scar forms.