An electron microscope and immunohistochemical study of the intracellular location of Toxoplasma tissue cysts within the brains of mice with congenital toxoplasmosis.

The wall of intact Toxoplasma tissue cysts within the brains of mice with congenital toxoplasmosis was investigated. Smaller cysts were identified within the soma of neurones. With larger cysts, the contained cystozoites were shown by ultrastructural examination to be surrounded by a layer of microtubules; immunohistochemical staining revealed that this layer contained neurofilament protein. Interior to this layer was a much convoluted parasitophorus vacuole membrane; exterior was the host cell membrane. In most cases, synaptic plates were noted on the outer plasma membrane. In no instance were tissue cysts observed either within neuroglial cells or in the absence of host cells. These observations are discussed in relation to the pathogenesis of congenital toxoplasmosis in the brain.