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Morphological Studies on the Development of the Toxoplasma Cyst^*

A strain of high virulence of Toxoplasma grows rapidly in HeLa cells and destroys them within a few days. Parasites very often form rosettes or disperse in the cytoplasm of the host cell, which soon bursts and liberates them. The limiting membrane of the vacuole in which the parasites are situated is rather obscure when examined with the light microscope. When observed with the electron microscope the limiting membrane of the vacuole is conspicuous but very thin. Granular, or sometimes filamentous, precipitates are scattered throughout the interior of the vacuole. Cysts are not produced in this type of infection in HeLa cells.

A strain of low virulence of Toxoplasma grows rather slowly in HeLa cells and the cultures survive for a long period (3 weeks or more). Parasites do not form rosettes but make relatively compact clusters, each of which is surrounded by a thick membrane and can be regarded as a cyst. In electron-micrographs the limiting membrane of the vacuole in which the parasites are situated is fairly thick even at the earliest stage of the development of the parasite. There are granular precipitates scattered throughout the interior of the vacuole. They are deposited on the inner surface of the limiting membrane and fuse together to produce an electron-dense cyst wall which is almost homogeneous in structure.

In the mature cyst, the cystic cavity is packed with organisms and the small spaces and interstices between organisms are filled with the same kind of material as that in the cystic membrane. This material between organisms is continuous with the cyst wall and must have been secreted from the body of the parasites as granular or filamentous precipitates that later fuse together to form a homogeneous structure.

The production of the Toxoplasma cyst is largely dependent upon the rate of the reproduction of the parasites. When they grow rapidly, as is the case of the RH strain in HeLa cells or mice, the vacuole in which the parasites are located may soon burst and cysts cannot be produced. When the reproduction is slow, as in the case of the Beverley-strain, the vacuole expands slowly and the precipitates have time enough to be deposited on the inner surface of the vacuole in a thick layer which probably protects the vacuole from rupture. This layer constitutes the cystic wall, and the cyst is thus formed. Immunity, treatment with antitoxoplasmic substances, and natural resistance of the host can all stimulate cyst-production by retarding the rate of reproduction of the parasite.

^* The research reported in this document has been made possible through the support and sponsorship of the U. S. Department of Army, through its Far East Research Office.

A part of this paper was read at the UNESCO Regional Symposium on Scientific Knowledge of Tropical Parasitology which was held on November 5 through 9, 1962, in Singapore.