Ultrastructural studies on the sporulation of oocysts of Toxoplasma gondii. I. Development of the zygote and formation of the sporoblasts.

The initial stages of sporulation in oocysts of Toxoplasma gondii were examined in samples sporulated at 27 degrees C for 0, 6, 12, 16 and 24 hours. The initial zygote was roughly spherical and was limited by a single unit membrane. A few micropores of the inactive type were present on this membrane. The cytoplasm contained a large nucleus with a nucleolus, a number of polysaccharide granules, lipid globules, mitochondria, and Golgi bodies together with a few strands of rough endoplasmic reticulum. After the initiation of sporulation little change was noted in the cytoplasm except for an increase in protein synthesis as evidenced by the augmentation of the amount of rough endoplasmic reticulum and the appearance of polyribosomes. Nuclear division occurred twice giving rise to four nuclei which were situated close to the cell periphery and well separated from each other. At this multinucleate stage a second limiting membrane was formed. The cytoplasmic mass then divided to form the two sporoblasts. This was accomplished by an invagination of the limiting membranes in combination with internally formed membranes. The two binuclear sporoblasts were roughly spherical. They were limited by two unit membranes and contained the same cytoplasmic organelles as described for the zygote.