An ultrastructural study on the excystation of the sporozoites of Toxoplasma gondii.

The ultrastructural changes which occur during the in vitro excystation of the sporozoites of Toxoplasma gondii were examined. The excystation was carried out at 37 degrees C on suspensions of oocysts which had been ground and then treated with an excysting medium containing 0.25% trypsin and 0.75% sodium taurocholate in phosphate buffered saline, pH 7.3. It was found that sporocysts within intact oocysts were unaffected while sporocysts exposed to the medium ruptured. The sporocyst wall consisted of two layers and during excystation the four plates which form the inner layer started to curl inward. At the same time changes were seen at the specialized junctions between these plates. When the junctions finally break, the plates separate. The outer layer of the sporocyst wall is then ruptured at points directly above where the plates were joined. Each of the four portions of the sporocyst wall curled inward to form a tightly wound whorl. The sporozoites can escape through the openings created between the portions of the sporocyst wall.