Sporozoites enter host cells by constructing a unique vacuolar structure.
Although sporozoites do not replicate within this vacuole, once inside the host cell they immediately begin to differentiate into the tachyzoite stage. The absence of fluorescence in the vacuole demonstrates this vacuole lacks molecular pores, and thus the dye is excluded. Bright staining regions result from entrapment of lucifer yellow within the host cell nucleus. Approximately 24 h into their development, sporozoites move out of the first vacuole and construct a second vacuole. Within this second vacuole, sporozoites complete their transformation into the tachyzoite stage and replication begins.