Apicomplexan Parasite

Apicomplexan parasites actively secrete proteins at their apical pole as part of the host cell invasion process in response to free Ca++ in the parasite's cytoplasm. When present the conoid is located in the center of the polar rings, with the short narrow end pointing anteriorly. The conoid intermittently protrudes beyond the apical end of the microtubules. Protrusion of the conoid is sensitive to parasite cytoplasmic calcium concentration. The rhoptries are club shaped secretory organelles, often located near the apical end of Apicomplexan intracellular parasites. Rhoptries are secreted during host cell invasion, and rhoptry proteins are found within the lumen and the membrane of very early stages of the forming parasitophorous vacuole.