The Phylum Apicomplexa comprises protozoan parasites of medical and veterinary relevance such as Plasmodium, Toxoplasma, Cryptosporidium and Eimeria. The cytoskeleton of these organisms consists of an array of subpellicular microtubules radially disposed around an apical polar ring that has characteristics of a microtubule organizing center. The conoid, a cylindrical hollow structure made of tubulin in a very particular array is present in several genera and can move up and down above the apical polar ring during invasion and egress. Secretory structures, as rhoptries, release their contents through the conoid. Three membrane units form the pellicle around these protozoans. The outer membrane completely surrounds the zoites, while the two inner membranes limit a compartment starting below the polar ring and ending at the posterior end being fenestrated at some points. Between the plasma membrane and the inner membranes, a dynamic arrangement of actin, TG myosin, aldolase and TRAP proteins are responsible for the gliding motility of the parasites, being also indirectly associated to the subpellicular microtubules. A third component of the cytoskeleton is the subpellicular network, a network of 8-10 nm filaments containing articulin-like proteins usually found in Euglena. Gliding, invasion and egress of these parasites rely on the coordinated assembly and activity of these three cytoskeletal systems (microtubules, actin and subpellicular network).

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