The Apicomplexa are parasites whose various species cause malaria or coccidiosis. They are intracellular, and transmit from host to host by use of a vector such as a mosquito or by ingestion of food or water contaminated by oocysts. A common name for this group is the sporozoans.
Most apicomplexans have alternate cycles of sexual and asexual reproduction. The sexual phase involves production of microgametocytes and macrogametocytes, which produce gametes that fuse to form a zygote. The zygote develops within an oocyst. Sporozoites develop asexually within the oocyst and are the usual stage to infect a new host. The sporozoite infects a host cell, and becomes a trophozoite, which feeds and grows. Then it becomes a schizont by undergoing nuclear division, which is followed by cytokinesis to form a number of new merozoites. Merozoites infect new cells and either repeat the process or become gametocytes.
Sporozoites and merozoites of apicomplexans are banana-shaped bodies which possess an apical complex, the unifying morphological feature of the group. This intricate structure, located at the anterior end of the organism, aids in the penetration of host cells, but is visible only with aid of an electron microscope. The only structure visible with light microscopy is the nucleus. Trophozoites are uninucleate cells which may have a large vacuole present as well as various other inclusions. Schizonts are multinucleated cells. Gametocytes are usually large cells, uninucleate, and often containing granules. Oocysts are surrounded by a cyst wall, which may be impermeable in those species that pass through the external environment. Oocysts shed by the host may contain a number of sporocysts, which each contain a number of sporozoites.