AN INTRODUCTION TO THE APICOMPLEXA

Apicomplexans are a monophyletic group composed entirely of parasitic species. Of the more than 5000 described species seven genera infect humans: Plasmodium, Babesia, Cryptosporidium, Isospora, Toxoplasma, Cyclospora and Sarcocystis. Here we will consider only the first 5 genera.

 Electron microscopic studies of apicomplexans demonstrated the presence of  distinctive organelles at the anterior end of the organism, the apical complex. The apical complex consists of several secretory organelles: rhoptries and  micronemes, as well as conical structure, the conoid, surrounded by  polar rings. The apical organelles play a role in attachment and invasion of host cells.

 Apicomplexans have no specialized locomotor organelles (except for the flagellated microgamete) yet they are capable of gliding movements.  

Apicomplexans have complex life cycles that consist of 3 distinct processes: sporogony, gamogony, and merogony (=schizogony).

The generalized life cycle may vary in its details between genera.

 Sporogony, an asexual reproductive phase,  produces  haploid sporozoites, and follows the sexual phase, gamogony. During gamogony male and female gametes are formed, and  upon fusion  a diploid zygote (called an ookinete or sporoblast) results, and this is where genetic recombination  can take place. .Sporogony is followed by a phase of asexual reproduction called merogony, and the cellular products of this phase are called merozoites. Merogony and schizogony are synonyms. Merozoites can invade host  cells and undergo multiple rounds of merogony or they may differentiate into gametocytes/gametes (= gamogony.)  Parasite amplification, i.e. increase in numbers, takes place during merogony and sporogony. Only the invasive stages--merozoite, sporozoite, and ookinete—contain an apical complex.

 VARIATIONS  ON  THE  APICOMPLEXAN  LIFE  CYCLE

Click here to see Powerpoint slides of a lecture by Dr. P. Johnson on "Introduction to the Apicomplexa".