INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

ALVEOLATES

PHYLUM APICOMPLEXA

Intracellular or, less commonly, coelomic animal parasites. This phylum comprises about 6,000 species.

Formerly, these Protozoans were referred to as Sporozoans, as they are characterized by The formation of a specialized developmental stage—the spore—which serves for the parasite's dispersal in the external environment when transitioning from one host to another. Unlike free-living protozoans, they lack locomotor Organelles for the greater part of their life cycle, with flagella present only in Gametes. Sporozoans differ from other parasitic protozoans in their life cycle features, spore Structure, and the specific structural plan of their active life-cycle phases—zoites (merozoites and sporozoites).

The most distinctive feature of this phylum is the unique structure of merozoites and sporozoites (Fig. 43). These are elongated Cells covered by a pellicle consisting of three Plasma Membranes (one outer membrane, and two forming the walls of highly flattened alveoli). A Cytoskeleton of longitudinal microtubules lies beneath the membranes, providing structural support. At the anterior end of The Cell lies the apical complex (hence the phylum's name), equipped with specialized organelles for penetrating the host cell.

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Fig. 43. Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF a sporozoite (a), micropore (b), and conoid (c) according to Electron Microscopy data (after Hausmann, modified):

1 - conoid; 2 - micronemes; 3 - rhoptries; 4 - micropore; 5 - Golgi apparatus; 6 - Nucleus;

7 - mitochondrion; 8 - Endoplasmic reticulum; 9 - pellicle

The apical complex includes the conoid, rhoptries, and micronemes (Fig. 43).

The conoid is a supporting structure shaped like a truncated cone. It consists of tightly packed, spirally wound fibrils. Rhoptries are two to three flask-shaped formations. Upon penetration of the host cell by the zoite, the conoid mechanically damages it, while the lytic Enzymes of the rhoptries presumably dissolve the host cell membrane, allowing the parasite to enter. Thread-like micronemes surround the rhoptries and likely also contain compounds that facilitate the parasite's invasion of the host cell.

At one or more sites on the cell, there are micropores where only a single-layered membrane is retained; these may function as a cell Mouth—an ultrastasome—to mediate pinocytosis.

Life Cycle. Apicomplexans are characterized by metagenesis with zygotic chromosomal reduction (Fig. 44). The infective stage is the sporozoite, which is released into the external environment enclosed within spores, each containing two to four sporozoites. The spores, in turn, are housed inside an oocyst (four, eight, or more).

Fig. 44. Generalized diagram of The life cycle of Apicomplexa

(after Hausmann, modified)

Upon entering the host Organism, sporozoites emerge from the oocyst and spore, penetrate into cells where they feed, grow (trophozoite or schizont stage), and reproduce via schizogony, resulting in the formation of merozoites. The latter infect new host cells, where The process of schizogony and merozoite production is repeated. After several generations, gamonts are formed instead of merozoites; through mitotic divisions, these give rise to gametes. Gametes fuse (copulate) to form a diploid zygote. Copulation occurs either via heterogamy—the fusion of differently sized motile gametes (micro- and macrogametes)—or oogamy, where a motile spermatozoon fuses with a non-motile egg cell. The zygote becomes surrounded by a wall (oocyst stage), inside of which Meiosis takes place to produce sporozoites. The process of sporozoite formation is known as sporogony.

This phylum includes three classes.