Many parasitic Protozoa (e.g. Giardia, Entamoeba histolytica, Coccidia) are capable of undergoing encystation, which involves formation of a cyst wall either outside or inside the cell membrane. This cyst wall may be single- or multilayered. Walls formed outside the plasmalemma are produced by exocytosis of materials (wall-forming bodies). Cyst walls have two main tasks: (1) to protect the organism against unfavorable environmental conditions when passing from one host to another, and (2) to create spaces for reorganization and nuclear division. Cyst walls may also aid the parasite in its transmission from one host to another by facilitating attachment to host cell surfaces.
The cyst wall has one layer in cysts of Entamoeba histolytica, Giardia and some ciliates, such as the fish parasite Ichthyophthirius multifiliis and the human parasite Balantidium coli. There are usually 2 types of cysts in the stages of coccidia in feces: the oocysts and the sporocysts. The oocyst wall is usually 2-layered but in a few species it may have 4 layers. It is formed by 2 types of cyst wall-forming bodies (Eimeria) (Fig. 1).
The chemical composition of cyst walls varies according to the species, although proteins are usually the basic component. The cyst walls of E. histolytica and G. lamblia contain proteins which are keratin-like or elastin-like albuminoids, composed of lysine, histidine, arginine, tyrosine, glutamic acid and glycine.
The 2-layered oocyst wall of the sporozoans is periodic acid-Schiff-positive (PAS-positive). The outer layer of these oocysts consists mainly of fatty alcohols (e.g. hexacosanol), some phospholipids and fatty acids. It contains no carbohydrates or proteins. The inner layer is composed of glycoproteins and contains most of the carbohydrates found in the oocyst wall. These carbohydrates are composed of mannose, galactose, glucose and hexosamine. The oocyst wall is highly resistant to the passage of potassium dichromate, sodium hypochloride, sulphuric acid and sodium hydroxide and therefore these chemicals are used in the storage and cleaning of oocysts. It is permeable to O2, CO2, NH3, methylbromide, carbon disulphide and various organic solvents. The oocyst wall is highly susceptible to mechanical pressure and therefore may be easily ruptured by shearing forces. Thus, the mechanical rupture of oocysts in the gizzard of the avian host is likely to be the normal method of excystation of avian coccidia. The micropyle (a pre-formed opening) is probably rarely used as a passage for the escape of sporocysts. The sporocysts in the oocyst are bound by a 2-layered wall, the inner layer of which is relatively smooth (Fig. 2). The sporocysts of eimerians have an opening which serves as an exit for the sporozoites. This is closed by the Stieda body, which can be dissolved by trypsin. The sporocysts of Isospora and the tissue cyst-forming coccidia (e.g., Sarcocystis, Toxoplasma) have sutures on which the excystation fluids act, causing collapse of the sporocyst wall (Fig. 2).
The cyst walls of the members of the Myxozoa and Microspora are at least double-walled in most species (Fig. 3), but the layers can only be resolved by electron microscopy. In Myxozoa, the wall consists of two valves that open at preformed sites to release the infectious sporoplasm (Fig. 3A). The Microspora have developed a hollow tube that protrudes from the surface of the wall (Fig. 3B; Fig. 4). It penetrates into a host cell and the infectious sporoplasm is passed through it. The exospore layer in Microspora is proteinaceous and is 15–100 nm thick, depending on the species. The endospore layer is chitinous and ca. 150–200 nm thick (Fig. 4). The spores are Gram-positive (i.e. stain reddish purple with the Gram stain), a fact that is of diagnostic value. They are also stained light blue by Giemsa. Besides these typical cysts, some genera such as Toxoplasma and Sarcocystis, form so-called tissue cysts. They develop from a “normal parasitophorous vacuole”, the membrane of which becomes fortified to a so-called primary cyst wall, which starts to form species-specific protruism. Such tissue-cysts, while inside divisions go on, may become covered by host defense cells so that a secondary cyst wall is produced (e.g. Sarcocystis ovifelis, cf. tissue cyst).
Encystation is also done by cercariae of some digenean trematodes (Digenea) which excrete (via cystogenous glands) material to produce the wall of metacercariae, Fasciola. Further similar processes are seen in Acanthocephala/Cystacanth.