The second portion of VPARA 5333 is concerned with those protozoan organisms that infect man and animals.
The protozoa comprise many different organisms with four classes that are of interest to us. These are the Sarcodina (amoebae), Ciliophora (ciliates, ) Zoomastigophora (flagellates) and the Apicomplexa (sporozoa). In each of these, individuals can be readily differentiated microscopically using a variety of anatomical and morphologic characteristics. One major difference between each class is the type of locomotion employed. In amoebae, movement is by pseudopodia or false feet, while ciliates utilize cilia and flagellates posses flagella. There is no notable means of movement in the Apicomplexa with most members being intracellular parasites .However, limited movement can be accomplished by contraction of intracellular microfilaments.
Amoebae produce pseudopodia which are used for both locomotion and food-acquiring. The production of these temporary body extensions depends on the association of actin and myosin. In a manner similar to that seen in vertebrate muscle contraction, actine and myosin combine at the tip of the forming psuedopodium, following activation by cations such as calcium and magnesium. This produces a contractile response causing the cytoplasm to evert at the plasma membrane, forming an outer zone of ectoplasm. At the other end of the cell, the actin and myosin dissociate and reverse the process resulting in a relaxed state. The more fluid endoplasm moves forward due to the pressure of the contracting ectoplasm resulting in a recurrent outward flow of ectoplasm from the rear of the cell in the direction of the forming pseudopod. Pseudopods are assigned to one of several types with the lobopodia (blunt and composed of both ectoplasm and endoplasm) being the most common in parasitic species.
Although pseudopods are a distinguishing characteristic of amoebae trophozoites, some flagellates can also produce pseudopodial movement (some amoebae also have flagella during some stage of their development). Amoebae require a substrate on which the organisms must glide, and thus can not exist in liquid environments. Thus, most parasitic species are found in the alimentary tract of their hosts associated with the epithelial lining. At least seven species of amoebae belonging to three genera are known to parasitize humans. These include Entamoeba histolytica, E. hartmanni, E. coli, E. polecki, E. gingivalis, Endolimax nana, and Iodamoeba butschlii. All inhabit the large intestine except E. gingivalis which is found in the mouth. At least three genera (Naegleria, Acanthamoeba, and Hartmannella) usually free-living organisms, are known to also parasitize humans and other mammals.
The Mastigophora (flagellates) include all protozoans that posses at least one flagella in their trophozoite stage. Unlike amoebae that require a substrate on which to move, flagellates are well adapted to liquid medium and thus are found in blood, lymph, cerebrospinal fluid, etch of the hosts that they infect. A flagellum is a cytoplasmic projection seen as a sheath consisting of a cytoplasmic matrix enclosed in a plasma membrane. A axoneme, which extends the length of the flagellum consists of a series of microtubules arranged in a pattern of two central microtubules surrounded by an outer circle of nine pair of microtubules. The flagellum is anchored to the cytoplasm by a basal body (also called a blepharoplast or kinetosome). Most flagellum extend from the basal body to the exterior. In some instances, the flagellum extends into the cytostome and aids in food procurement. Some flagellum are attached to the plasma membrane which, during the beating process, pulls the plasma membrane and portions of the cytoplasm away from the body of the cell, producing a undulating membrane. Thus, a flagellum not only propels and directs the cell but may also assist in food procurement and promote tactility.
The cilia which occur in all ciliated organisms are identical to flagella. They are considered miniature flagella with a major difference being in the number that are present. Flagellates usually posses less than 10 flagella while thousands of cilia may occur on the surface of a ciliate. The basal bodies of cilia are also interconnected by a series of microfilaments (neurofibrils) forming a infraciliature that is believed to coordinate ciliary beats and/or provide support for the beat.
The Apicomplexa are a taxonomic group that possess structures known collectively as an apical complex. The apical complex is found in the sporozoite and merozoite stage of all genera of malaria, piroplasms, coccidia (and related organisms) and Pneumocystis carinii. The apical complex (seen only at the ultrastructural level) consists of one or two electron dense polar rings at the anterior end of the cell, a conoid (seen in some coccidia) that lies within the polar ring , and two or more rhoptries located within the polar ring extending posteriorly from the plasma membrane. Subpellicular microtubules extend from the polar ring parallel to the long axis of the cell (except in the genus Babesia) presumably functioning as support for the cell. Micronemes lie parallel to the rhoptries and often merge with them at the apex of the cell. Rhoptries and micronemes probably are secretory organelles and facilitate penetration into the host cell.
The first group of protozoa that will be covered are the amoebae. The amoebae are common protozoa in most free-living situations. Most species posses both a trophozoite and cyst stages although exceptions occur. Some species have developed a parasitic mode of existence in various tissues and organs of animals and it is these that we will discuss in the next section.