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Despite immense diversity, organisms of the kingdom Protista share a number of characteristics. Perhaps the most distinctive of these characteristics are that the organisms
Despite these broad criteria, however, sufficient diversity exiss, even within the subkingdom Protozoa, to give rise to freuent confusion in taxonomy. The taxonomic scheme adopted for this text is one whereby protozoans parasitic to animals and humans are assigned to three phyla-Sarcomastigophora, Apicomplexa, Ciliophora and microsporida with method of locomotion being one basis for identification. For example, within the phylum Sarcomastigophora, the amoebae (subphylum Mastigophora) use flagella as their primary locomotor apparatus; and the ciliates (phylum Ciliophora) propel themselves by cilia. Members of the phylum Apicomplexa are primarily intracellular parasites and except at some stages of their life cycles (flagellated gametes in some species), do not possess locomotor organelles. In some species limited movement is accomplished by contraction of intracellular microfilaments.
A terminology unique in many ways to the protozoans has developed; based on light microscopy studies. Increased use of electron microscopy, however, has shown that many of these structures are ubiquitous to all eukaryotic cells. To preclude confusion resulting from the use of two sets of terms, those used in cell biology are adopted herin with attempts to correlate them to their older counterparts.
The Mastigophora, commonly known as flagellates, include all protozoans usually exhibiting in their trophozoite (motile) stage one or more flagella. The ability to swim has facilitated the flagellates’ adaptation to a variety of habitats in their hosts. Unlike amoebae, which rquire a substrate on which to move, flagellates thrive in a liquid medium and thus are well adapted for survival in the blood, lymph, and cerebrospinal fluid of the host. Their elongate, torpedo-shaped form enables them to swim in the host’s body fluids with little resistance, further adaptation for life in a liquid medium.
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While flagella are also found in the developmental stages of some amoebae and in the microgametes of some members of the phylum Apicomplexa, they are an invariable characteristic of the trophozoite stage of all flagellates. The number of flagella per organism varies widely according to species.
The single flagellum is a filamentous cytoplasmic projection. When examined with an electron microscope, this projection is seen to be sheath consisting of a cytoplasmic matrix enclosed by a plasma membrane with in which is embedded an axial filament or axoneme. The axoneme, extending the length of the flagellum, consists of a series of regularly oriented microtubules arranged in a specific pattern of two central microtubules surrounded by an outer circle of nine pairs of microtubules or doublets. Each flagellum is anchored in the cytoplasm by a basal body (also called a blepharoplast or kinetosome). Recent ultrastructural studies demonstrate that the basal body is morphologically identical to the centriole of the cell, and it is from this organelle that flagellum (or cilium) originates. The centriole and basal body consist of microtubules arranged in a circle of nine triplets, with two members of each triplet probably giving rise to and extending distally as one of the peripheral doublets of the flagellum.
In most flagellated cells, the flagellum extends from the basal body to the exterior; in some, however, one or more flagella may loop back in a complete reversal of their original direction. A flagellum of this type is known as a recurrent flagellum. A recurrent flagellum may extend into a cytostome, where it aids in the procurement of food. Or it may be attached to the plasma membrane by a series of desmosomes, in which case, during the beating process, it pulls the plasma membrane and a portion of the cytoplasm away from the body of the cell, producing an undulating menbrane.
It is seen, then, that flagellar movement propels and directs the organism and at times assists in procuring food. The movement may also promote tactility and secretion of mating substances. The so-called "beat" of a flagellum (or cilium) is actually the propagation of a series of wavelike bends along the length of the organelle that are associated with the connections between outer microtubule and the inner sheath containing the two central microtubules. Energy to fuel this movement comes from the ATPase activity of the dynein arms associated with one member of each of the outer doublets. This energy allows the doublet microtubules to slide past each other; however, since this action is restricted by chenical cross-links between the two microtubules, a bending occurs, which becomes a wave as the cross-links are broken and reformed along the axoneme. When this phenomenon occurs sequentially, a regular beat pattern emerges, resulting in directional movement of the cell.
The fine structure of the axoneme of flagella and cilia is identical. Cilia may therefore be considered miniature flagella. In addition to noticeable differences in length, however, there are other fundamental differences between these two types of organelles, the most obvious of which is their number. Flagella usually number no more than ten on a given cell surface, while there may be literally thousands of cilia on a surface. An exception is seen in the flagellate order Hypermastigida, whose memberg poossess greater numbers of flagella. In ciliates the numerous basal bodies are also interconnected by a series of subpellicular microfilaments or neurofibrils, forming an infraciliature believed to be responsible for either coordinating the ciliary beat of the cell or providing support for the ciliary beat. Nevertheless, the exact control mechanism of this coordination is not presently understood.
Amoebae are usual1y capable of producing pseudopodia, which are used as locomotor and food-acquiring organelles. These transitory body extensions depend for their function on the association of actin andmyosin. These two molecules function in a manner similar to their roles in the contraction of vertebrate muscle.
Activated by ATP-derived energy and certain cations, such as calcium and magnesium, actin and myosin become intimately associated at the tip of the forming pseudopodium. This association produces a localized contractile response in the cytoplasm, whereupon the cytoplasm everts at the plasma membrane and moves posteriad in the cell, forming an outer zone of cytoplasm known as the ectoplasm. At the rear of the cell, actin and myosin become dissociated; the ectoplasm reverts to the relaxed state, becoming more fluid, and moves inward to form endoplasm. When the endoplasm streams forward under the pressure of the contractile ectoplasm, actin again becomes associated with myosin, producing anew the contractile state. The overall effect, then, is a recurrent outward and posteriad flow of ectoplasm away from the direction of movement and a concmitant movement of the endoplasm from the rear of the cell in the direction of the forming pseudopod.
Morphologically,pseudopodia can be assigned to one of four types: filopodia, lobopodia,rhizopodia, and axopodia. Lobopodia, the most common form among parasitic amoebae, are blunt and may be
composed of both ectoplasm and endoplasm or of ectoplasm only. In most species, lobopodia form slowly. Observation of living specimens clearly shows the gradual flow of granular endoplasm, when present, into the broad projection. Entamoeba histolytica, an important parasite of the human intestine, is exceptional in that the lobopodia are produced abruptly ad withdrawn almost as quickly. Another exception is observed among some amoebae that appear to move on a substratum with no obvious cytoplasmic protrusions. Such amoebae are termed limax forms after the slug, Limax spp, whose movement they appear to mimic.
Although the formation of pseudopodia by trophozoites usually is considered a distinguishing characteristic of amoebae, some flagellates also are capable of pseudopodial movement at some stage during their life and, conversely, some amoebae possess flagella during their developmental stages (Naegleria, for example). As a general rule, however, among flagellates the principal means of locomotion is flagellar while among amoebae it is pseudopodial.
Most amoebae cannot swim because pseudopodial locomotion requires a substrate on which these organisms can glide. Parasitic species, therefore, are commonly found in the alimentary tracts of their hosts, intimately associated with the epthelial lining.
Structurally, protozoa are unicellular organisms with each cell a self-sufficient unit capable of carrying out all the metabolic functions of which multicellular organisms are capable. each protozoan is surrounded by a unit membrane chemically similar to the plasma membranes common to all eukaryotic cells - a bilipid layer associated with a variety of proteins. Among the sarcodinans, this tends to be a very thin, flexible layer often called the plasmalemma. On the other hand, a more rigid body wall, usually supported by microtubules and characteristic of some flagellates and most ciliates is termed a pellicle. It results in a more constant and uniform shape than that of the more amorphic amoebae.
The cytoplasm is usually divided into two areas: the peripheral ectoplasm and the medullary endoplasm. The consistency, extent, and appearance of these two zones differ among species.
Typically, the semisolid ectoplasm is a gel containing the basal bodies of cilia or flagella, microfilaments, and, in s0me protozoa, microtubules for rigidity and/or contractility. The semiliquid endoplasm, or sol, is more fluid than ectoplasm and contains such organelles as nuclei, mitochondria, and vacuoles and vesicles of various types.
Well-defined nuclei bounded by nuclear envelopes are a feature of all protozoa. Some protozoa have a single nucleus, others have two or more essentially identical nuclei, and still others, such as the ciliophorans, have two different types of nuclei: a macronucleus and one or more micronuclei. As the name indicates, the macronucleus of ciliophorans is larger than the micronuclei and is involved with trophic activities of the cell. Micronuclei, whether one or more, are concerned with reproductive activities, both asexual and sexua1.
In addition to distinctions as macro- or micronuclei, nuclei may be defined morphologically as either vesicular or compact. This classification is often used to aid in the identification of species infecting humans, since such species are commonly characterised as having the vesicular type.
The nuclear envelope of a vesicular nucleus, although delicate in appearance, is visible by light microscopy. The term vesicular denotes numerous clear areas resulting from the irregular distribution of chromatin, creating the impression of many small sacs or vesicles. The chromatin areas may be concentrated peripherally or internally. The nucleoplasm contains one or more endosomes or karyosomes, which are DNA-negative and are probably analogous to metazoan nucleoli; unlike nucleoli, however, they do not disappear during mitosis..
The compact nucleus appears to contain a large amount of more densely packed chromatin than does the vesicular nucleus. A nucleus of this type is generally larger than a vesicular nucleus and may vary in shape from round to ovate. Compact nuclei are found in the ciliophorans, where they are involved in the sexual process called conjugation. During this process, one micronucleus of each cell undergoes meiosis, and some of the resulting haploid micronuclei are exchanged, fusion of the micronuclie occurs, resulting in a genetically new set of micronuclei for each partner cell. During the meiotic division, exchange, and fusion of micronuclei, the macronuclei disappear and subsequently reform. They seem to serve a directors of the phenotypic expressions of the cells. Following conjugation, the two cells separate and then usually divide mitotically.
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These double-unit membrane-bound organelles serve as the sites of intracellular aerobic metabolism and are similar in ultrastructure to those of most eukaryotes. One feature peculiar to protistan mitochondria is the tubular shape of the cristae. Similar cristae are observed in mitochondria of multicellular eukaryotes but not as consistently as in those of protists. The significance of this structural variation is presently not understood. It should be noted that a number of parasitic protozoans (such as Entamoeba histolytica) do not possess mitochondria. Such a deficiency is associated with anaerobic metabolism.
The Golgi complex is another cytoplasmic structure whose specific function in protozoans is essentially identical to that observed in other eukaryotes. The Golgi is the seat of glycosylation of a number of secretory products of the cell. It is in the cisternae of the Golgi complex, for instance, that the final carbohydrate moieties are added to the particular cell coat assoicated with the plasma membrane. The arrangement and number of Golgi complexes vary during the life cycle of many protozoans. Thus, cyst-forming protozoans may lose their Golgi complexes during encystation, only toe resynthesize them when they excyst. The so-called parabasal body of protozoans is homologous to the Golgi complex of other eukaryotic cells but with several morphological differences, the most notable of which is the frequent presence of a fibril, the parabasal filament, running from the criternae of the Golgi complex to one or more basalbodies.
The lysosome, an organelle ubquitous among eukaryotes, is bounded by a single-unit membrane enclosing various hydrolytic enzymes whose optimum activities occur in the acid pH range. Such enzymes, therefore, are designated acid hydrolases. Lysosomes, with their battery of acid hydrolases, function in autophagy as well as in intracellular digestion of exogenous foodstuffs. In amoebae, for instance, food vacuoles are formed by the engulfment of exogenous food-including host cells, in certain parasitic species. Then the food vacuoles fuse with lysosomes, forming a digestive vacuole, and the lysosomal enzymes mix with and degrade the ingested material. Among a number of protozoans, ingestion occurs at a specialized site on the plasma membrane called the cytostome. After digestion, undigested residues are egested through the plasma membrane. Among amoebae such egestion may occur anywhere on the plasmalemma, while in ciliates a permanent pellicular site, the cytopyge, exists through which undigested residues are emitted.
Reserved food inclusions are seen in various species of parasitic protozoa. The naturE and amount of stored nutrients vary with the environment and the species involved. For example, glycogen and/or amylopectin are sometimes found in cysts of amoebae that inhabit the human intestine. This stored food, accumulated shortly before encystation and used during the nonfeeding stage, is usually completely exhausted by the time of excystation. In addition to these polysaccharides, lipid droplets and nuleic acid reserves may be observed in both trophozoites and cysts.
These organelles, the sites of cellular protein synthesis, are part of the organelle population of all eukaryotes and occur abundantly in those cells that actively synthesize protein for either secretion or internal use. They may be seen in association with the endoplasmic reticulum or free, either single or in clusters (polyribosomes), in the cytoplasm. During encystation, much of the ribosomal constituency of the cell is exhausted.
Associated with te basal bodies of many flagellates is a prominent, striated rod, the costa. This structure usually courses from one of the basal bodies along the base of the undulating membrane. It can best be described as a modified, striated rootlet. Rootlets are found at the bases of many cilia and flagella in other eukaryotes, penetrating deep into the cytoplasm where they are believed to serve as anchors. In addition to the costa, a sheath of microtubules in the shape of a tube, the axostyle, is observed in many flagellates. This structure extends posteriorly from the basal body and may actually appear to protrude through the plasma membrane. The function of this organelle is unknown, and it has no counterpart in other eukaryotic cells.
Some parasitic protozoans, such as the ciliate Balantidium coli, possess fluid-filled vesicles sometimes called contractile vacuoles. In protozoas in general, these are considered osmoregulatory organelles, ridding the cell of excess water; some dissolved metbolic wastes are also eliminated. However, since most parasitic protozoa, like their marine counterparts, are isoosmotic to their environment, contracile vacuoles are not common in parasitic protozoa.
Many parasitic protozoa are capable of encystation, during which the rounded cytoplasic mass is surrounded by a rigid or semirigid cyst wall secreted by the organism. The cyst wall may be single or multilayered. Cysts of parasitic protozoa serve three primary functions:
Examples of the first function are seen in the human and animal pathogens Entamoeba histolytica, an amoeba, and Giardia a flagellate, which form cysts in the intestinal tract and pass out in fecal material. Such cysts may remain viable for many weeks under normal conditions and for days at higher and lower temperatures and during periods of dessication. Further, the cyst wall protects the ingested organisms as it passes through the host’s hostile gastric fluids.
As previously stated, cysts of many parasitic protozoa also serve as sites for nuclear and cytoplasmic reorganization and division. Shortly after the trophozoite encysts, cytoplasmic reorganization occurs, sometimes followed by nuclear divisions-after which the mature cyst may enclose from one (in the absence of nuclear division) to eight vesicular nuclei. In E. histolytic, for instance, two consecutive mitotic divisions result in four vesicular nuclei. If mature cystgs are reintroduced into a suitable host, excystation occurs, and the escaping motile trophozoite usually divides once more, resulting in eight small trophozoites produced from the single, tetranucleated mass of cytoplasm encased within the cyst wall. Following excystation, the newly excysted trophozoites begin a period of active feeding, followed by rapid growth and binary fission.
Finally, intestinal protozoa are transmitted to a new host (or become reestablished in the same host) when that host swollows the cysts. Thus, the cysts serve as a means of transmission.
The precise environmental conditions that cause encystation are not totally defined. In many species, the process often occurs in response to a deficiency in the host of nutrients useful to the parasite. In addition, increased osmotic pressure, temperature changes. low pH’s, accumulation of waste products in the medium, and crowding all appear to stimulate encystment.
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Parasitic protozoa most commonly reproduce by means of an asexual process called fission, a form of mitosis whereby each parent forms two progeny. The plane of division is random among amoebae, usually longitudinal in flagellates, and transverse in ciliates. The sequence of division in a typical protozoan is as follows:
In apicomplexans, multiple fission or schizogony occurs. This type of asexual reproduction is characterized by rapid organelle and nuclear divisions, followed by multiple cytokinesis. The multinucleated cell is called the schizont or segmenter. After cytoplasmic division, each mucleus, with its attendant cytoplasm, forms a separate organism, a merozoite, which usually breaks away from the aggregate to infect a new host cell. Once a merozoite enters a new host cell, it may either enter anoter schizogonic cycle or become a macro or microgametocyte. Syngamy, the union of gametes derived from the gametocytEs, initiatEs the sexual cycle. The resulting zygote undergoes sporogony, which results in the production of sporozoites. The organisms that produce malaria are apicomplexans capable of both schizognic (asexual) and sporogonic (sexual) reproduction. Infact, the apicomplexans are considered unique among protists in displaying alternation of generations, a characteristic more commonly encountered in plants and some invertebrate animals, for example, cnidarians.
Conjugation, the specialized sexual mechanism in the ciliatEs, has already been discussed; it is distinguishable from syngamy in that conjugation involves nuclear exchange and union, whereas syngamy involves the union of entire cells (e.g., gametes).
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