Paired Organelles Research Articles

The transformation of the Plasmodium gallinaceum oocyst in Aedes aegypti mosquitoes.

Sporoblast and sporozoite formation from oocysts of the avian malarial parasite, Plasmodium gallinaceum, after the seventh day of infection in Aedes aegypti mosquitoes offers an interesting example of differentiation involving the appearance and modification of several cellular components. Sporoblast formation is preceded by (a) invaginations of the oocyst capsule into the oocyst cytoplasm, (b) subcapsular vacuolization and cleft formation, (c) the appearance of small tufts of capsule material on the previously noted invaginations, and (d) linear dense areas located just below the oocyst plasma membrane which predetermine the site of emerging sporozoites from the sporoblast. The subcapsular clefts subdivide the once-solid oocyst into sporoblast peninsulae. Within the sporoblast, nuclei migrate from the random distribution seen in the solid oocyst and come to lie at the periphery of the sporoblast just below the linear dense areas noted in the earlier stage. A typical nuclear fiber apparatus occurs in most of the nuclei seen in random sections at this stage although such a fiber apparatus may occasionally be seen in the solid oocyst stage. The nucleus, its associated fiber apparatus, and the overlying dense area appear to induce the onset of sporozoite budding from the sporoblast as well as the formation of the sporozoite pellicular complex and the paired organelle precursor. Several mitochondria are present in each sporozoite, in contrast to the single mitochondrion seen in the merozoites of the erythrocytic and exoerythrocytic stages of avian malaria infection. The paired organelles and associated dense inclusion bodies are formed by condensation of an irregular meshwork of membrane-bound, coarse, dense material. The nature of small, particulate cytoplasmic inclusions is described.

The living, endogenous stages of the rat coccidium, Eimeria nieschulzi.

SYNOPSIS. The living, endogenous stages of Eimeria nieschulzi Dieben, 1924 (Landers isolate) were studied under the phase contrast microscope. Active sporozoites were found as early as 2.5 hours after exposure and as late as 48 hours after exposure. The first generation schizont was recognized by the presence of a refractile globule remaining from the sporozoite. First and second generation merozoites were only weakly motile and had small paired organelles. Third generation merozoites were seen 48–120 hours after exposure and were strongly motile from 72 hours after exposure onward. The paired organelle consisted of 2 intertwining portions, one 5.5 μ long, the other tapering to a slender filament and continuing to about the posterior quarter of the parasite. The fourth generation merozoites were short, curved, and weakly motile. A paired organelle about 3 μ long was seen. Gametocytes and gametes were seen 144–192 hours after exposure. Macrogametes appeared to elaborate refractile granules in the vicinity of the nucleus. No motility of any type was seen in the macrogametes. Microgametocytes were recognized when nuclear material moved to the periphery of the parasite for the formation of microgametes. Observations on living organisms agreed generally with those made on fixed and stained organisms with the exception that the living merozoites were about 20% larger.

The fine structure of the exoerythrocytic stages of Plasmodium fallax.

The fine structure of the exoerythrocytic cycle of an avian malarial parasite, Plasmodium fallax, has been analyzed using preparations grown in a tissue culture system derived from embryonic turkey brain cells which were fixed in glutaraldehyde-OsO(4). The mature merozoite, an elongated cell 3- to 4-micro long and 1- to 2-micro wide, is ensheathed in a complex double-layered pellicle. The anterior end consists of a conoid, from which emanate two lobed paired organelles and several closely associated dense bodies. A nucleus is situated in the mid portion of the cell, while a single mitochondrion wrapped around a spherical body is found in the posterior end. On the pellicle of the merozoite near the nucleus a cytostomal cavity, 80 to 100 mmicro in diameter, is located. Based on changes in fine structure, the subsequent sequence of development is divided into three phases: first, the dedifferentiation phase, in which the merozoite loses many complex structures, i.e. the conoid, paired organelles, dense bodies, spherical body, and the thick inner layers of the pellicle, and transforms into a trophozoite; second, the growth phase, which consists of many nuclear divisions as well as parallel increases in mitochondria, endoplasmic reticulum, and ribosomes; and third, the redifferentiation and cytoplasmic schizogony phase, in which the specialized organelles reappear as the new merozoites bud off from the mother schizont.

Fine Structure of First-Generation Merozoites of Eimeria bovis

The fusiform merozoite is enclosed by a cell membrane. Another membrane underlying the cell membrane encloses the cytoplasm except at the anterior end of the merozoite where this inner membrane terminates forming the polar ring. Approximately 22 subpellicular fibrils extend posteriorly from the polar ring. A conoid, consisting of one or more fibrils wound in a tight helix, is situated within the polar ring. A paired organelle extends posteriorly through the conoid from the anterior end. Each member of the paired organelle is club-shaped having a narrow neck within the conoid region and a wider posterior portion. A median rod parallels the necks of the paired organelle. The region of the merozoite between the conoid and the ovoid glycogen bodies is tightly packed with many tortuous structures having indistinct borders. Numerous ribosomes as well as one or two mitochondria are scattered among these structures. A dense, membrane-enclosed body, possibly a lysosome, is occasionally seen near the mitochondria. The Golgi apparatus lies next to the flattened anterior edge of the nucleus. In some specimens a punctate invagination of the cell surface was seen near the level of the Golgi apparatus. Several cisternae of rough-surfaced endoplasmic reticulum are found both anterior and posterior to the nucleus. The merozoites lie free in a vacuole of the host cell. Blebbing of the host cell's vacuolar membrane releases vesicles into the vacuole. The outer surface of the host cell has numerous microvilli. A fine, fibrous layer exists in the host cell cytoplasm surrounding the vacuole. The cytological characteristics of the firstgeneration merozoites of Eimeria bovis, as revealed by light microscopy, were reported by Hammond, Ernst, and Goldman (1965). The large number of merozoites (about 120,000) contained within the macroscopic host cell and the ease of collection of these host cells provided a simple means for studying the fine structure of this stage of the coccidial life cycle. Electron microscope studies of the merozoites of E. intestinalis (Mossevitch and Cheissin, 1961), E. perforans, and E. stiedae (Scholtyseck and Piekarski, 1965) have been reported. These species differ from E. bovis, however, in the size of the schizont and number of merozoites produced. This study was initiated to provide a detailed description of the ultrastructure of mature first-generation merozoites of E. bovis and the host cell in which they are located. The morphological characters of this stage of E. bovis will be compared with those of the corresponding stages of other species of Eimeria and of other Sporozoa. Received for publication 16 February 1966. * U. S. Department of Health, Education, and Welfare, Public Health Service, NIAID, Laboratory of Parasitic Diseases, Bethesda, Maryland 20014. t Department of Zoology, Agricultural Experiment Station, Utah State University, Logan, Utah. MATERIALS AND METHODS First-generation merozoites of E. bovis were obtained from infected calves as described by Hammond et al. (1965). Immediately after collection, by scraping the intestinal mucosa, the material was placed in a large volume of fixative without washing. Fixation was accomplished by one of three methods. Three per cent glutaraldehyde in Sorensen's phosphate buffer (Sabatine, Bensch, and Barrnett, 1963) was used for some specimens. The material was fixed for 2 hr and then rinsed in buffer with sucrose. This was followed by postfixation in veronal-acetate-buffered Os04 for 2 hr. Some specimens were fixed for 2 hr in veronalacetate-buffered OsO (Caulfield, 1957); others were fixed for a similar period in Dalton's chromeosmium fixative (Dalton, 1955). All fixatives were buffered at pH 7.4 and used at 0 to 4 C. Since it was necessary to mail specimens after fixation from one laboratory to another, they were left for several days in 70% alcohol, or, in the case of glutaraldehyde-fixed material, in the rinsing buffer. All specimens were dehydrated in a graded series of ethyl alcohol. They were then embedded in Epon (Sporn, Wanko, and Dingman, 1962) after being passed through propylene oxide to remove the alcohol. Polymerization was carried out for 16 to 18 hr at 60 C. Sectioning was done with an LKB Ultrotome using a diamond knife. Sections were mounted on bare, 400-mesh grids and stained with lead (Karnovsky, 1961). An RCA EMU-3G electron icroscope operating at 50 kv was used to view and photograph the specimens.

The Fine Structure of Lankesterella garnhami

SYNOPSIS. Thin sections of Lankesterella garnhami were examined by electron microscope and the morphology of trophozoites and sporozoites is described. The envelope of the organism consists of a double membrane, which (in the presumed sporozoite) is broken at one point posteriorly by the microphyle. The anterior end possesses characteristic organelles, which include an open apical ring leading to a collar or conoid and to 30 peripheral fibrils, a paired organelle, and numerous “lankesterellonemes.” Typical mitochondria and a Golgi apparatus are found in the cytoplasm and a nucleus of heterogeneous nature.

Electron microscope studies of motile stages of malaria parasites II. The fine structure of the sporozoite of Laverania (= Plasmodium) falcipara

1. 1) Thin sections of the sporozoites of Laverania (= Plasmodium) falcipara were examined by electron microscopy. 2. 2) The sporozoite is covered by a thick pellicle which merges at the blunt anterior tip into a specialized formation consisting of concentric rings surrounding a shallow cup. 3. 3) Leading up to the anterior cup is a paired organelle, the lobes of which are sinuous and extend back towards the nucleus. 4. 4) Near the nucleus there is a well-marked pit or micropyle, through which it is suggested the “sporoplasm” emerges in the liver cell, to initiate pre-erythrocytic schizogony. 5. 5) Fourteen fibrils are distributed around two-thirds of the periphery, with an extra one placed by itself in the remaining third. They extend from the apical cup to at least the region of the nucleus. 6. 6) Mitochondria are numerous in the cytoplasm of the sporozoite.