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Fine Structure of the Sporozoite of Schellackia occidentalis

1974; American Society of Parasitologists; Volume: 60; Issue: 4 Linguagem: Inglês

10.2307/3278735

1937-2345

Robert E. Sinden, Johnnie N. Moore,

Helminth infection and control

Sporozoites of Schellackia occidentalis isolated from the peripheral circulation of laboratory specimens of Sceloporus occidentalis are described at the ultrastructural level. The parasite is found in large parasitophorous vacuoles in both erythrocytes and leukocytes, but predominantly in the latter. The subcellular morphology of the sporozoite is compared to that of Eimeria and Lankesterella. One striking and possibly unique organelle which we term the paralamellate body is described. During the course of a study on the fine structure of Plasmodium mexicanum (Moore and Sinden, 1973) it was observed that the host, Sceloporus occidentalis, was simultaneously infected with another sporozoan parasite. Identification of this parasite as the hemogregarine, Schellackia occidentalis (Bonorris and Ball, 1955), was based entirely on the blood stages as the other tissue phases were not available for our examination. Although there have been several ultrastructural studies on members of the subfamily Cryptosporidiinae, these have been largely confined to the genus Lankesterella. This paper reports the first observations on the ultrastructure of a member of the genus Schellackia. MATERIALS AND METHODS Sceloporus occidentalis infected with both Schellackia occidentalis and Plasmodium mexicanum were obtained from Dr. G. H. Ball of the University of California at Los Angeles. Blood samples of 0.1 ml were obtained by cardiac puncture, and rapidly expressed into 1.25% glutaraldehyde with 4% sucrose in 0.05 M phosphate buffer pH 7.3. Fixation was for 1 hr at 4 C. The blood was centrifuged and the pellet washed 3 times with 0.05 M phosphate buffer over a period of 1 hr. Postfixation was with 1% osmium tetroxide in 0.05 M phosphate buffer for 1 hr at 4 C. The pellets were dehydrated in ethanol and embedded in araldite. Sections were stained in aqueous uranyl acetate and lead citrate. Material was examined in a Philips EM 300. Received for publication 17 August 1973. * Permanent address: Biology Department, California State University, Northridge, California 91324. t Reprint requests should be addressed to J. Moore, Biology Department, California State University, Northridge, California 91324. OBSERVATIONS Sporozoites of S. occidentalis were seen within erythrocytes (Fig. 7), but were more commonly encountered in other cells (Figs. 1, 2, 4-6, 8-10) of the blood of Sceloporus occidentalis including those infected with P. mexicanum (Fig. 9). These intracellular forms were contained within a large parasitophorous vacuole (Fig. 2). This vacuole contained numerous small membrane-bound vesicles and swollen ribosomes (Figs. 2, 7, 10, rb). No extracellular sporozoites were observed in this investigation. The sporozoites were fusiform in shape with a truncated anterior end; the posterior end in all specimens observed was sharply recurved (Figs. 1, 2), yet the host cell was never distorted by the enclosed parasite. The pellicle consists of two unit membranes and a microtubule layer. The outer unit membrane is clearly visible as a tripartite structure; the inner, which is less densely stained and poorly defined, only occasionally may be resolved as a unit membrane (Fig. 4). At the anterior end of the organism the subpellicular microtubules can be seen to converge toward the polar ring (Fig. 5, pr) and terminate near the anterior border of the conoid (Fig. 6). We are uncertain of the number, length, and position of these microtubules beyond that there appear to be between 10 and 20 microtubules unevenly distributed around the parasite, and that no microtubules have been seen within the recurved portion of the tail (Figs. 1, 2, 7). The hollow conoid is composed of one or more helically wound filaments of ca. 30 nm diameter (Fig. 5); the structure is tapered slightly toward the anterior end where it has a diameter of ca. 220 nm. There is in one