Encyclopaedia Britannica, 11th Edition, "Electrostatics" to "Engis" / Volume 9, Slice 3

by Various

From Lankester’s Treatise on Zoology, vol. Protozoa.
Fig. 9.—Spores of Actinomyxidia (after Stolc).
a, Hexactinomyxon psammoryctis (par. Psammoryctesbarbatus). b, Synactinomyxon tubificis (par. Tubifex rivulorum);the mass of united spores.	c, Triactinomyxon ignotum (par. Clitellio, sp.). d, Upper portion of Hexactinomyxon, showing twoof the three polar capsules, one with filamentdischarged.

From Wasielewski’s Sporozoenkunde.
Fig. 10.—A. Sarcosporidiain the ox; a transverse sectionof the oesophagus, natural size,showing the parasites in theouter (a, b, c, d, e) and inner(f, g, h) muscular coats.
B. Longitudinal section ofa muscle-fibre containing aSarcosporidian parasite, × 60.

3. Order—Sarcosporidia. With the exception of one or two forms occurring in reptiles, these parasites are always found in warm-blooded Vertebrates, usually Mammals. They are of common occurrence in domestic animals, such as pigs, sheep, horses and (sometimes) cattle. A Sarcosporidian has also been described from man. The characteristic habitat is the striped muscle, generally of the oesophagus (fig. 10, A) and heart, but in acute cases the parasites overrun the general musculature. When this occurs, as often happens in mice, the result is usually fatal. Unless, however, the organisms thus spread throughout the body, the host does not appear to suffer any serious consequences. In addition to the effects produced by the general disturbance to the tissues, the attacked animals have apparently to contend—at any rate in the case of Sarcocystis tenella in the sheep—with a poison secreted by the parasite. For Laveran and Mesnil (27) have isolated a toxine from this form, which they have termed sarcocystin.

In the early stages of growth, a Sarcosporidian appears as an elongated whitish body lodged in the substance of a muscle-fibre; this phase has long been known as a “Miescher’s tube,” or Miescheria. The youngest trophozoites that have been yet observed (by Bertram, 1) were multinucleate (fig. 11, A), but there is no reason to doubt that they begin life in a uninuclear condition. The protoplasm is limited by a delicate cuticle. With growth, organellae corresponding to the Myxosporidian pansporoblasts are formed by the segregation internally of little uninuclear spheres of protoplasm. At the same time, a thick striated envelope is developed around the parasite, which later comes to look like a fur of fine filaments. The probable explanation of this feature (given by Vuillemin, 44) is that it is due to the partial breaking down of a stiff, vertically (or radially) striated external layer (fig. 12, A), such as is seen in Myxidium lieberkühnii. Immediately internal to this is a thin, homogeneous membrane, which sends numerous partitions or septa inwards; these divide up the endoplasm into somewhat angular chambers or alveoli (fig. 12). In each chamber is a pansporoblast, which divides up to produce many spores; hence the spores formed from different pansporoblasts are kept more or less separate. The pansporoblasts originate, in a growing Sarcosporidian, at the two poles of the body, where the peripheral endoplasm with its nuclei is chiefly aggregated. More internally, spore-formation is in progress; and in the centre, pansporoblasts full of ripe spores are found.

By this time the parasite has greatly distended the muscle-fibre in which it has hitherto lain, absorbing, with its growth, practically all the contractile-substance, until it is surrounded only by the sarcolemma and sarcoplasm. It next passes into the adjacent connective-tissue, and in this phase has been distinguished from Miescheria as Balbiania, under the impression that the two forms were quite distinct. In the later stages, the parasite may become more rounded, and a cyst may be secreted around it by the host’s tissue. In these older forms, the most centrally placed spores degenerate and die, having become over-ripe and stale.

After Bertram, from Wasielewski’s Sporozoenkunde.
Fig. 11.—Stages in the growth ofSarcocystis tenella of the sheep. A,Youngest observed stage in whichthe radially striated outer coat hasnot appeared; the body of thetrophozoite is already divided intoa number of cells or pansporoblasts(k). B and C, Older stages withnumerous pansporoblasts and twoenvelopes, an inner membrane andan outer radially striated layer.

With regard to the spores themselves and what becomes of them, our knowledge is defective. Two kinds of reproductive germ have been described, termed respectively gymnospores (so-called sporozoites, “Rainey’s corpuscles”) and chlamydospores, or simply spores. It seems probable that the former serve for endogenous or auto-infection, and the latter for infecting fresh hosts. Unfortunately, however, both kinds of germ are not yet known in the case of any one species. The gymnospores, which are the more commonly found (e.g. in S. muris, S. miescheriana of the pig, and other forms), are small sickle-shaped or reniform bodies which are more or less amoeboid, and capable of active movement at certain temperatures. They appear to be naked, and consist of finely granular protoplasm, containing a single nucleus and one or two vacuoles. The chlamydospores, or true spores, occur in S. tenella of sheep (fig. 13), and have been described by Laveran and Mesnil (26). They also are falciform, but one extremity is rounded, the other pointed. There is a very thin, delicate membrane, most unlike a typical, resistant spore-wall; and the spores themselves are extremely fragile and easily acted upon and deformed by reagents, even by distilled water. The rounded end of the spore contains a large nucleus, while at the other end is an oval, clear space, which, in the fresh condition, shows a distinct spiral striation. The exact significance of this structure has been much debated. In position and appearance it recalls the polar-capsule of a Myxosporidian spore. The proof of this interpretation would be the expulsion of a filament on suitably stimulating the spore; while, however, some investigators have asserted that such a filament is extruded, this cannot be regarded as at all certain. Hence it is still doubtful whether this striated body really corresponds to a polar-capsule.

From Wasielewski’s Sporozoenkunde.
Fig. 12.—A, Sarcocystis miescheriana(Kühn) from the pig: late stage inwhich the body has become dividedup into numerous chambers or alveoli,each containing a number of germs.
B, Sarcocystis of the ox: section of a stage similar to fig. 12. a,Substance of muscle-fibre; b, envelope of parasite; c, nuclei of themuscle; d, parasitic germs (gymnospores); e, walls of the alveoli.In the peripheral alveoli are seen immature germs.

(After Laveran and Mesnil, from Lankester’s Treatise on Zoology, vol. Protozoa.)

Fig. 13.—Spores of Sarcocystis tenella, Raill., from the sheep.

a, Spore in the fresh condition, showing a clear nucleus (n) and a striated body or capsule (c). b, Stained spore; the nucleus (n) shows a central karyosome; the striations of the polar capsule (c) are not visible.

Nothing whatever is known as to the natural means by which infection with Sarcosporidia is brought about. Smith (39) showed that mice can be infected with Sarcocystis muris by simply feeding them on the flesh of infected mice. It is not very likely, however, that this represents the natural mode, even in the case of mice; and it certainly cannot do so in the case of Herbivora. The difficulty in the way is the delicacy of the spores, which seem totally unfitted to withstand external conditions. It may be that some alternative (intermediate) host is concerned in dispersal; but this has yet to be ascertained.