Formation of Arrowhead Complexes With Heavy Meromyosin in a Variety of Cell Types

Abstract

The parasitic protozoan Toxoplasma gondii has been examined with the electron microscope in order to study the fine structure and the formation of the membranes surrounding the cell . The study of the ultrastructure of the membranes covering the parasite shows the existence of a three-membraned complex . Only the outer membrane is considered to be the plasma membrane ; the two membranes below it form an inseparable whole of changeable molecular architecture (modifications in appearance depending on the methods of fixation, local differentiation) . During reproduction, which takes place by fission or more often by endogeny, the membranes of the daughter individuals are formed from the membranes of the parent . At first the middle and inner membranes of the parent extend, separating the cytoplasm of the daughter cells from that of the parent . The three-membrane complex of the endozoites is completed at the time of their liberation ; the external membrane of the parent covers the leaving endozoites ; thus, the plasma membrane of the daughter cells derives also from that of the parent . These findings on the origin and role of limiting membranes during reproduction differ entirely from those described so far for other cells .

Fine structure of merozoites ofEimeria ? beauchampiLeger and Duboscq, 1917 inGlossobalanus minutus(Kow.) (Enteropneusta)

“…The thick-walled vacuoles which occur near the refractile-like body are similar to those described in the sporozoan and mainly, in the coccidian (Vivier and Henneré, 1965;Vivier and Petitprez, 1969;Porchet-Henneré, 1972;Porchet-Henneré and Ormieres, 1973;Boulard et al, 1982) where functional relationship with the nucleus has been attributed to these structures.…”

“…beauchampi is a typical feature of the Apicomplexa motile stages (Vivier and Petitprez, 1969;Vivier et al, 1970;Porchet-Henneré and Vivier, 1970;Porchet and Torpier, 1977;Dubremetz and Torpier, 1978, Vivier, 1979, Porchet et al, 1982. In the present study the outer membrane of the pellicle shows infoldings which probably are artifacts.…”

Cytoskeleton of Apicomplexan Parasites

“…This preserves the ability of replicating tachyzoites to invade throughout the cell cycle. The internal daughter cells are delimited by an IMC and associated subpellicular microtubules, and each contains (in addition to the nucleus, mitochondrion, Golgi, and plastid) a complete set of apical organelles (100,154,183). When the daughter cells are fully mature, the maternal apical complex is disassembled and the daughter parasites bud from the mother, adopting her plasma membrane.…”

“…The IMC lies directly below the parasite plasma membrane and is closely associated with it, creating a three-layered pellicle characteristic of the Apicomplexa (26,104,183). In Plasmodium sporozoites, the IMC is constructed from a single large flattened vesicle joined by a single suture line traversing the long axis of the parasite (44).…”

“…In addition to the apical complex, the apicomplexans have other unique structural features, such as an essential chloroplast-like organelle called the apicoplast (87,99,169,194). The parasites are bounded by the pellicle, a composite structure consisting of the plasma membrane and the closely apposed inner membrane complex (IMC) (2,4,26,43,44,58,103,104,177,183). The pellicle is intimately associated with a number of cytoskeletal elements, including actin, myosin, microtubules, and a network of intermediate filamentlike proteins (Fig.…”