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The cytoskeleton

Transmission electron microscopy of thin sections of trypanosomatids showed the presence of subpellicular microtubules distributed throughout the cell body, except in the flagellar pocket region. Profiles of the endoplasmic reticulum can be seen in between and below the subpellicular microtubules (Fig. 18.9).33

In addition to the subpellicular microtubules, other sets of a few microtubules have been visualized in trypanosomatids. First, a triplet of microtubules is associated with the cytostome—cytopharynx complex. These microtubules start underneath the cytostome membrane and follow part of the cytopharynx, sometimes disappearing when the cytopharynx lumen becomes thinner. Second, a set of four microtubules appears to originate (or to end) underneath the opening of the flagellar pocket, follow the path of the preoral ridge, and associate with the cytopharynx. These two sets of microtubules appear to be a characteristic feature of T. cruzi (and

Deep-etching view reveals that subpelicular microtubules connecting with endoplasmic reticulum (A), with plasma membrane (B) and between them (arrows—C)

Figure 18.9 Deep-etching view reveals that subpelicular microtubules connecting with endoplasmic reticulum (A), with plasma membrane (B) and between them (arrows—C).

Bar, 100 nm.

Source: After Souto-Padron T, De Souza W, Heuser JE. Quick-freeze, deep-etch rotary replication of Trypanosoma cruzi and Herpetomonas megaseliae. J Cell Sci 1984;69:161—8.

possibly other members of the Trypanosomatidae family that contain a cytostome). Third, another set of four microtubules is also observed in T. brucei33; these microtubules originate close to the basal bodies and underlie a portion of the flagellar pocket, forming a channel through which extracellular components may have access to the lumen of the flagellar pocket. This microtubule complex has been designated as MtQ (microtubule quartet) in T. brucei.33 In T. cruzi, this complex also surrounds a domain of the flagellar pocket, following a helicoidal pattern and approaching the subpellicular microtubules near the opening of the flagellar pocket.34 Fourth, single microtubules were observed running through the cytoplasm. One of these microtubules passed through a tunnel-like structure of the mitochondrion (Fig. 18.10).

Microfilaments have never been observed in the cytoplasm of T. cruzi. However, it has been shown35,36 that cytochalasin B treatment leads to morphological alterations in the cytoskeletal elements associated with the cytostome—cytopharynx complex, which is responsible for transferrin uptake. Actin and actin-binding proteins were characterized in T. cruzi?1 Cevallos and colleagues38 demonstrated that

D model of T. cruzi amastigotes demonstrating a tunnel-like mitochondrion structure in green

Figure 18.10 3D model of T. cruzi amastigotes demonstrating a tunnel-like mitochondrion structure in green. Nucleus and kinetoplast are represented in blue, microtubules are represented in yellow, flagelum is represented in orange, and cell body in gray.

Source: After Girard-Dias W, Alcantara CL, Cunha-e-Silva N, de Souza W, Miranda K. On the ultrastructural organization of Trypanosoma cruzi using cryopreparation methods and electron tomography. Histochem Cell Biol 2012;138(6):821—31.

T. cruzi genome encodes for a diverse group of actins, actin-like and actin-related proteins that are conserved among trypanosomatids.

 
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