Although the predilection for
Toxoplasma gondii
to form cysts in the nervous system and skeletal and heart
muscles has been described for more than fifty years, skeletal muscle cells (SkMCs) have not been explored as a host
cell type to study the
Toxoplasma-host cell interaction and investigate the intracellular development of the parasite.
Morphological aspects of the initial events in the
Toxoplasma-SkMC interaction were analysed and suggest that
there are different processes of protozoan adhesion and invasion and of the subsequent fate of the parasite inside the
parasitophorous vacuole (PV). Using scanning electron microscopy,
Toxoplasma tachyzoites from the mouse-virulent
RH strain were found to be attached to SkMCs by the anterior or posterior region of the body, with or without
expansion of the SkMC membrane. This suggests that different types of parasite internalization occurred. Asynchronous
multiplication and differentiation of
T. gondii were observed. Importantly, intracellular parasites were seen to
display high amounts of amylopectin granules in their cytoplasm, indicating that tachyzoites of the RH strain were
able to differentiate spontaneously into bradyzoites in SkMCs. This stage conversion occurred in approximately 3%
of the PVs. This is particularly intriguing as tachyzoites of virulent
Toxoplasma strains are not thought to be prone
to cyst formation. We discuss whether biological differences in host cells are crucial to
Toxoplasma stage conversion
and suggest that important questions concerning the host cell type and its relevance in
Toxoplasma differentiation
are still unanswered.
