The protozoan parasite Toxoplasma gondii is representative
of a large group of parasites within the phylum Apicomplexa, which
share a highly unusual motility system that is crucial for locomotion
and active host cell invasion. Despite the importance of motility in
the pathology of these unicellular organisms, the motor mechanisms for
locomotion remain uncertain, largely because only limited data exist
about composition and organization of the cytoskeleton. By using
cytoskeleton stabilizing protocols on membrane-extracted parasites and
novel imaging with high-resolution low-voltage field emission scanning
electron microscopy (LVFESEM), we were able to visualize for the first
time a network of actin-sized filaments just below the cell membrane. A
complex cytoskeletal network remained after removing the actin-sized
fibers with cytochalasin D, revealing longitudinally arranged,
subpellicular microtubules and intermediate-sized fibers of 10 nm,
which, in stereo images, are seen both above and below the
microtubules. These approaches open new possibilities to characterize
more fully the largely unexplored and unconventional cytoskeletal
motility complex in apicomplexan parasites.