Surface spines of human blood flukes (Schistosoma mansoni) contain bundles of actin filaments having identical polarity.

The spines of Schistosoma mansoni have crystalline structures that have been suggested to consist of actin filaments. In this ultrastructural study, binding of heavy meromyosin to the actin filament spines strongly supports this view. Moreover, we reveal that all the packed actin filaments in the spines have the same polarity pointing away from the apical plasma membrane toward the basal membrane of the surface syncytial epithelium of the parasites and that the spine filaments interact indirectly with both the apical and basal membranes.