Fine structure of the ventral disk apparatus and the mechanism of attachment in the flagellate Giardia muris.

The topography of the Giardia trophozoite is dominated by the large domed sucking disk of the ventral surface. Attached to the host duodenal epithelium, the rim of this disk penetrates the enteric surface coat and interdigitates with microvilli of the epithelial cells, approaching to within 20 nm of the host surface membrane. Distortion of the host brush border within the disk suggests an applied suction force. A mechanical explanation of disk action is sought in a detailed description of the fine structure of components of the ventral surface but is found to be untenable. The disk is supported by a platform of modified 25-nm microtubules, linked to the ventral membrane by side arms and bearing heavily cross-linked vertical dense ribbons. It is argued that such is the architecture of rigidity rather than relative movement. Around the disk a mobile cytoplasmic flange is supported by 2 lateral plates of periodic substructure. The flange has no clear mechanical role in attachment; a likely evolutionary origin from a component of the anterior axonemal axis is suggested. The cavity of the ventral disk leads posteriorly through a portal into the ventrocaudal groove: a shallow depression that houses the ventral flagella. Observation of isolated living trophozoites suggests that attachment depends on the continuing activity of the ventral flagella, which normally beat synchronously in a sinusoidal waveform. Electron micrographs confirm that this waveform is maintained in situ on the host epithelium. Of the 4 pairs of flagella, the ultrastructure of the ventral flagella is notable for additional components in the flagellar shaft, including an intraflagellar dense rod linked to 3 axonemal doublets by fine connectives. From a consideration of analogous macroscopic systems, a preliminary hydrodynamic analysis is advanced in which the suction force of attachment follows from the pattern of fluid flow induced by the beating ventral flagella. The significance of the conclusion that cytoplasmic microtubules (or structures derived from them) apparently maintain cell shape in the face of an applied external force is discussed.