Endocytosis and streaming of highly gelated cytoplasm alongside rows of arm-bearing microtubules in the ciliate Nassula.

The microtubular cytopharyngeal basket acts as a jet engine when Nassula ingests filaments of blue-green algae. Prolonged and highly directed cytoplasmic streaming forms the main mechanical basis for propulsion of algal filaments through the basket. Cytoplasm surrounding such filaments streams at the same rate as that at which filaments travel through the basket's interior. This cytoplasm appears to be very highly gelated. The arm-bearing surfaces of certain rows of microtubules seem to be located in the active shear zone where the forces responsible for cytoplasmic streaming are generated. Algal ingestion represents a rapid and large scale form of endocytosis. An algal filament is contained in a long membranous invagination from the ciliate's cell surface as it is drawn through the basket. The invagination moves through the basket at the same velocity as the algal filament and surrounding cytoplasm. The arm-bearing microtubules do not appear to bind directly to the invagination which consists of two unit membranes. One of these is probably an invagination of the cell surface membrane. The origin of the other membrane has not been ascertained. There are indications that microtubule sliding and a contractile ring of microfilaments operate antagonistically during feeding to evert and re-invaginate a cell surface depression where the top of the basket is located.