Fine structure and mechanical design of cylindrical tension-transmitting cytoskeletons

Abstract

All cells contain cytoskeletal components which perform static 'spatial organizing' and dynamic 'motile' functions. The studies described in this thesis concern two types of cytoskeletal component of universal biological importance: microtubules and microfilaments, and various other structures which are associated with them. The general properties of microtubules and microfilaments and of other cytoskeletal components are reviewed in the introduction. Examples of the functions they perform in various systems are described, with emphasis on Protozoa and metazoan morphogenetic systems. The ciliate Nassula aurea was the first system studied. The cytopharyngeal basket is a tube shaped feeding organelle through which rapid cytoplasmic streaming occurs. The basket becomes deformed during transport of a food vacuole through its lumen. Ultrastructural examination of deformed baskets reveal that part of the basket, the tubular 'sheath', becomes stretched, probably in an elastic manner. Links between the microtubules in the sheath also appear to become stretched, so these could be responsible for the elastic properties. The nature of the links between microtubules within the 'rods', components of the basket responsible for its longitudinal rigidity, is also discussed, as are some other features relevant to the function of the basket as a feeding organelle. The second system studied was oogenesis in Drosophila melanogaster. Several stages in the development of the oocyte and the surrounding follicle cells and nurse cells were examined ultrastructurally. The arrangement of microtubules and microfilaments is described, as are the changes that occur in these and other structures during oogenesis, and the related changes in morphology. A circumferentially oriented layer of microfilaments was discovered in vitellogenic oocyte follicles. The oocyte expands rapidly during vitellogenesis, as cytoplasm flows into it from the nurse cells, and as yolk proteins and nutrients flow in across the follicle. The microfilamentous layer could cause the oocyte to elongate as it expands by restricting its increase in diameter. The layer is destroyed by cytochalasin B, and can be decorated by heavy meromyosin. Therefore it is concluded that the microfilaments are actin-like. Various other structures in the developing egg chamber (the oocyte plus follicle and nurse cells) are described.