Ultrastructure of Trypanosoma lewisi: Flagellum, Microtubules, and the Kinetoplast

SYNOPSIS. With the technique of preselecting pure suspensions of trypanosomes for subsequent electron microscopy and with epoxy resin embedding after glutaraldehyde fixation, we have re-examined the ultrastructure of Trypanosoma lewisi with particular reference to the flagellum and pellicle. The peripheral flagellar tubules exhibit clockwise asymmetry; however, sub-tubule B of the peripheral tubules is divided into a doublet by a diagonally running anti-clockwise arm that originates from the median diaphragm. Projections which seem to be continuous with the counter-clockwise arms extend into a “basket shaped” intra-flagellar structure. This structure, originating distal to the flagellar pocket and running anteriorly within the flagellar membrane, may play a role in maintaining the rigidity of the flagellum. The central flagellar tubules have a double helical substructure and dense cross-striae (diameter 50 A), and are regularly arranged at distances of approximately 250 A. The central tubules arise from separate kinetosomal plates that lie at two different elevations. One central flagellar tubule originates from a flattened kinetosomal plate which makes contact with the peripheral tubules. The other central tubule passes through the flattened plate to fuse with an underlying “disc-like” plate. The primary kinetosome exhibits the uniform triplet pattern of tubules at its most proximal region. A pair of tubules closely associated with one of the triplets represents two of the four subpellicular tubules which penetrate the cytoplasm in the region of the flagellar pocket. Sub-pellicular tubules of dimensions similar to the flagellar tubules form a uniform cytoskeleton in all regions of the cell except in the area of the flagellar pocket. A desmosome-like structure maintains continual cell membrane contact with the flagellum and the pellicle. This structure is designated the attachment zone. The fine structure of the kinetoplast is demonstrated. The dense intra-mitochondrial elements (presumably containing DNA) are actually tubules, embedded in a moderately dense matrix. Morphological observations indicate that this organelle is probably primarily concerned with the ontogenesis of mitochondria, and several mitochondria extend from it into the cytoplasm of the cell.