Leishmania major: in vitro ultrastructural study of the paraxial rod of promastigotes.

The flagellum of Leishmania major promastigotes has an intraflagellar structure known as the paraxial rod (PAR) which extends from a point halfway in the flagellar pocket to the tip of the flagellum, lying opposite the axonemal microtubule doublets 4-7. An expansion of the axonemal plasma membrane envelops the PAR and may provide desmosomal attachment at the orifice of the flagellar pocket. The complex organization of the 4-6 nm thick filaments in the PAR was studied by us in cross, oblique, longitudinal and tangential sections by electron microscope. These filaments are disposed in two parallel lamellae, one alongside the axoneme (ca. 45 nm thick), and the other alongside the plasma membrane (ca. 65 nm thick), with an interlamellar gap of about 22-28 nm. In each lamella, 8-12 parallel series of longitudinal filaments at ca. 30 nm intervals interdigitate with coplanar parallel series of oblique filaments at ca. 25 nm intervals and inclined to the long axis of the flagellum at ca. 48 degrees, and ca. 55 degrees, in the inner (paraxonemal) and outer lamella, respectively. The parallel filaments in each of the longitudinal and oblique series are spaced at ca. 8 nm intervals. They are cross-striated at ca. 30 nm intervals by transverse filaments which terminate occasionally on adjacent axonemal microtubules 5 and 6 in the inner lamella, and the plasma membrane in the outer lamella. Extending across the interlamellar gap is a set of parallel rows of 7-12 nearly parallel filaments at ca. 20 nm intervals. The part of the flagellar plasma membrane enclosing the PAR has a subplasmalemmal cytoskeleton consisting of a layer of longitudinal 2 nm filaments at 8 nm intervals, obliquely striated by parallel 2 nm filament doubles at ca. (-65) degrees with the long axis of the flagellum and ca. 20 nm periodicity. Each filament doublet stria apparently gives origin to collinear short filament doublet extensions that curve into juxtaposed meshes of the outer lamella. Microtubules of the axonemal doublets 5 and 6 are connected to electron-dense (ca. 12 nm thick) strips of the inner lamella of the PAR by longitudinal series of ca. 4 nm cross-links across a ca. 12 nm cleft.