Fig 1. Rings in the apex of an apicomplexan parasite.

The front (apical) end of apicomplexan parasites is specialized for secretion of proteins involved in motility and host cell invasion. Secretory vesicles that contain different sets of proteins are encased by microtubules and concentric rings that provide an aperture at the tip for secretion. The numbers and shape of the rings and vesicles vary considerably between parasites. Shown here are selected sections from a tomographic reconstruction of the front end of a Plasmodium sporozoite, the form of malaria parasites transmitted by mosquitos. Plasmodium sporozoites are unusual as they show apical complex rings reoriented so that they are not perpendicular to the cell longitudinal axis, but face the substrate. The colored images represent 3D models derived from the electron microscopy data. Visible structures and organelles are indicated by arrowheads with matching color in the models unless otherwise stated. White and pink arrowheads indicate the apical rings, which are colored in pink; red arrowhead indicates a sporozoite specific kink, green: microtubules, blue: plasma membrane, yellow: the inner membrane complex (alveoli) that defines alveolates, cyan and magenta: vesicles releasing proteins involved in migration (micronemes, cyan) and invasion (rhoptries, magenta). Scale bars: 100 nm. Images from [10].