Ultrastructural insights into the polar tube invasion organelle from microsporidian parasites

Microsporidia are single-celled intracellular parasites that infect a wide range of animals, including humans. To invade host cells, microsporidia have evolved a harpoon-like invasion apparatus called the polar tube (PT). Initially neatly coiled inside dormant spores, when triggered, the PT rapidly fires out of the spore, transforming into a linear tube that impales or mediates attachment to a nearby host cell. The linear PT then serves as a conduit for the delivery of the infectious parasite cell to the interior of the host cell. While the PT was discovered more than 120 years ago, the composition, structure, and mechanism of this fascinating invasion organelle remain poorly understood. Recent developments in cryoelectron tomography have enabled new investigations of the PT, yielding fresh insights into its organization and ultrastructure. Here, we review our current understanding of the PT invasion organelle, with a focus on its ultrastructure in the pre-firing state (when coiled in the dormant spore) and post-firing state (after exit from the spore).