As a member of the phylum Microspora, Nosema algerae is a small obligate intracellular parasite. Its free invasive stage is a spore with a characteristic cellular organization, including an apically anchored polar tube that serves as a tool for the transmission of genetic material into the host cell. By detailed electron micrographie documentation of the spore ultrastructure we present the aspects related to the biologic process of spore extrusion. Our ultrastructure findings confirm that the extrusion process of microsporidian spores is based on extreme changes in their organization. This study is the first complete ultrastructural documentation of N. algerae concerning the extrusion process, which can be subdivided into different stages: the breakdown of the microsporidian cellular compartmentation; the filling of a preformed polar tube with modified sporoplasm; the uncoiling of the polar tube, which in this stage has reached its final length; and, finally, its extrusion and screw-like movement. © Springer-Verlag 1998.
