[Observation on metacercariae of Paragonimus westermani by transmission electron microscopy]

The ultrastructure of metacercariae of Paragonimus westermani was observed by transmission electron microscopy. The worm is composed of integument, parenchyma, gut and excretory sac. The integument consists of tegument, muscular layer and tegumental cell. The tegument, a syncytium, includes tegumental membrane, tegumental cytoplasm, and basal plasma membrane. On the surface of the tegument, many finger-like processes with branches and spines can be readily identified. The spines projecting beyond the surface of the tegument are completely enclosed within the outer tegument membrane and the basal plasma membrane; in the transverse section, the spine shows a crystalline lattice structure. The tegumental cytoplasm consists of an electron dense granular matrix, in which two types of inclusion bodies, spherical and rod-like bodies may be recognised, both of them comprise a unit membrane and dense granular contents, the rod-like bodies are arranged along the edge of the spines. The tegument nuclei are located not within the surface syncytium but within sunken tegumental cells situated beneath the muscle layer, the tegumental cells contain a few cell organelles, and the two types of characteristic tegument inclusion bodies. The cells are joined to the tegument by one or more long, tortuous cytoplasmic tubules, responsible for the synthesis of the tegumental inclusion bodies, and that these inclusion bodies are translocated into the tegument by the cytoplasmic tubules. It suggests that the tegument of metacercaria is the main absorptive site of nutrients.