Proliferation and metastases formation of larval Echinococcus multilocularis. II. Ultrastructural investigations.

The larval stage (metacestode) of Echinococcus multilocularis was studied by means of electron microscopy (SEM, TEM) before and after subcutaneous transplantation to jirds (Meriones unguiculatus) and in their lymph nodes and lungs with parasite metastases. It was found that the metacestode consists of a network of solid, cellular protrusions (buds) of the germinal layer which transform to tube-like and cystic structures devoid or with a laminated layer. Proliferation of the metacestode apparently occurs by protruding filamentous solid cell columns (buds) from the germinal layer. Their tips have diameters of only one cell: they are covered with a smooth syncytial tegument without microtriches and are filled with undifferentiated cells which contain a nucleus with a large nucleolus. The tegument is constantly enlarged by fusion with the underlying undifferentiated cells that divide repeatedly. At some distance from the tip a cavity develops inside the protrusion, thus finally giving rise to a tube-like structure which may transform to a cystic expansion. Simultaneously, the surface of the protrusion changes by the formation of microtriches and the occurrence of an amorphous laminated layer. The latter is concentrically covered by connective tissue cells and large amounts of collagen. Within cyst-brood capsules, finally protoscoleces are formed from accumulations of undifferentiated cells beneath the tegument. The study has unequivocally proven the cestode nature of the cellular protrusions, and it is assumed that detachment of cells from these structures and their subsequent distribution via the circulation may play a role in the formation of metastases. The origin of the laminated layer is discussed.