Formation and excretion of calcareous bodies by the metacestode (Tetrathyridium) of Mesocestoides vogae.

The function of calcareous bodies, commonly found in the parenchyma of cestodes and trematodes, is relatively poorly understood. The present histochemical ultrastructural study of the proliferative tetrathyridia of Mesocestoides vogae revealed that calcareous corpuscles begin to form as organic (lipid-protein) masses that coalesce in parenchymal (calcareogenic) cells. Concentric accretion of organic and inorganic crystalline material then leads to the formation of typical refractile calcareous bodies. The precise composition of such bodies, determined by x-ray diffraction, revealed that their major inorganic constituent is indeed calcium, with significant amounts of phosphorus, silicon, and zinc as well. Emission of calcareous bodies through the tegument was observed by scanning electron microscopy, explaining their accumulation in the intracapsular spaces around worms embedded in liver tissue of the host. Following their emission, the crystalline substance of corpuscles dissolves, leaving only nonrefractile, membrane-bound cytoplasmic debris. These observations do not preclude the possibility that calcareous bodies may play some role as buffers or reservoirs of inorganic ions. However, it is difficult to accept such a function for unencapsulated worms in the coelom or intestinal lumen; we suggest that excretion is a more likely role in such sites.