| The ultrastructure of trophozoites and developing spores of Myxidium serotinum, a gall bladder parasite of Eurycea bislineata and other amphibians, was studied with both scanning (SEM) and transmission electron microscopy (TEM). Trophozoites possessed a microvillous surface underlain by a felt-like ectoplasm. A mitochondria-rich region lies within the reticular endoplasm, adjacent to the ectoplasm. Paired spores begin development within pansporoblasts formed from envelopment of the sporont cell by the envelop cell. Subsequent replication of sporonts produces 10 new cells, five synthesize each spore; two valves (valvogenic cells), two polar capsules (capsulogenic cells), and the binucleate sporoplasm occupying the spore center. Capsules develop at opposite poles. All stages of pansporoblast development were concurrent in each sporulating trophozoite; the two spores in each pansporoblast developing synchronously. Mature spores were concentrated endoplasmically in trophozoites. The envelop cell did not divide, but actively contributed microtubules to spore valve formation.; Histological and histochemical staining procedures revealed that spore valves, capsules, and filaments were protein. The sporoplasm contained glycogen, an observation confirmed by TEM. Polysaccharides also were found in the ectoplasm and reticula of the endoplasm. A "stopper" in the filament eversion pore of polar capsules stained with acid phosphatase, indicating lysosomes, whose suggested role is initiating filament eversion.; Spores were found in intestines of hosts with sporulating trophozoites in their gall bladders, demonstrating how spores leave the host. In one gall bladder, elongated, sporulating trophozoites, 1.25 cm and 1.50 cm, were found; such large specimens suggest prolonged association with the host, and also indicate that spore emergence is not dependent upon trophic death and disintegration.; A membrane-bound body, seen in one thick TEM section, appeared to be emerging through a pore in the trophozoite, which contained spores although none was seen in the body. Membrane-bound spores were often seen free in bile showing that spores emerge from live trophozoites. Experimental transmission of the parasite to larval amphibians was unsuccessful. |
