ENDOCYTOSIS AT THE MICROPORE OF TOXOPLASMA-GONDII

Ultrastructural studies were undertaken to reexamine the structure and function of the micropore of Toxoplasma gondii. By incubating tachyzo ites with the tracer horseradish peroxidase (HRP), we showed for the f irst time cytochemically that an extracellular tracer was internalized into vacuoles at the micropore. Our morphological observations also d emonstrated that the base of the micropore in both tachyzoites and bra dyzoites was sometimes covered by a clathrin-like bristle coat. A coat ed vesicle was observed in continuity with a bradyzoite micropore, and large (150-nm) coated vesicles were occasionally present just beneath the micropore. These results suggest that receptor-mediated endocytos is occurs at the micropore. In other micrographs, however, the micropo re appeared uncoated. In some bradyzoites, the uncoated micropore was greatly dilated, and it contained vesicles like those found in the cys t matrix associated with debris from disintegrated parasites. We had p reviously observed such debris from fragmented organisms in cysts prep ared in vivo. These results indicate that residues from dead bradyzoit es may provide nutrients for younger, developing parasites in the same cysts. Our observations also suggest that either receptor-mediated or bulk endocytosis can occur at the micropore, perhaps depending upon t he availability of specific ligands. Investigation of a receptor-media ted pathway may reveal a means for targeting therapy selectively to th e parasites to benefit patients with disseminated toxoplasmosis.