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.