Fig. 1. Toxoplasma tachyzoites have two discrete populations of microtubules. (A) A diagram of the microtubules of Toxoplasma tachyzoites: the subpellicular microtubules and spindle microtubules. The sequence illustrates the behavior of microtubules during replication by endodyogeny. (A1) The subpellicular microtubules (red) are nucleated from the apical polar ring (green) and are tightly apposed to the inner membrane complex (gray dashed lines) that underlies the plasma membrane. (A2) The spindle microtubules (red) are nucleated from a spindle pole plaque embedded in the nuclear membrane (green). Extranuclear centrioles (red) are closely associated with the spindle pole plaques and daughter buds. (A3) Tachyzoites replicate by endodyogeny, a form of internal budding. The two daughter parasites (each containing a collection of apical organelles) are enclosed by individual sets of inner membrane complex and associated subpellicular microtubules. The mother parasite's subpellicular microtubules and apical complex are retained so that parasites are competent for invasion throughout the cell cycle. (B) Immunofluorescent labeling of the subpellicular microtubules (B1, arrow), spindle microtubules (B2, arrow) and daughter subpellicular microtubules (B3, arrows). The left-hand panels correspond to tubulin labeling and the right-hand panels are phase-contrast images of the parasites growing within primary fibroblasts.