Life cycle of T. gondii.

Schematic drawings of a tachyzoite (left) and a bradyzoite (right) of T. gondii. The drawings are composites of electron micrographs.

Transmission electron micrograph of a negatively stained apical complex of a tachyzoite. Ar1 and Ar2, apical rings 1 and 2; Co, conoid consisting of coiled microtubules; Im, internal microtubules; Pr 1, ring 1; Pr2, polar ring 2; Sm, subpellicular microtubules. Reprinted from reference 121 with permission of the publisher.

Transmission electron micrograph of a tachyzoite of the VEG strain of T. gondii undergoing endodyogeny within a mouse peritoneal macrophage to form two daughter tachyzoites. Cd, conoid of developing tachyzoite; Co, conoid of mother tachyzoite; Dg, electron-dense granule; Ga, Golgi adjunct; Hm, host cell mitochondrion; Hn, host cell nucleus; Id, inner membrane complex of developing tachyzoite; Im, inner membrane complex of mother tachyzoite; Mi, microchdrion; Mn, microneme; Nu, nuclei of daughter tachyzoites; Pl, plasmalemma of mother tachyzoite; Pm, parasitophorous vacuolar membrane; Pv, parasitophorous vacuole; Rh, rhoptries of daughter tachyzoites.

Transmission electron micrograph of a mouse peritoneal macrophage containing several tachyzoites of the VEG strain of T. gondii, one of which is escaping from the macrophage near the top of the micrograph (arrow). Note that the PV is no longer evident and the tachyzoites are free in the macrophage cytoplasm.

Tissue cysts of T. gondii in mouse brains. (A) Tissue cyst with three bradyzoites, each with a terminal nucleus (arrows). Note the thin cyst wall (arrowhead). Impression smear with silver impregnation and Giemsa stain. (B) Three tissue cysts with well-defined cyst walls (arrowheads). Note the tissue cyst with two bradyzoites, each with a terminal nucleus (arrows). Impression smear with silver impregnation and Giemsa stain. (C) Intracellular tissue cyst in section. Note the thin cyst wall (arrow) and the host cell nucleus (arrowhead). Hematoxylin and eosin stain. (D) Tissue cyst with numerous PAS-positive bradyzoites (arrowheads) enclosed in a PAS-negative cyst wall (arrow). PASH stain. (E) Tissue cyst freed from mouse brain. Note the cyst wall (arrow) enclosing hundreds of bradyzoites. Unstained impression smear.

Tissue cyst in the brain of a mouse that was inoculated 8 months earlier with oocysts of the VEG strain of T. gondii. This ultrathin section of the cyst shows approximately 110 bradyzoites (Bz). The tissue cyst is surrounded by a relatively thin cyst wall (Cw) and is situated within the host cell cytoplasm near the host cell nucleus (Hn). A few of the bradyzoites appear to be degenerate (Dz). The area within the rectangular box is shown at a higher magnification in Fig. 14.

Bradyzoite in the tissue cyst shown in Fig. 13. Am, amylopectin granule; Ce, centrioles; Co, conoid; Dg, electron-dense granule; Ga, Golgi adjunct (apicoplast); Go, Golgi complex; Im, inner membrane complex; Mi, mitochondrion; Mn, microneme; Nu nucleus; Pm, plasmalemma; Rh, rhoptry.

Coccidian cycle of T. gondii. Reprinted from reference 47 with permission of the publisher.

Transmission electron micrograph of type D T. gondii schizonts. (A) Intermediate schizont showing several nuclei (Nu). Mv, microvilli of host enterocyte. (B) Mature schizont. (C) Longitudinal section of a merozoite. Co, conoid; Dg, dense granule; Mi, mitochondrion; Mn, microneme; No, nucleolus; Nu, nucleus; Rh, rhoptry.

Oocysts of T. gondii. (A) Unsporulated oocyst. Note the central mass (sporont) occupying most of the oocyst. (B) Sporulated oocyst with two sporocysts. Four sporozoites (arrows) are visible in one of the sporocysts. (C) Transmission electron micrograph of a sporulated oocyst. Note the thin oocyst wall (large arrow), two sporocysts (arrowheads), and sporozoites, one of which is cut longitudinally (small arrows).

Transmission electron micrograph of a sporulated oocyst of the VEG strain of T. gondii. (A) Oocyst in the late stage of excystation, showing several free sporozoites (Sz) and collapsed sporocyst walls (Sw). Box B (see panel B) shows a micropyle in the oocyst wall (Ow). (B) High magnification of portion of panel A showing details of the oocyst wall and the micropyle, the oocyst wall consists of an outer electron-dense layer (Ol) and an inner electron-lucent layer (Il), which are separated by an electron-lucent line (El). At the micropyle, the outer layer of the oocyst wall becomes electron lucent and extremely thin (arrow); the electron-lucent line runs continuously through the micropyle, and the inner layer of the oocyst wall becomes slightly thicker near the micropyle and is continuous with the micropyle, which consists primarily of a curved, moderately electron-dense disc (∗).

Schematic drawing of a T. gondii sporozoite.

Section of the cerebrum of a patient with AIDS. Note the large area of necrosis (large arrow) and several small satellite areas (small arrows), probably due to tissue cyst rupture and subsequent growth of tachyzoites. Intact tissue cysts (arrowheads) are also present. All the black dots are T. gondii. Immunohistochemical stain with anti-T. gondii serum.

Encephalitis in the brain of a mouse 87 days after being fed oocysts of the VEG strain of T. gondii. Hematoxylin and eosin stain. (A) Coronal section showing grossly visible areas of necrosis (arrows) in the cerebellum and pons. (B) Necrosis (arrowhead) in the cerebrum and numerous tachyzoites (small arrows) in a meningeal blood vessel with severe meningitis and a group of extravascular tachyzoites (large arrow). (C) Four tissue cysts, two of which (arrowheads) are degenerating. (D) Numerous tissue cysts (arrowheads) and tachyzoites (arrows).

Section of the brain of a mouse 10 days after it was fed T. gondii oocysts. There are individual (arrowheads) and a large group of (large arrow) BAG-5 positive organisms with different intensities of reactivity. Numerous BAG-5-negative organisms (small arrows) are also present. Immunohistochemical staining with anti-BAG-5 antibody.

Tachyzoites of T. gondii. A dividing tachyzoite (arrowheads) and single tachyzoites (arrows). Impression smear feline lung, stained with Giemsa stain.

Schematic representation of the apical complex of T. gondii. Modified from reference 51 with permission of the publisher.

Transmission electron micrograph of a tachyzoite of the VEG strain of T. gondii in a mouse peritoneal exudate cell. Am, amylopectin granule; Co, conoid; Dg, electron-dense granule; Go, Golgi complex; Mn, microneme; No, nucleolus, Nu, nucleus; Pv, parasitophorous vacuole; Rh, rhoptry.

Transmission electron micrograph of four tachyzoites of the VEG strain of T. gondii in the final stages of endodyogeny that are still attached by their posterior ends to a common residual body (Rb); note that several host cell mitochondria (∗) are situated close to the parasitophorous vacuole (Pv), which contains extensively developed tubulovesicular membranes (Tv). Am, amylopectin granule; Co, conoid; Dg, electron-dense granule; Hn, host cell nucleus; Mn, microneme; Mp, micropore; Nu, nucleus; Rh, rhoptry.