Structures of Toxoplasma gondii tachyzoites, bradyzoites, and sporozoites and biology and development of tissue cysts

Abstract

Infections by the protozoan parasite Toxoplasma gondii are widely prevalent world-wide in animals and humans. This paper reviews the life cycle; the structure of tachyzoites, bradyzoites, oocysts, sporocysts, sporozoites and enteroepithelial stages of T. gondii; and the mode of penetration of T. gondii. The review provides a detailed account of the biology of tissue cysts and bradyzoites including in vivo and in vitro development, methods of separation from host tissue, tissue cyst rupture, and relapse. The mechanism of in vivo and in vitro stage conversion from sporozoites to tachyzoites to bradyzoites and from bradyzoites to tachyzoites to bradyzoites is also discussed.

FIG. 1

Life cycle of T. gondii.

FIG. 2

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

FIG. 3

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

FIG. 4

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

FIG. 5

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 with permission of the publisher.

FIG. 6

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.

FIG. 7

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.

FIG. 8

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.

FIG. 9

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.

FIG. 10

Transmission electron micrograph of a tachyzoite of the VEG strain of T. gondii penetrating a neutrophil in mouse peritoneum; note the moving junction (Mj) at the site of penetration into the neutrophil and the extraordinary early development of the tubulovesicular membranes (Tv) in the space between the neutrophil and the tip of the tachyzoite. Am, amylopectin granule; Dg, electron-dense granule; Rh, rhoptry; Nu, nucleus.

FIG. 11

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.

FIG. 12

Transmission electron micrograph of two T. gondii tissue cysts in the brain of a mouse 6 months after infection with the Me-49 strain. Note that the tissue cyst on the left is younger than the one on the right because of the differences in their bradyzoites. The bradyzoites in the tissue cyst on the right contain more micronemes and amylopectin than do those in the one on the left. Additionally, the contents of rhoptries in the bradyzoites in the older tissue cyst are electron dense (arrowheads) whereas those in the bradyzoites in the younger tissue cyst are honeycombed (arrows). The cyst wall (cw) also is more branched and prominent in the older tissue cyst than in the younger tissue cyst. Reprinted from reference .

FIG. 13

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.

FIG. 14

High magnification of a portion of Fig. 13 showing the cyst wall and part of a bradyzoite with an active micropore. The cyst wall is approximately 0.25 to 0.75 μm thick and consists of a parasitophorous vacuolar membrane (Pm) that has numerous indentations (arrows); the rest of the wall consists of membrane-bound vesicles and moderately course granular material. Note that the micropore of the bradyzoite consists of an indentation in the parasite plasmalemma (Pl) and of an electron-dense collar (Ec) that arises from the inner membrane complex (Im); also note the presence of an electron-dense layer (EI) of material on the surface of the plasmalemma within the micropore. Am, amylopectin granule; Mn, microneme; Sm, subpellicular microtubule.

FIG. 15

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.

FIG. 16

Anterior end of a bradyzoite in Fig. 13 showing the apical complex. Am, amylopectin; Ar, apical rings 1 and 2; Co, conoid; Im, inner membrane complex; Mn, microneme; Nr, neck of rhoptry; Pl, plasmalemma; Pr, polar rings 1 and 2; Rh, rhoptry; Sm, subpellicular microtubule.

FIG. 17

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

FIG. 18

Enteroepithelial stages of T. gondii in a small intestinal villus of a cat fed tissue cysts. Note the heavy parasitization of entrocytes, containing schizonts (type D) (small arrows) and male (large arrow) and female (arrowheads) gamonts. Hematoxylin and eosin stain.

FIG. 19

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.

FIG. 20

Transmission electron micrograph of T. gondii gamonts. (A) Mature microgamont. Fl, flagellum of microgamete; Mg, body of microgamete; Pv, parasitophorous vacuole. (B) Zygote in the early stage of oocyst wall formation. Am, amylopectin granule; Lb, lipid body; Nu, nucleus of zygote; Ow, oocyst wall in the early stage of formation; Wf, wall-forming bodies.

FIG. 21

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).

FIG. 22

Transmission electron micrograph of an oocyst of the VEG strain of T. gondii exposed to excysting fluid (trypsin and bile salts) showing one sporocyst in an early stage of excystation. The oocyst is surrounded by a fine reticulate veil (Ov) and an oocyst wall (Ow). The sporocyst wall (Sw) consists of a continuous thin outer layer and an inner layer composed of four curved plates joined at sutures (arrows) containing an interposed strip of electron-dense material. During excystation, the excysting fluid acts on the sutures, causing the plates of the inner layer to separate and curl inward (double arrow), releasing the sporozoites. Am, amylopectin; Co, conoid; Dg, electron-dense granule; Lb, lipid body; Mn, microneme; Rh, rhoptry.

FIG. 23

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 (∗).

FIG. 24

Freshly excysted sporozoite still within an oocyst. Am, amylopectin; Co, conoid; Dg, electron-dense granule; Go, Golgi complex; Im, inner membrane complex: Lb, lipid body; Mi, mitochondrion; Mn, microneme; Ow, oocyst wall; Pl, plasmalemma; Rh, rhoptry; Sm, subpellicular microtubule; Sw, sporocyst wall.

FIG. 25

Schematic drawing of a T. gondii sporozoite.

FIG. 26

A highly stretched tissue cyst estimated to contain more than 1,000 bradyzoites in an impression smear of brain homogenate from a rat 14 months after infection with the VEG strain of T. gondii. The cyst wall (arrow) is barely visible.

FIG. 27

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.

FIG. 28

Cluster of T. gondii tissue cysts in the brain of a mouse which in life showed no apparent clinical signs. Unstained squash smear.

FIG. 29

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).

FIG. 30

Transmission electron micrograph of a T. gondii tissue cyst in human foreskin fibroblast culture (HS68) 6 days after inoculation with the VEG strain zoites. Note the three bradyzoites with honeycombed rhoptries (arrowheads) and a thin cyst wall (large arrows). The small arrows point to the plasmalemma of the bradyzoites.

FIG. 31

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.