The tachyzoite

The tachyzoite is crescent shaped and is approximately 2 x 6 •m in size. The tachyzoite has a pellicle, sub-pellicular microtubules, a polar ring, a conoid, rhoptries, micronemes, mitochondria, endoplasmatic reticulum, Golgi apparatus, ribosomes, rough surface endoplasmatic reticulum, micropores, apicoplast, and a well defined nucleus.

The nucleus is situated in the central or posterior part of the cell. The pellicle consists of three membranes. The polar ring encircles the conoid, a cylindrical cone which consists of six to eight fibrillar elements arranged like a compressed spring. The 22 sub-pellicular microtubules originate from the polar ring and run longitudinally for almost the entire length of the cell (Sulzer et al. 1974) and probably provide a frame for the parasite.

The rhoptries are 4-10 club-shaped, gland-like structures with an anterior narrow neck and posterior saclike end reaching as far as the nucleus. The rhoptries have a secretory function associated with host cell penetration. When the parasite has attached to the host cell, the contents of the rhoptries are discharged through the conoid (Nichols et al. 1983). The micronemes are rice-grain-like structures, usually less than 100 in number, situated at the conoidal end of T. gondii without any defined function but may participate in the invasion of the host cell (Joiner and Dubremetz 1993). In addition to the rhoptries and the micronemes, the parasite contains dense granules which also appears to have a secretory function (Charif et al. 1990).

The functions of the conoid, rhoptries, and micronemes are not fully known. The conoid can rotate, extend, and retract and is important when the parasite searches for an attachment site at the host cell, as the parasite can rotate, glide, and twist. Myosin has been found in the apical end of the parasite (Schwartzman and Pfefferkorn 1983), and actin has been found both at the apical end and distributed throughout the cytoplasm (Endo et al. 1988). The motion observed during parasite entry corresponds to the orientation of the sub-pellicular microtubules, and it is likely that the microtubules are the basis of the motility system.

After entry into the host cell, the parasite is surrounded by a parasitophorous vacuole membrane (PVM). The PVM contains numerous intra-vacuolar tubules (Sibley et al. 1985; Sibley and Krahenbuhl 1988). Toxoplasma gondii enters the host cell by active invasion (Werk 1985).

Endodyogeny is a process in which two progenies form within T. gondii and consume it from within. The Golgi apparatus divides first, and the anterior cell membranes of the progenies are formed at the anterior end. The nuclear membranes remains intact and the chromosomes do not condense at metaphase. The progenies move towards the cell membrane of

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the parent parasite as they continue to grow, and at last the inner membrane of the parent parasite disappears and the outer membrane fuses with the inner membrane of the progenies, and two new tachyzoites are formed.

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