320 



SARCOCYSTIS, TOXOPLASMA AND RELATED PROTOZOA 



The posterior third of the body con- 

 tains the nucleus. It is an ellipsoidal 

 vesicle almost as wide as the body, and 

 contains a relatively small number of 

 chromatin granules and an endosome which 

 stains with Bodian silver. The nucleus is 

 surrounded by a large number of small 

 vacuoles and granules, many of which con- 

 tain glycogen, and these extend to the 

 posterior end of the body. Among them 

 lie 1 to 3 serpentine mitochondria 0. 15 to 

 0.2/i in diameter and 2/i or more long. 



In addition to the above structures, a 

 network of fibrils forming a characteristic 

 rectangular pattern can be seen on the sur- 

 face following silver impregnation by the 

 Klein or Chatton technics. 



Life Cycle : Several differing accounts 

 have been given of the life cycle oiSarco- 

 cystis. Pitfalls in its study have been dis- 

 cussed by Scott (1943). There is now gen- 

 eral agreement that the life cycle is sim- 

 ple, without sexual stages, and that no 

 intermediate host is involved. 



Animals become infected by ingesting 

 trophozoites, either in unbroken cysts in 

 the muscles or free in the feces of other 

 animals. Smith (1901, 1905) was the first 

 to show that infection took place by the 

 oral route, and was able to maintain the 

 infection with S. maris in mice for 7 years 

 by feeding infected mouse muscle. 



The trophozoites presumably pass 

 thru the intestinal wall, enter the blood 

 stream and are carried to the striated 

 muscles, where they enter the muscle 

 cells. They are found in the striated and 

 heart muscles. They are especially com- 

 mon in the wall of the esophagus, but are 

 also found in the tongue, masseter muscle, 

 diaphragm, throat, neck, body and limb 

 muscles, and even in the eye muscles and 

 Purkinje fibers of the heart among other 

 places. In ducks they are most commonly 

 found in the breast muscles. 



There is a latent period of a month to 

 6 weeks or more during which almost 

 nothing is known of what happens. The 

 first stage in the muscle cell is a one- 

 celled, irregularly rounded ("amoeboid") 

 naked parasite. This divides by repeated 



binary fissions (Scott, 1943) into a number 

 of rounded cells 4 to 8^t in diameter which 

 are enclosed in a cyst wall. Betegh and 

 Dorcich (1912), Erdmann (1914) and Arai 

 (1925) thought that schizogony takes place 

 at this stage, but Scott (1943) did not 

 agree, and Frenkel (1956a) considered its 

 existence doubtful. 



The rounded cells have been called 

 sporoblasts, pansporoblasts or prosporo- 

 blasts, but these names all carry the con- 

 notation that the trophozoites are spores, 

 and the cells are better called cytomeres 

 (Grasse', 1953) or trophoblasts. They con- 

 tinue to reproduce by binary fission, and 

 become pressed together and polygonal. 

 Later they change into ellipsoidal and then 

 into banana-shaped trophozoites. 



As multiplication proceeds, the cyst 

 grows and is divided into chambers or 

 compartments by septa arising from the 

 inner layer of the cyst wall. The process 

 continues, new trophoblasts are formed 

 at the periphery of the cysts, produce new 

 trophozoites, and new septa are laid down 

 and new compartments formed. 



The trophozoites themselves also re- 

 produce by binary fission. This process 

 was described by Ludvik (1958). The 

 nucleus first begins to enlarge and the 

 dispersed chromatin forms large granules 

 and variously curved structures. The 

 nucleus is indented in the middle of its 

 anterior edge and becomes horseshoe- 

 shaped. The cell loses its banana shape 

 and becomes broadly spindle-shaped, with 

 a rounded posterior end. The central 

 granules become dispersed thru the whole 

 cell and diminish in size. A medial sac- 

 like structure begins to be separated off 

 from the posterior part of the horseshoe- 

 shaped nucleus, and the central granules 

 disappear. The sac -like structure be- 

 comes detached from the nucleus and 

 gradually divides into 2 halves which later, 

 after the true nuclear division has been 

 completed, disappear. The horseshoe- 

 shaped nucleus divides into 2 longitudinal 

 segments. The conoid and cj^oplasm in 

 the anterior third of the cell also divide 

 into 2 longitudinal halves with a clear 

 streak between them. The newly formed 

 nuclei become shorter and their chromatin 



