Electron microscopical study on gamogony of Sarcocystis suihominis in human tissue cultures

Sexual stages and oocysts of Sarcocystis suihominis were developed in human tissue cultures and studied with the electron microscope. This development was extremely rapid, being completed about 18--22 h post infection and there were no preceding schizogonic processes, thus confirming the earlier observations that schizogony is obligatorily restricted to the intermediate host in the sarcosporidian life cycle. Micro- and macrogamonts could be distinguished about 12 h post infection and were situated in a parasitophorous vacuole bounded by two membranes. These gamonts reached diameters of up to 10 micrometer. The large nucleus of every microgamont gave rise simultaneously to about 20--30 microgametes. Only dense projections of the nucleus were used as nuclei of microgametes. The microgametes were slender, about 4--5 micrometer long, and several were found to have three flagella, one of which was attached to the body for some distance. Besides these flagella additional microtubules were found and in several cases the attached flagellum was not complete and contained various numbers of single or paired microtubules. The macrogametes were bounded by two membranes and contained two types of inclusions similar to the wall-forming bodies known from the genus Eimeria. The oocysts were bounded by a wall consisting of a dense outer layer and four membranes, under which two other membranes covered the cytoplasm. Beginning from the 22nd h post infection a development similar to sporulation was noted inside these oocysts. This sporulation, i.e., the formation of two sporocysts inside an oocyst, was, however, not completed, probably due to the rapid degeneration of the parasitized host cell. The oocyst itself even appeared intact five days later.