[An electron microscopic study of the natural death of cyst cells in Sarcosporidia. I. Ultrastructural changes in the cytoplasm of the cyst cells of Sarcocystis muris and S. bovifelis]

Abstract

A complicated development of Sarcocystis muris and S.bovifelis, within tissue cysts (sarcocysts), was examined in terms of a phenomenon of programmed cell death well-known in Metazoa. It looks likely that this phenomenon of versatile significance in living organisms has not yet been followed in parasitic protozoa. This is the first attempt to find out a general picture of morphological changes, occurring in the course of natural death in parasitic protozoa. With electron microscope, a study was made of sarcocysts of S. bovifelis isolated from oesophagus muscles of a naturally infected cow. In 6-month old sarcocysts of S. muris, three morphofunctional cell types are commonly distinguished: little differentiated metrocytes, intermediate cells, and highly differentiated <<cyst merozoites>>, homologous to gamonts of other Sporozoa. Among numerous cyst cells, looking overtly normal, some <<abnormal>> cells tend to be encountered, which increase in number in 10 month old cysts, i.e. as sarcocysts are getting older. At least three stages of morphological degradation are to be distinguished in the cyst cells, with special attention being paid to changes in their cytoplasm. The first stage degradation involves ultrastructural changes in cell organelles, primarily in rough endoplasmic reticulum, whose membranes form vacuoles, with various kinds of membranous and non-membranous materials inside. At the second stage, apical organelles of cyst cells are involved: rhoptries are seen to lose their contrast, suggesting that their proteinaceous content may be discharged into the cell cytoplasm; eventually, membranes of rhoptries disappear. Micronemes seem to break into composing channels of endoplasmic reticulum. Ultimately, in the cells committed to death, the main body of cell organelles looks indecipherable, and the whole cells become filled with numerous vacuoles of different sizes and configuration. At the third statge of morphologoical degradation, cell trans