Identification of a subpopulation of merozoites of Sarcocystis singaporensis that invades and partially develops inside muscle cells in vitro

The affinity of merozoites of Sarcocystis singaporensis obtained from the l ungs of acutely infected rats to muscle cells and other cell lines grown in vitro was examined. Two distinct types of mature schizonts developed in th e lungs 11-13 days p.i. with sporocysts: those containing PAS(-) merozoites (type 1) which mainly reacted with antibodies prepared against sporozoites , and others containing PAS(+) merozoites (type 2) which were antigenically close to bradyzoites. When inoculated onto cell cultures, type 1-merozoite s induced schizogonic development in brain capillary endothelial cells of t he rat. In contrast, type 2-merozoites invaded L6 myoblasts. In long-term c ultures (50 days) of L6 cells, zoites transformed to a 8-15 mu m long uninu cleate stage which, tentatively, could be unizoite sarcocysts. Although the observed dichotomy in merozoite development is unprecedented in this form, evidence from previous work suggests that these observations are relevant to other Sarcocystis species. The presented cell culture system could be a first step towards successful growth of sarcocysts in vitro.