Ultrastructure of shizonts and merozoites of Sarcocystis falcatula in the lungs of budgerigars (Melopsittacus undulatus).

Transmission electron microscopy was used to study the ultrastructure of schizogony of Sarcocystis falcatula in the lungs of budgerigars (Melopsittacus undulatus). Schizogony occurred exclusively by endopolygeny within endothelial cells of pulmonary capillaries, venules, and small veins. Early schizonts were elongate with a large nucleus and nucleolus, surrounded by a pellicle consisting of a plasmalemma and an inner single membrane, and contained most of the organelles and inclusion bodies found in merozoites of Sarcocystis species. As development proceeded, schizonts increased in size and conformed to the shapes of the pulmonary blood vessels. As micronemes, dense granules, the conoid, and subpellicular microtubules disappeared, there was an increase in the size and number of mitochondria, Golgi complexes, and Golgi adjuncts (apicoplasts). As the nucleus elongated, there was a progressive increase in the number of spindles located at various intervals along the nuclear envelope. Eventually, 2 merozoites formed internally immediately above each spindle. During endopolygeny, a portion of the nucleus was incorporated into each merozoite bud along with 1 or 2 Golgi adjuncts, a Golgi complex, mitochondria, endoplasmic reticulum, and ribosomes. During merozoite formation, micronemes appeared in close association with the Golgi complex and gradually increased in number. The pellicle invaginated around the merozoites so they budded at the schizont surface leaving behind a small, central residual body. Dense granules appeared after merozoites were completely formed. Schizonts were 24 x 6.8 microm and contained 24-96 merozoites. Merozoites were 5.1 x 1.8 microm and were found free in the pulmonary air passages and pulmonary capillaries and within nearly all cells of the lung except red blood cells.