Ultrastructure of schizonts and merozoites of Sarcocystis falcatula in thelungs of budgerigars (Melopsittacus undulatus)

Transmission electron microscopy was used to study the ultrastructure of sc hizogony of Sarcocystis falcatula in the lungs of budgerigars (Melopsittacu s undulatus). Sckizogony occurred exclusively by endopolygeny within endoth elial cells of pulmonary capillaries, venules. and small veins. Early schiz onts were elongate with a large nucleus and nucleolus, surrounded by a pell icle consisting of a plasmalemma and an inner single membrane, and containe d most of the organelles and inclusion bodies found in merozoites of Sarcoc ystis species. As development proceeded, schizonts increased in size and co nformed 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 G olgi adjuncts (apicoplasts). As the nucleus elongated, there was a progress ive increase in the number of spindles located at various intervals along t he nuclear envelope. Eventually. 2 merozoites formed internally immediately above each spindle. During endopolygeny, a portion of the nucleus was inco rporated into each merozoite bud along with 1 or 2 Golgi adjuncts, a Golgi complex, mitochondria, endoplasmic reticulum, and ribosomes. During merozoi te formation, micronemes appeared in close association with the Golgi compl ex and gradually increased in number The pellicle invaginated around the me rozoites so they budded at the schizont surface leaving behind a small, cen tral residual body. Dense granules appeared after merozoites were completel y formed. Schizonts were 24 x 6.8 mu m and contained 24-96 merozoites. Mero zoites were 5.1 x 1.8 mu m and were found free in the pulmonary air passage s and pulmonary capillaries and within nearly all cells of the lung except red blood cells.