Ca. Speer et Jp. Dubey, Ultrastructure of schizonts and merozoites of Sarcocystis falcatula in thelungs of budgerigars (Melopsittacus undulatus), J PARASITOL, 85(4), 1999, pp. 630-637
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.