The fine structure of Leucocytozoon simondi. III. The ookinete and mature sporozoite

Abstract

The pellicles of ookinetes and sporozoites of Leucocytozoon simondi measure about 40 mμ and are similar in appearance. The parasites are bounded externally by a trilaminar plasma membrane beneath which lies a fibrillar zone. Below this zone and forming the inner surface of the pellicle is a second membrane-like layer. The specialized apical region of the ookinete is modified into a thickened "cap-like" structure. The pellicle in this apical cap region appears wavy and the inner layer appears allernately thick and thin in transverse section. In the subpellicular space lie numerous long thickened structures suggestive of struts which line the inner surface of the apical cap. Beneath these, about 70 microtubules ring the cytoplasm. Numerous dense elongate micronemes extend anteriorly toward the apex of the ookinete. The anterior tip of the pellicle of sporozoites is modified into a cup-like conoid and a polar ring with which about 35 subpellicular microtubules are associated. Elongate paired organelles extend anteriorly from the nuclear region to the polar ring. Numerous dense, ellipsoidal granules and one or more mitochondria are seen in the cytoplasm. A crystalline material which corresponds with the "vacuoles" observed in Giemsa-stained ookinetes and sporozoites lies in two or more zones within the cytoplasm and may be a stored food material or possibly a virus.