Fine Structure of Oocyst Transformation and the Sporozoites of Leucocytozoon dubreuili*

Abstract

The sporogonic stages of L. dubreuili in the midgut and salivary glands of the simuliid [Simulium aureum, S. latipes, S. quebecense, Prosimulium decemarticulatum, Cnephia ornithophilia] vectors was studied by electron microscopy. Young uninucleate oocysts have a pellicle that initially resembles that of the ookinete. Numerous electron-dense bodies and microtubules in the peripheral cytoplasm may be involved in the formation of the cyst wall. The dense bodies appear to give rise to the amorphous material of the wall. The tubules which run circumferentially beneath the oocyst boundary probably serve as a skeletal support for the cell surface during deposition of the wall material. A subcapsular space which provides area for expansion of the developing sporozoites is formed in early multinucleate oocysts. The subcapsular space appears to be formed through a condensation of the peripheral cytoplasm, resulting in an osmotic gradient across the oocyst limiting membrane. Consequently, water diffuses out creating a fluid-filled space. Sporozoite formation begins with localized thickenings on the oocyst limiting membrane. Subsequent extension of the thickened regions into the subcapsular space marks the onset of sporozoite budding. The process is highly synchronized and culminates with the production of up to 150 sporozoites about the sporoblastoid body. The structure of sporozoites from mature oocysts and of the salivary glands of the vector is basically similar, although salivary gland sporozoites are more elongate and have numerous electron-dense micronemes. The paired rhoptries in the latter sporozoites are more elongate and uniformly electron-dense than in oocyst sporozoites.