Electron microscopy studies of a developing Pythium oogonium

Abstract

The young Pythium oogonium is filled with mitochondria, endoplasmic reticulum, several nuclei, electron-dense ground plasm, and four types of vesicles. The vesicles responsible for the production of the reserve globule contain a spherical electron-dense globule which enlarges from a tiny drop until it occupies a large part of the vesicle. The reserve vesicles coalesce in the central region of the oogonium and form the reserve globule. Other vesicles probably lipid in nature enlarge and fuse with each other and aggregate in the outer zone. In the mature oospore, the small amount of cytoplasm and one nucleus are sandwiched between the vesicles and the reserve globule. Vesicles containing microtubules, and numerous small vesicles underlying the oogonial plasmalemma and wall are also present. The nuclei move to the outer zone of the oogonium before fertilization of the privileged nucleus with the antheridial nucleus. All except the privileged nucleus degenerate in the periplasm. The periplasm is delimited at the time of fertilization and contains a great deal of endoplasmic reticulum at first. Large vacuoles soon occupy much space in the outer periplasm and they continue to enlarge as the oospore wall is formed. By the time the oospore wall is completed the zone between it and the oogonial wall is almost empty.