Ultrastructural Aspects of Oocyte Development and Shell Formation in Ascaris lumbricoides

Oocytes of Ascaris lumbricoides are attached throughout their development to an axial rachis by means of cytoplasmic bridges, the walls of which are thickened and in which lie many microtubules. Oogonia, as well as the rachis in the region of very young oocytes, contain numerous lamellar bodies of unknown function. Development of the oocytes is synchronous. They contain many lipid droplets which appear to arise by coalescence of small vesicles. The largest organelles, excepting the nucleus, are the membrane-bounded refringent granules. During their formation, these granules coalesce with small vesicles, as well as with dense granules which form in the apical cytoplasm. In a late prematuration state the refringent granules are intimately associated with rough endoplasmic reticulum and Golgi complexes. They contain ascaroside esters, and respond strongly to protein stains. Label from absorbed, tritiated ascarosides is deposited only in refringent granules and lipid droplets. Fertilization immediately stimulates shell formation, in which three layers originate from the egg, and a fourth from uterine secretions. The first (vitelline) layer appears immediately and beneath it appears a chitinous layer the elements of which are probably synthesized in the dense particulate cytoplasmic cortex. The refringent granules are then extruded from the cytoplasm and coalesce to form the third (ascaroside) layer, in which the first polar body is often enclosed. Meanwhile, uterine cells secrete material which adheres to the outer surface of the egg, forming the fourth layer. The fine structure of the uterine cells is that of typical secretory cells.