Synthesis and Deposition of Oocyte Envelopes (Vitelline Membrane, Chorion) and the Uptake of Yolk in the Dragonfly (Odonata: Aeschnidae)

Abstract

Light and electron-microscope studies on dragonfly ovarioles reveal evidence that the precursor vitelline membrane and chorion secretions are synthesized within the follicle cells. It is suggested that the sequence of synthesis and deposition of the vitelline membrane occurs as follows. The vitelline membrane presecretion appears to be synthesized by the rough surfaced endoplasmic reticulum, giving rise to intracisternal granules. These appear to migrate in the cisternae to the region of the Golgi complex where the endoplasmic reticulum loses most of its ribosomes and the intracisternal granules move into the Golgi region where they appear within small vesicles. These seem to find their way into the Golgi cisternae where they may be incorporated with the secretions from the Golgi cisternae to produce the definitive previtelline secretion. The previtelline secretion bodies are eventually discharged into the space between the oocyte and follicle cells, forming rows of secretion bodies between the microvilli. These fuse into progressively larger bodies until a complete membrane is established. Follicle cells actively secreting precursor vitelline membrane substance show many disk-shaped, relatively clear vesicles in the cytoplasm. After the vitelline membrane is laid down, the follicle cells take on an entirely different function; namely, the synthesis and deposition of the chorion. The first visible chorion secretion appears in profile as elongate dense bodies within the Golgi cisternae which tend to coil, and in so doing, expand the cisternae. As this occurs, the enlarged cisterna, loaded with concentric coiled secretion material, separates from the remainder of the Golgi cisternae and becomes free in the cytoplasm as a prechorion secretion body. These migrate to, and collect below, the surface of the cell where they are eventually ejected between the surface folds and become incorporated into the developing chorion. Uptake of yolk in the dragonfly seems to be predominantly by micropinocytosis. The oocyte surface during active vitellogenesis bears many pits which contain an extracellular material closely applied to the outer surface of the plasma membrane. Thin, radially oriented bristles are continuous with the inner surface of the plasma membrane in this region. The pits continue to invaginate until they are cut off from the plasma membrane and come to lie in the oocyte cortex as coated vesicles. These appear to lose their coats gradually and fuse with one another to produce definitive yolk spheres.