Electron microscopic and autoradiographic studies on vitellogenesis in Necturus maculosus.

Electron microscope studies on Necturus maculosus oocytes ranging in size from 1.1-1.5 mm in diameter indicate the primary proteinaceous yolk to arise within structures referred to in other amphibian oocytes as yolk precursor sacs or bodies. The origin of these yolk precursor sacs appears to result from the activity of the Golgi complexes which form multivesicular and granular-vesicular bodies, the limiting membrane of which is at times incomplete. During differentiation, the yolk precursor sacs contain small vesicles similar in size to Golgi vesicles, larger vesicles similar to vesicular elements of the agranular endoplasmic reticulum and, on occasion, a portion of a mitochondrion. The interior of these sacs becomes granular, perhaps by a dissolution of the components just described, and soon becomes organized into a crystalline configuration. In oocytes 2.0-2.5 mm in diameter, an extensive micropinocytotic activity begins, continues throughout vitellogenesis, and constitutes the primary mechanism for the formation of secondary yolk protein. Numerous coated and smooth-surfaced vesicles, as well as electron-dense and electronlucent ones, fuse in the cortical ooplasm to form progressively larger yolk platelets.