The origin of protein and fatty yolk in Rana pipiens. III. Intramitochondrial and primary vesicular yolk formation in frog oocytes.

The precise origin of the primary yolk precursor complex or primary vesicular yolk is obscure but in its earliest recognizable stage it is a typical multivesicular body which first acquires a moderately electron-dark matrix. Following this, an extremely electron-dark amorphous material, the yolk protein, appears within the precursor. This yolk protein increases in amount as the yolk vesicle grows and by the time the precursors are about 1 micrometer in diameter this protein is partly to almost completely crystalline. Yolk originating within mitochondrial cristae unlike that in the yolk precursor complexes is crystalline from its earliest appearance. Intracristae mitochondrial yolk crystals have a spacing of 70--85 A. Their molecular organization appears in some sections as electron-dark lamellae and in others as light cylinders surrounded by an electron-dark matrix.