The mitochondrial cloud of Xenopus oocytes: the source of germinal granule material

The mitochondrial cloud is a prominent mass in the cytoplasm of previtellogenic oocytes of Xenopus laevis. It is shown here that the cloud contains both mitochondria and electron-dense granulofibrillar material (GFM). Using a combination of light microscopical, fluorescence, time-lapse filming, and electron microscopical techniques, the ontogeny of these components is reported and their fate is studied. It was found that the cloud is stationary in previtellogenic stages and fragments into islands of mitochondria and GFM during stage II (using the staging system of J. N. Dumont [1972) J. Morphol. 136, 153-180). These islands become localized in the peripheral cytoplasm at one pole of the stage III oocyte. By studying successive stages, GFM was followed through oogenesis and it was found localized only at the vegetal pole of stage IV and V oocytes. Furthermore, it was found that it bears a striking resemblance in position, appearance, and associations with mitochondria to the "germinal granules" of unfertilized eggs. Germinal granules have been shown by others to become incorporated into germ-line cells. It is concluded that the GFM is the precursor of this material and that the mitochondrial cloud is the site of its accumulation and localization in the previtellogenic oocyte.