Cytochemical evidence of an organized microtubular cytoskeleton in Xenopus laevis oocytes: involvement in the segregation of mitochondrial populations.

An organized microtubular cytoskeleton was discovered in the cytoplasm of Xenopus laevis oocytes. The microtubules were observed in 10- to 30-micron cryostat sections by indirect immunoperoxidase labeling using an antibody to tubulin. A gradual extraction of cells with a nonionic detergent was essential for good penetration of the antibody into the cells. In the cytoplasm of all previtellogenic oocytes, a dense network of criss-crossed long microtubules was associated in a basket-like structure surrounding the mitochondrial mass. At the beginning of vitellogenesis, the network meshes enlarged, while clusters of mitochondria migrated, in close association with microtubule bundles. At the beginning of vitellogenesis, the reorganization of the microtubular network, mostly in the vegetal hemisphere, occurred during the segregation of the mitochondrial populations. Reorganization is characterized by (1) a temporary enlargement of the network and close association of mitochondrial clusters with microtubular bundles, and (2) a progressive organization of a ring-shaped microtubular structure in the crown elaboration area. It is hypothesized that these modifications of the microtubular cytoskeleton contribute to the maintenance of cell shape and the polarized organization of the cell.

Segregation of mitochondria in the cytoplasm of Xenopus vitellogenic oocytes.

In actively growing vitellogenic oocytes of Xenopus laevis mitochondria segregate into 2 populations. One stays around the nucleus, actively replicates mitochondrial DNA (mtDNA), and builds up most of the stock of the mitochondria in the full-grown oocyte. The other moves toward the vegetal pole and stops replicating mtDNA early in vitellogenesis. Organelles of this population are components of the germ plasm of the cell.

Heterogeneous distribution and replication activity of mitochondria in Xenopus laevis oocytes.

In early diplotene oocytes of Xenopus laevis mitochondria are not dispersed all over the cytoplasm but gathered in a well described mitochondrial mass [18]. When tracing these organelles during active vitellogenesis we observe that some of them are involved in the elaboration of a cortical layer at the vegetative hemisphere of the cell while others stay around the nucleus. The latter contribute to the transient formation of a mitochondrial crown throughout active mitochondriogenesis. Autoradiographic studies of thymidine incorporation into mtDNA suggest a differential participation of each organelle to the final population of a full-grown oocyte according to its position in the cytoplasm.

Changes in D-loop frequency and superhelicity among the mitochondrial DNA molecules in relation to organelle biogenesis in oocytes of Xenopus laevis.

Changes in mitochondrial DNA (mtDNA) replication activity are known to occur during oogenesis in Xenopus laevis. Electron microscopic and electrophoretic analyses carried out on mtDNA molecules at different vitellogenic stages show that 1. The frequency of displacement loop (D-loop) forms is correlated with the intensity of mitochondrial biogenesis. 2. Most of the native molecules as well as the D-loop carrying ones are superhelical. 3. Four families of different superhelicity may be distinguished and D-loops are found only in the most superhelical ones. To account for the changes in the frequencies of the D-loop forms and of the different topological types during cell differentiation, it is suggested that the initiation of a new replication occurs only on the most superhelical molecules and that some control of superhelicity may exist in mitochondria.

[Evidence for catalase activity in peroxisomes of Micrasterias fimbriata (Ralfs)]. / Mise en evidence d'une activité catalasique dans les peroxysomes de Micrasterias fimbriata (Ralfs).

By ultrastructural methods, cytology, and cytochemistry it is shown that peroxysomes are present during all stages of the life cycle of the green unicellular alga Micrasterias fimbriata, cultivated on mineral medium. These organelles, surrounded by a single membrane, are in connection with endoplasmic reticulum. In full-grown cells, they are preferentially situated near chloroplasts and cell walls. The number of peroxisomes increase before cellular division and the organelles flow into the young bulge in front of the chloroplast.Application of a modified Graham and Karnosky's medium using DAB at pH 9 shows that an important activity of catalase is present not only at the level of peroxisomes but also at the level of the cell walls and certain Golgi vesicles.The topographic relations of peroxisomes with different cellular organelles and their possible functions in cell wall or mucus synthesis are discussed.