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Much is known about determinative events in early amphibian embryos, perhaps more than any other animal group. However, as yet, little attention has been focused on the cytoarchitecture of the oocyte, and the way in which this could regulate asymmetries in the egg, which in turn could lead to developmentally important interactions. The changing cytoarchitecture of the Xenopus oocyte is described with the emphasis on the following: -firstly the polarity; the oocyte is not radially symmetrical at early stages of oogenesis, but shows marked polarity. Secondly, several cytoskeletal elements change their distribution during oogenesis, and again during maturation to form a fertilizable egg. Thirdly, monoclonal antibody methods show that the oocyte develops several asymmetries which are retained in the egg and early embryo, and may be lineage related.
Fig. 1. Nomarski images of living oocytes at early (A), mid (B) and late (C) stage I and
early stage II (D). The mitochondrial cloud is arrowed in (A), (B), (D) and can be seen
spreading to one pole in (D). The connection of the mitochondrial cloud to the rest of
the cytoplasmic matrix can be seen in (B). Mitochondrial aggregates around the
nucleus, and connected to the cytoplasmic matrix, are seen in (C). Scale bars = 25 jwm.
Fig. 2. Living stage I oocyte stained with rhodamine 123 to show distribution of
mitochondria. Staining is seen in the mitochondrial cloud, perinuclear aggregates, and
in the follicle cells round the outside. Scale bar = 25 jum.
Fig. 3. Anti-tubulin (kindly donated by Dr K. Fujiwara) staining stage I oocyte. Staining
is seen throughout the cytoplasm, but highly concentrated in the cortex, the
mitochondrial cloud, and perinuclear aggregates. Scale bar = 40/im.
Fig. 4. Anti-vimentin (kindly donated by Dr R. O. Hynes) staining of early (A), mid
(B) and late (C) previtellogenic oocytes. Staining first appears in a perinuclear ring (A)
and becomes concentrated in the mitochondrial cloud (B) perinuclear aggregates (C)
and a network of increasing complexity through the cytoplasm (C). Scale bars: (A) = 20
/an, (B) = 47/im, (C)
Fig. 5. Anti-cytokeratin (kindly donated by Dr E. B. Lane) staining of early (A and B)
stage I and vitellogenic stage III (C) oocytes. Cytokeratin filaments first appear cortically
(A). They surround and compartmentalize the mitochondrial cloud (B), and
eventually spread throughout the cytoplasm (C). Scale bars (A) and (B) = 40jum,
(C) = 50jim.
Fig. 6. Anti-vimentin (A) and anti-tubulin (B) staining of the mitochondrial cloud
fragments as it breaks down spreads to the future vegetal pole. Scale bars = 40jum.
Fig. 7. Stage I oocytes treated with 10- 1M DAC for 18 h. (A) Mitochondrial pattern
shown by rhodamine 123, (B) anti-vimentin pattern, (C) Nomarski image of living
oocyte, (D) anti-tubulin pattern. Scale bars: (A) = 25 jum, (B), (C), (D) = 40jum.
Fig. 8. (A) and (B) show anti-vimentin staining of late vitellogenic oocyte in animal (A)
and vegetal (B) hemispheres. V = position of vegetal pole, A = position of animal pole.
(C) and (D) show anti-cytokeratin staining in animal and vegetal hemispheres respectively.
Scale bars (A) and (B) = 25 pm, (C) and (D) = 50 [mi.
Fig. 9. Shows the germinal granules found in the germ plasm of the egg (A) and
mitochondrial cloud of stage I oocyte (B). The increased contrast seen in (B) is due to
the addition of saponin and tannic acid in the fixative (See Heasman et al. 1984 for
details). Anti-vimentin staining of the germ plasm of the fertilized egg (C) and 32-cellstage
embryo (A) shows that the vimentin-containing germ plasm islands aggregate
during this period. Scale bars: (A) = 150 nm, (B) = 300 nm, (C) = 50/im, (D) = 40Jum.
Fig. 10. Shows staining patterns in oocytes and embryos with the monoclonal
antibodies VC4 (A and B), VC1 (C-E), and MC3 (F-H). (A) and (B) show VC4
staining in the central region of the full-grown oocyte and gastrula respectively. The
superficial region, seen in (A) is unstained. The distribution of VC1 is shown in the
vegetal hemisphere of the stage VI oocyte (C), the animal hemisphere of the fertilized
egg (D), and in the swimming tadpole (E). In (E) primordial germ cell is stained
(straight arrow) as are the mesonephric ducts and the gut lining (curved arrow).
The distribution of MC3 is shown in the previtellogenic oocyte (F), swimming tadpole
(G) and larval oesophagus (H). In (G) only a primordial germ cell is stained. In (H) only
certain cells of the oesophagus at the same stage are stained. Scale bars (A)-(G)
H) = 30jum.
