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Fig. 12. Models of the effect of depletion of Xlsirts and VegT RNAs on the
cytokeratin network and germinal granules. (A) The effect of Xlsirts RNA and
VegT mRNA depletion, and of anti-cytokeratin antibody on the cytokeratin
network and the germinal granules. In the cortex of stage VI oocytes, the
cytokeratin (green) forms a complex, multi-layer network. The particles of
VegT mRNA (red) and Xlsirts RNA (blue) are integrated into the network in
RNA-specific pattern (1). The ablation of VegT mRNA or injection of
anti-pancytokeratin antibody causes a severance of the network (2), which in
turn, results in a premature aggregation of the germinal granules (3). The
ablation of Xlsirts RNA results in the collapse of the network into a
flattened, less complex sheet (2). The means of association of cytokeratin
with germinal granules are unknown, but one possibility is that cytokeratin
anchors germinal granules (pink spheres) indirectly, via an unknown binding
protein(s) (yellow, 3). (B) The effect of VegT mRNA depletion
and anti-cytokeratin antibody on the cytokeratin network and the germinal
granule aggregation in oocytes and embryos. Control (left panel), and
antisense VegT ODN or anti-cytokeratin C11 antibody injected (right panel)
oocytes and embryos. Part of the vegetal cortex is shown. For simplicity, the
cytokeratin is shown only in part of the cortex. In stage VI control oocytes,
the germinal granules (pink spheres) are located in germ plasm islands
(yellow) within the cortical network of cytokeratin filaments (green). In
antisense VegT and anti-pancytokeratin antibody injected stage VI oocytes, the
cytokeratin network is severed. In control mature oocytes, the cytokeratin
network is replaced by cytokeratin foci with normal germinal granules and germ
plasm. However, in mature antisense VegT- and anti-pancytokeratin
antibody-injected oocytes, the germinal granules coalesce into long stringy
aggregates. Upon egg activation, the cytokeratin network reconstitutes in
control but not in antisense VegT-injected eggs, and only partially in
anti-pancytokeratin antibody-injected eggs. In addition, the germ plasm
islands aggregate into larger islands. In activated eggs, the germinal
granules remain as small, separate entities in control eggs, and as stringy
aggregates in antisense VegT and anti-pancytokeratin antibody-injected eggs.
During early cleavage, the cytokeratin network is present both in control and
in antisense VegT or anti-pancytokeratin antibody-injected embryos, and the
germ plasm islands segregate to the vegetal apex of vegetal blastomeres. In
control embryos, the germinal granules aggregate into larger, more complex
germinal granules, and in antisense VegT or anti-pancytokeratin
antibody-injected embryos, the stringy aggregates of germinal granules that
were present in matured and activated eggs aggregate even further, resulting
in the formation of extremely large aggregates.
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