The ultrastructural organization of the isolated cortex in eggs ofNassarius reticulatus (Mollusca) |
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Authors: | Johanna E Speksnijder Kees de Jong Heleen A Wisselaar Wilbert A M Linnemans M René Dohmen |
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Institution: | (1) Department of Experimental Zoology, University of Utrecht, Padualaan 8, NL 3584 CH Utrecht, The Netherlands;(2) Department of Molecular Cell Biology, University of Utrecht, Padualaan 8, NL 3584 CH Utrecht, The Netherlands |
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Abstract: | Summary We have studied the organization of the cortex in fertilized eggs ofNassarius reticulatus by examining rotary-shadowed whole mounts of isolated cortices in the transmission electron microscope. The following components
were distinguished: (a) the plasma membrane, with clathrin-coated areas and coated pits, (b) microfilaments and microtubules,
and (c) a tubulovesicular network of endoplasmic reticulum. Microfilaments were identified by labeling with heavy meromyosin,
and microtubules with a monoclonal anti-tubulin antibody, using both immunofluorescence microscopy and immunogold labeling
for transmission electron microscopy. The microfilaments are organized in a network parallel to and closely associated with
the plasma membrane, with typical Y- and X-shaped intersections. The endoplasmic reticulum is associated with this microfilamentous
lattice. The microtubules also run parallel to the plasma membrane, but they are located at a greater distance, as can be
inferred from stereo images. In the uncleaved egg, numerous microtubules are present in the egg cortex. Shortly before polar
lobe formation, at the onset of mitosis, the microtubules disappear almost entirely. They reappear again at the end of first
cleavage, as the polar lobe is being resorbed. The synthesis of cortical microtubules at this stage appears to depend on the
presence of microtubule-organizing centers in the animal hemisphere of the egg, since microtubules do not reappear in isolated
polar lobes. Clathrin-coated areas are present in both the animal and vegetal hemisphere before polar lobe formation. During
mitosis, the clathrin-coated plaques and pits are found almost exclusively in the animal hemisphere. After resorption of the
polar lobe, at the two-cell stage, no clathrin-coated areas were found at all. |
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Keywords: | Egg cortex Molluscan egg Polarity Polar lobe Cytoskeleton |
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