title: The ultrastructural organization of the isolated cortex in eggs ofNassarius reticulatus (Mollusca).
abstract: 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.
authors.affiliation: Department of Experimental Zoology, University of Utrecht, Padualaan 8, NL 3584 CH, Utrecht, The Netherlands.
authors.forename: Johanna E
authors.initials: JE
authors.lastname: Speksnijder
authors.affiliation: Department of Experimental Zoology, University of Utrecht, Padualaan 8, NL 3584 CH, Utrecht, The Netherlands.|Department of Molecular Cell Biology, University of Utrecht, Padualaan 8, NL 3584 CH, Utrecht, The Netherlands.
authors.forename: Kees
authors.initials: K
authors.lastname: de Jong
authors.affiliation: Department of Experimental Zoology, University of Utrecht, Padualaan 8, NL 3584 CH, Utrecht, The Netherlands.
authors.forename: Heleen A
authors.initials: HA
authors.lastname: Wisselaar
authors.affiliation: Department of Molecular Cell Biology, University of Utrecht, Padualaan 8, NL 3584 CH, Utrecht, The Netherlands.
authors.forename: Wilbert A M
authors.initials: WA
authors.lastname: Linnemans
authors.affiliation: Department of Experimental Zoology, University of Utrecht, Padualaan 8, NL 3584 CH, Utrecht, The Netherlands.
authors.forename: M René
authors.initials: MR
authors.lastname: Dohmen
keywords: Cytoskeleton; Egg cortex; Molluscan egg; Polar lobe; Polarity