Fertilization-induced changes in concanavalin A binding to mouse eggs

The binding of concanavalin A (ConA) to zona-free unfertilized and fertilized mouse eggs has been investigated using tritiated ConA. At low lectin concentrations (1–5 μg ml^?1) the fertilized egg shows a higher affinity for [³H]ConA than does the unfertilized egg. In saturation conditions, however, unfertilized and fertilized eggs show the same binding capacity (1.55 × 10⁸ ConA molecules/egg). The results indicate that ConA-binding sites change qualitatively following fertilization; possible connections between this change and other fertilization-induced changes in the egg surface are discussed.     

Transmission electron microscopy observations of sonication-induced changes in liposome structure.

Freeze-fracture Transmission Electron Microscopy (TEM) was used to show that sonication does not homogeneously disrupt liposome dispersions to form vesicles. Many large multilamellar particles remain intact after sonication and small, unilamellar vesicles are present after just 10 s of exposure. Small vesicles appear to coexist with large liposomes even before sonication. The mechanical and thermal stresses induced by sonication nucleate liquid crystalline defects in the liposomes, including edge and screw dislocations and +1 disclinations, but the Dupin cyclide structure of unsonicated liposomes is still recognizable in the larger particles after sonication. Defects in the bilayer organization may provide pathways for enhanced transport within the liposome, as well as from the liposome interior to exterior. A screw dislocation-catalyzed mechanism of liposome-to-vesicle conversion is proposed that accounts for the TEM observations.     

The fine structure of nuclei as revealed by electron microscopy

Summary The process of nucleolus formation has been studied by electron microscopy in spermatogonia of new-born, 15-day-old mice. One of two heteropycnotic sex chromosomes is concerned with nucleolus formation in the type A spermatogonia. The evidence for such formation has been presented with regard to behaviour and fine structure of both sex chromosome and nucleolus, Nucleolar material appears at one of two heteropycnotic sex chromosomes which are closely attached to the nuclear envelope. The two sex chromosomes approach each other, and subsequently one of them migrates into the central part of the nucleoplasm, being related to the nucleolar material which develops to show a nucleolar configuration. The sex chromosomes are homogeneously electron dense during the nucleolus formation, but assume a vesicular form at the middle stage of its development. The nucleolus is mostly of fibrillar and amorphous components at early stages of its development, but the granular components increases in amount as development proceeds. The final, mature nucleolus is composed of irregularly twisted nucleolonemata consisting of granular components, separated from fibrillar and amorphous areas. The compactly dense sex chromosome remains closely connected with the mature nucleolus.     

The contractile ring. II. Determining its brief existence, volumetric changes, and vital role in cleaving Arbacia eggs

The first cleavage furrow in eggs of Arbacia (sea urchin) is accompanied by a uniform ring of aligned microfilaments, called the contractile ring. Individual contractile ring filaments measure 35–60 A and occasionally appear "hollow." The contractile ring exists from about 20 sec after anaphase to the end of furrowing activity, i.e., 6–7 min at 20°C. It is closely associated with the plasma membrane at all times, and is probably assembled there. It is about 8 µ wide and 0.2 µ thick throughout cleavage. Its volume decreases, however, suggesting a contraction-related disassembly of contractile ring filaments, rather than a sliding-filament mechanism in the strict sense. Cytochalasin B (>10^-6 M) arrests cleavage within 60 sec, by which time contractile ring filaments are no longer visible ultrastructurally. The furrow may be seen to recede within this time. Karyokinesis is unaffected. Simultaneous disruption of furrowing activity and of the contractile ring largely confirms the vital role of the contractile ring as the organelle of cell cleavage.     

Scanning electron microscopy of the infective eggs of Hydatigera taeniaeformis.

Scanning electron microscopy of the outer surface coat of the infective eggs of Hydatigera taeniaeformis examined at high magnifications revealed the presence of scale-like features. At low magnifications the surface of eggs appeared smooth. Eggs that were fractured showed a thick inner surface layer of ridges and striations. A second layer characterized by a smooth membrane surface presumably the basement membrane was observed beneath the innermost surface layer. When the eggs were treated with 0.02 M of EDTA the outer surface coat became distorted and the emerging hooks of the embryo could be seen. Small, spherical bosses were observed on the surface of some eggs. Other eggs possibly at an earlier stage of development contained pit-like depressions.     

Thymine in the acid-soluble fraction of Arbacia eggs

Mature Arbacia eggs were extracted with cold dilute perchloric acid, the extract concentrated, and the concentrate digested in hot perchloric acid. Thymine was recovered from the digest by paper chromatography, and the amount per egg found to be about 5 times the amount per sperm. This was the amount expected from previous experiments and is believed to represent all or almost all of the thymine in the egg. The result supports previous observations that DNA is absent from the mature egg although present in the nucleus of the egg in the germinal vesicle stage. No thymine could be recovered from a similar extract of 5,000 times as many sperm of the same species. The observations are consistent with the theory that DNA and its derivatives act as metabolic antagonists of the corresponding ribose compounds.     

Freeze-fracture electron microscopy of membrane changes in progesterone-induced maturing oocytes and eggs of Xenopus laevis

Using freeze-fracture electron microscopy, compositional changes were analysed in the surface membrane of Xenopus oocytes during maturation after in vitro progesterone treatment, as well as in eggs before and after fertilization. Investigated stages were as follows: (1) defolliculated full-grown oocytes; (2) defolliculated oocytes after 5 min exposure to 5 micrograms/ml progesterone; (3) ditto at germinal vesicle breakdown (GVBD) after 5 h progesterone treatment; (4) unfertilized eggs at oviposition and (5) zygotes 30 min post-fertilization. Comparing the patterns of intramembranous particle (IMP) density and IMP size during these stages the following changes were found: a transient decrease in IMP density was found after 5 min progesterone treatment; a 48% increase during maturation; a further 17% increase after fertilization. In defolliculated oocytes tight-junction-like structures were found, but no gap junctions. These results are discussed with reference to progesterone action, membrane remodelling, protein synthesis and membrane lipid organization.     

Post-fertilization changes in the chorion of winter flounder, Pseudopleuronectes americanus Walbaum, eggs observed with scanning electron microscopy

Little has been reported on the fine structure of the outer membrane of fish eggs during and after fertilization. When observed in the scanning electron microscope, the unfertilized egg of the winter flounder, Pseudopleuronectes americanus , is characterized by a crisscross pattern of depressions. These depressions radiate in all directions across the membrane surface creating a wrinkled appearance. After fertilization, the surface of the chorion becomes regular with a smoother appearance. The pores of the unfertilized egg are flush with the chorion surface, but become thickened and elevated after fertilization. While the chorion of the unfertilized egg is also smooth and uniformly textured, the chorion of the fertilized egg appears granular by first cleavage of the blastodisc. Although no apparent change occurs in the distance between pores after fertilization, statistically significant decreases in pore diameter occur 5 min after fertilization. These results are compared to those on egg membranes of other species of fish and invertebrates.