Cecil E. Hall

The Structure of Certain Muscle Fibrils as Revealed by the Use of Electron Stains

Fibrils from certain molluscan muscles, in particular the adductor muscles of the clam Venus mercenaria, were examined with the electron microscope and found to possess periodic variations in structure. In order to make these structural variations visible, it was necessary to treat the fibrils with reagents of high electron scattering power (electron stains). Phosphotungstic acid was found to be particularly suitable. This stain combines with specific regions in the fibrils, forming a remarkably regular geometrical pattern of which the most prominent feature is a regular cross striation, representing a fiber‐axis spacing of about 145A. Within each stained band, the stain is more highly concentrated in spots spaced about 193A from center to center across the band. A line drawn through any such spot parallel to the fiber axis passes through other similar spots, spaced five cross bands apart, making the length of the fiber‐axis period precisely five times the fiber‐axis spacing. X‐ray diffraction data obtain...

The visualization of polyribosomal structure

Polyribosomes from rabbit reticulocytes have been studied in the electron microscope using positive- and negative-staining techniques. They can be seen as extended arrays of ribosomes connected by a strand approximately 10 A in width. The most frequent structure contains five ribosomes, although some larger units have six or even seven. The distance between ribosomal centers has been measured for different types of preparations. Some of these are tightly clustered with the ribosomes almost in contact, while other preparations have ribosomes separated by a gap of 100 to 150 A. This variability is probably associated with differences in sample preparation. The relation between the length of the polyribosome and the length of the messenger RNA for hemoglobin is discussed. Some evidence concerning the sub-structure of ribosomes is also presented.

Polyribosomes: Size in Normal and Polio-Infected HeLa Cells

HeLa cells normally contain a distribution of polysome sizes, and the largest polysomes contain over 40 ribosomes. After infection with polio virus and actinomycin-D treatment, a new class of polio-induced polysomes are found, some of which contain up to 60 ribosomes. Examinaton of these polysomes suggests a mechanism for protein synthesis with this polycistronic RNA.

Electron microscopy of wound-tumor virus.

Abstract The wound-tumor virus, causing disease of sweet clover, Melilotus sp. and other plants, is about 600 A in diameter and has the shape of an icosahedron. The surface consists of subunits about 75 A in diameter numbering 4 along an edge and 92 in all. A core about 350 A in diameter stains heavily with uranyl acetate, indicating that it consists of nucleic acid (RNA) or nucleoprotein. Disintegration of the virus envelope by removal of salt releases long strands about 30 A in diameter that are either RNA in a highly folded or coiled form or an RNA strand perhaps strengthened with combined protein.

Configuration of Inactive and Active Polysomes of the Developing Down Feather

Inactive four-ribosome polyribosomes, insensitive to ribonuclease, have the form of tight symmetrical squares and appear in feather cells during the early and intermediate periods of development. When, on the 13th day of incubation, the inactive polyribosomes become sensitive to ribonuclease and can then synthesize protein, the squares open up so that the four-ribosome polysomes are strung out in the configuration characteristic of functioning polysomes.

Scattering Phenomena in Electron Microscope Image Formation

Equations are derived for the relative intensities in electron microscope images formed without a limiting aperture in the objective lens. Three components of intensity at the image plane can be distinguished: IT, a transmitted intensity of very small relative aperture; IK, an intensity scattered within the effective aperture of the objective lens; and IB, a background intensity which does not contribute to the recognizable image. The equations derived involve two scattering cross sections, both of which are readily measurable under normal operating conditions. The form of the equations is verified experimentally; and cross sections are measured for Be, SiO, Cr, Ge, Pd, Pt, and U. The effect of altering the beam potential is discussed in the light of the results and the minimum detectable increment in specimen thickness due to scattering is estimated.

Dark‐Field Electron Microscopy. I. Studies of Crystalline Substances in Dark‐Field

Dark‐field images in the electron microscope were studied by means of an objective aperture system fixed to the object rather than to the objective lens. The resolution in images produced by the diffusely scattered component is in the range 100 to 200A, but the resolution in images produced by Bragg reflections approaches that obtainable in bright‐field operation. A resolution of about 50A was obtained with test objects consisting of evaporated films containing small crystallites, but it is estimated that crystallites having dimensions down to about 20A can be recorded with conditions prevailing in this work. Substances studied in dark‐field include evaporated films of metals and compounds and finely divided substances. It is concluded that the dark‐field method is of value for studying the size, location, and other characteristics of crystalline components in a specimen. Some of the theoretical factors influencing resolution in dark‐field are discussed.

Experimental Study of Electron Scattering in Electron Microscope Specimens

Effective scattering cross sections for polystyrene in electron microscope specimens were measured through the microdensitometry of the images of polystyrene latex spheres recorded with an RCA Type EMU 3B electron microscope. Measurements were made at both 50 kv and 100 kv beam potential without an objective aperture, with a 40‐μ aperture and with a 20‐μ aperture. Cross sections per gram vary from 1.9 to 11×104 cm2/g. The results are consistent with previous experimental data and are in reasonable agreement with theoretical calculations.

Studies on bacteriophage φX174 and its DNA by electron microscopy

In shadowed electron micrographs, bacteriophages φX174 and S13 appear to consist of twelve “knobs” arranged as at the vertices of an icosahedron. The existence of these “knobs” is confirmed by the appearance of both positively and negatively stained preparations of φX174. On heating or treatment with concentrated acetate or urea, φX174 extrudes strands up to 1·3μ in length. The strand diameter may range from less than 10 A to 40 A or above. The strands are disrupted by DNase or by heating to 80°C. They are believed to consist of DNA, irregularly coated by protein. Purified φX174 DNA, like other single-stranded nucleic acids, appears as inhomogeneous strands or “random coils” in electron micrographs. A model is proposed for the structure of φX174 in which the bulk of the phage consists of deoxyribonucleoprotein. The configuration of the DNA is thus maintained by direct combination with about 80% of the total protein. The remainder is divided into sixty identical units which are arranged in groups of five to form the twelve “knobs” of the icosahedrally symmetrical outer protein shell.

Dark‐Field Electron Microscopy: II. Studies of colloidal carbon

Several commercial carbon blacks were examined with dark‐field illumination in the electron microscope. Anomalous intensities in the dark‐field images are interpreted as resulting from coherent scattering by parallel layer groups of graphitic planes, which were previously identified by Biscoe and Warren using x‐ray diffraction. The dark‐field micrographs indicate that in a nonreinforcing and a semireinforcing black, the parallel layer groups near the surface of the roughly spherical particles are oriented with their representative vector approximately normal to the surface. No definite evidence was obtainable for such an arrangement in a reinforcing channel black with average particle diameter about 300A. After prolonged heat treatment to the point where graphitization occurs the channel black exhibits numerous diffraction images in darkfield, indicating the presence of crystals with dimensions in the range 20 to 90A.