Ribonucleic acids and the synthesis of cellular proteins

ELECTRON MICROSCOPY OF CULTURED SPHERULES OF COCCIDIOIDES IMMITIS

Spherules of C. immitis have been grown in vitro in modified Roessler's medium under CO₂ tension and continuous cultures now maintained for over 18 months. Transformation of hyphae and development of the spherule form have been studied by thin section electron microscopy. Cells of organisms in the hyphal stage have thin (ca. 50 mµ), apparently structureless walls and a cytoplasmic membrane. Many nuclei, elongated mitochrondria with both transverse and longitudinal cristae, and lipid particles are present. The hyphal wall thickens and the cell transforms into spherules. A large central accumulation of electrontransparent polysaccharide appears in the spherule. The peripheral cytoplasm contains nuclei, each enclosed in a double-layered membrane, mitochondria, and small dense particles. Prior to cleavage the polysaccharide droplets are lost, while mitochrondria become small and spherical. Endospores are formed and liberated when the spherule wall breaks. These begin to grow and repeat the cleavage cycle.

A METHOD FOR INTRACELLULAR AUTORADIOGRAPHY IN THE ELECTRON MICROSCOPE

A technic is described for high resolution intracellular autoradiography in the electron microscope. Cultures of LLC-MK₂ monkey kidney cells were incubated for 72 hours in a medium containing 0.4 µcurie per ml of thymidine-H³. After labeling, the cells were fixed with osmium tetroxide and embedded in methacrylate. Ultrathin sections of the labeled tissue were taken up on Formvar-coated and carbon-stabilized electron microscope grids. A 150 to 450 A layer of silver metal was then evaporated onto the tissue. The coated grids were exposed to bromine vapor for 1.5 to 2 minutes under red light, allowed to dry for 1 minute, and then covered with a thin film of 1 per cent aqueous gelatin applied by means of a fine wire loop lowered over the grid supported on a glass peg. For autoradiographic exposure, the grids were stored 50 days in a light-proof container at 4°C with calcium chloride desiccant. Development was carried out for 5 minutes at 20°C in Promicrol (May and Baker, England) diluted 1:1 with water, followed by a 1 minute water wash and fixation for 2.5 minutes in 15 per cent aqueous sodium thiosulphate. After removal of the gelatin by immersion for 16 hours in water at 37°C, the autoradiograms were dried and examined in the electron microscope. Ultrastructural detail was fairly well defined and the cytoplasm of each labeled cell was covered with an electron opaque deposit of silver, suggesting that a polynucleotide containing thymidine may be synthesized in the cytoplasm. The matter is discussed.

Fibroblasts and eosinophils in normal fibroplasia of ferret vulval skin, shown by light and electron microscopy; with special reference to eosinophil degranulation

In order to compare aspects of normal fibroplasia with reports for pathologically fibrotic tissue, light and electron microscopic observations were made on ferret vulval skin, which proliferates markedly with the onset of estrus and regresses equally fast after mating. Estrous tissue, which contained hypertrophied fibroblasts, new collagen, elastic fibers and ground substance, was invaded by numerous eosinophils with extensive release of granules. Eosinophil degranulation was both extra- and intracellular; some cells, completely disintegrated, had released all contents into the extracellular matrix. Extruded granules reacted positively for major basic protein. In regression, diminished fibroblasts incorporated collagen fibrils. Many of the features seen in this normal fibroplasia in ferret vulval skin are similar to those in pathologic situations. Thus, in particular, the presence of degranulating eosinophils in pathologic fibrosis does not necessarily indicate that they are the cause of the disease. Our information and that from other sources implies a role for eosinophils in connective tissue change.

Infective organisms in the cytoplasm of Amoeba proteus

Evidence from electron and phase microscopy is given which shows that infective organisms are present in the cytoplasm of Amoeba proteus. Vesicles containing living organisms have been observed after repeated washing and starvation of the amebae for a period of 2 weeks. Exposure to gamma-radiation in conjunction with starvation, repeated washing, isolation of single amebae, refeeding with contaminant-free Tetrahymena, and clone selection has produced clones with reduced cytoplasmic infection. These findings are discussed in regard to the autoradiographic studies of other investigators on Amoeba proteus. The controversies over whether DNA and RNA are synthesized in the cytoplasm may be resolved by the finding of cytoplasmic infection.

Kinetics of nucleoside incorporation into nuclear and cytoplasmic RNA

HeLa and conjunctiva tissue culture cells were incubated for various intervals with tritiated nucleosides and the incorporation into RNA was localized in different parts of the cell by means of autoradiography. In order to obtain quantitative measurements of incorporation from grain count data the influence of cell geometry on the absorption of the tritium beta ray was considered. Relative correction factors, E = g/g*, relating an idealized grain count in the absence of absorption, g, to the actual grain count, g*, were derived for the different cell compartments. For the average HeLa cell the factors for the nucleolus, n, non-nucleolar parts of the nucleus, N, and the cytoplasm, C, are in the ratio E(n)/E(N)/E(C)= 2.3/1.6/1.0. The kinetics of incorporation for cytidine and adenosine are similar. The n and N curves are characterized by a rapid rise and early saturation, whereas the C curves show an appreciable lag and no evidence of saturation for intervals as long as one generation time. Estimates of the relative amounts of RNA in each compartment were obtained from ultraviolet micro absorption measurements and used together with the kinetic data to calculate specific activities. For incubation periods of short duration the ratio of specific activities n/N for cytidine is approximately twice that for adenosine. Three hypotheses for the mechanism of ribonucleoside incorporation and RNA synthesis are discussed, and arguments favoring a transport of RNA or an RNA by-product from the nucleus and nucleolus to the cytoplasm are presented.

The ultrastructure and histochemistry of a nematode-induced giant cell

The development of giant cells induced by the nematode Meloidogyne in tomato roots has been followed under controlled growth conditions and the ultrastructure and histochemistry of these structures have been examined. Entry of the nematode larvae into the roots took place within 24 hours; giant cell formation started on the 4th day and involved breakdown of the cell walls accompanied by thickening of a surrounding giant cell wall and an increase in density and area of the cytoplasm. The nuclei increased in number by simultaneous mitosis throughout a single giant cell. The peak of cytoplasmic density was reached after moulting and during egg production. The rate of protein synthesis in the giant cell is correlated with the rate of growth of the nematode. The giant cell wall is a thick, irregularly surfaced structure which contains all the normal polysaccharide components of a cell wall. The cytoplasm is rich in protein and RNA and contains mitochondria, proplastids, Golgi bodies, and a dense endoplasmic reticulum. The nuclei are large and irregular in shape and contain large nucleoli and a number of Feulgen-positive bodies scattered irregularly along the nuclear envelope. The nucleolus contains RNA and fat as well as Feulgen-positive granules which are revealed after treatment with ribonuclease. It consists of a dense outer cortex surrounding a much lighter central core and is connected at times with the Feulgen-positive bodies in the nucleus. Speculation is provided on the role of these bodies in cytoplasmic protein synthesis.

Sequential structural response of lens epithelium to retina-conditioned medium

When lens epithelium is cultured with retina-conditioned medium, many of the cells undergo fibre differentiation within 6-10 days. Here we report the temporal sequence of structural events that characterize this change in an organ culture model system. Within an hour of exposure to conditioned medium, some cells withdrew from the epithelial monolayer and began migrating over stationary cells still attached to the capsule. By 24 hr, migratory activity was largely responsible for causing the explant to become multilayered, increasing its thickness while at the same time reducing its surface area. Control cells attached to each other along their lateral boundaries through interdigitations of microvilli. After BRCM treatment, microvilli flattened out and the membranes had a crinkled appearance. Eventually, as the cultured cells developed into fibres, membranes straightened and developed knob and socket junctions and large numbers of their organelles underwent degradation within autophagic vacuoles. Nucleoli began to enlarge by 16 hr and by the time cells had been exposed for 24 hr, some nucleoli were enlarged to seven-fold their original area, as measured on electron micrographs. This nucleolar change was followed over the next few days by a gradual increase in cytoplasmic protein, and cells became plump or elongated. The ultrastructural changes that we observed in culture are similar to those that can be seen in the intact lens. Such fidelity of change indicates that this cultured lens explant system is an excellent model for experimental intervention and analysis of the processes involved in terminal lens fibre differentiation.

OBSERVATIONS ON THE ORIGIN AND MULTIPLICATION OF PLASTIDS

Plastids were investigated in ultrathin sections of young developing tissues of corn endosperms, pea cotyledons, and potato. The electron photographs give support to the possibility that plastids are produced de novo, as well as by division. Some of the pictures suggest that, under certain conditions, plastids may also multiply by a process of delimitation brought about by invaginations of the plastid membrane.Several aspects of plastids and mitochondria can be directly compared, but the latter are much more limited in their response to environmental changes. An origin de novo for plastids is in agreement with known genetical behavior and results of recent biochemical investigations.