Small nuclear RNA localization during mitosis. An electron microscope study

The localization of small nuclear ribonucleic acids (snRNAs) during mitosis in Amoeba proteus was studied by high voltage (1,000 kV) electron microscope autoradiography. By suitable micromanipulations, the snRNA's, labeled with [3H]uridine, were made to be the only radioactive molecules in the cell and thus easy to follow autoradiographically. During interphase the snRNA label, which is almost exclusively nuclear, is distributed fairly uniformly through the nucleus with a slightly higher amount of label over chromatin than over nonchromatin areas. During prophase the snRNAs, which continue to be largely nuclear, become highly concentrated in the condensing chromosomes. At metapase, almost all of the snRNAs are cytoplasmic and essentially none are associated with the maximally condensed chromatin. Beginning in early anaphase, the snRNAs resume their association with the chromosomes, with the degree of association increasing throughout anaphase. Most of the snRNAs are back in the nuclei by telophase, but the intranuclear localization is hard to determine. We conclude that snRNAs have a great affinity for the partially condensed chromosomes of prophase and anaphase, but none for the maximally condensed chromosomes of metaphase. A minor amount of snRNA localizations in association with nucleoli and the nuclear envelope are also reported. On the basis of these findings a role of snRNAs in genetic "reprogramming" or chromosome organization is proposed.     

Cryo-electron microscopy of insect flight muscle thick filaments. An approach to dynamic electron microscope studies

Suspensions of isolated insect flight muscle thick filaments were embedded in layers of vitreous ice and visualized in the electron microscope under liquid nitrogen conditions. The unfixed, unstained, unsupported and fully hydrated filaments were observed under various biochemical conditions. We demonstrate here the first successful application of this method to thick filaments, and show that this is a possible approach to following dynamic processes by rapid freezing and electron microscopy.     

Ultrastructural and cytochemical observation of calcium ion localization in the black-beetle pleural muscles

The authors examined the infrastructure and distribution of calcium ions in the plexural muscles of a black-beetle (Blatta orientalis L.) after being fixed in a solution of osmiumtetroxide buffered with a cacodylate buffer (pH = 7.4) and according to the method of Carasso-Favard (1966). The infrastructure of these muscles differs from the muscles of other insects, first of all, in the amount and distribution of the sarcoplasmic net (SR) and mitochondria and also in the amount and topography of the location of lead precipitations which mark the calcium ions. The authors ascertained an intensive and permanent positive reaction to the presence of calcium in the mitochondria and sarcotubular systems of the sarcoplasmic reticulum, and in the intrafibrillar spaces and in the Z and M lines of the sarcomeres calcium concentrations are not detectable. The authors checked the results by using the method of Carasso-Favard (1966).     

Ultrastructural localization of calcium in post-mortem bovine muscle: A cytochemical and X-ray microanalytical study

Summary Calcium localization was demonstrated in bovine longissimus muscle using the antimonate precipitation technique in combination with electron probe X-ray microanalysis. Samples were taken each hour during the first 24 h post-mortem, and then after a storage period of 8 and 15 days. For all sampling times analysed, heavy precipitates were seen in dense parts of nuclei and on N-lines of myofibrils. Up to 18–20 h post-mortem, deposits were observed in sarcoplasmic reticulum at the level of triads. In comparison with the earlier post-mortem samples, myoplasmic precipitates were strongly increased at 4 h post-mortem, and just before rigor onset, at 19 h where intermyofibrillar spaces were completely blackened and triads were no more visible. These localizations of precipitates were still observed up to 15 days post-mortem. At these storage times, myofibril disruptions were seen at the level of N-lines. Wavelength-dispersive and energy-dispersive spectrometric analyses indicated that significant amounts of calcium occurred in the dense precipitates observed.     

An electron microscope study of myofibril formation in embryonic rabbit skeletal muscle

Trunk and limb muscles from fetal and newborn rabbits were investigated by means of light and electron microscopes. At 14 days gestation, the presumptive myoblasts migrate away from the myotome to form the anlage of the muscle of the trunk and limb. Among the population of undifferentiated cells, the myoblasts were recognized due to the presence of actin and myosin filaments. The aggregates of thin and thick filaments appear at the periphery of the cells. There is a great variety of filament assembly. The presence of Z band material appears to be essential for sarcomere formation. At 14 days of gestation the myotubes are more numerous in the limb than in the trunk. The presence of unmaturated fibrils with absence of the M line in the sarcomeres was observed. By day 18 of gestation the myotubes are wider and aggregate to form small bundles. The myofibrils were more numerous and the vesicles of the SR precursor, partly incrustated with ribosomes were dispersed among them. At day 22 of gestation the myotubes are thicker because of the myofibrils which are far more numberous. The sarcomeres were more fully developed, with the M line present. At day 28 of gestation and 3 days after delivery the already developed myofibers were present with a well organized SR system and fully developed sarcomeres.     

Ultrastructural and cytochemical localization of calcium in a rat cardiac muscle in normal and experimental conditions.

The experiment carried out by us on the, stimulated by adrenaline, cardiac muscle allowed the activation of calcium localized mainly in the mitochondria, MA and FA of the intercalated discs and SR to be translocated in the direction of the sarcomere myofilaments and this especially to the thin actin filaments. The authors' experimental proves, that during the contraction--relaxation function of the cardiac muscle there exists a circulation rythm or a oscillatory functional flow of calcium ions between the mitochondria, intercalated discs and SR and the contractile fibrillae of the sarcomere.     

Microvoids in Bombyx mori silk. An electron microscope study.

The size and distribution of microvoids in Bombyx mori silk were examined by transmission electron microscopy of silver sulphide 'stained' filaments. Silver sulphide deposited in voids and accessible regions of molecular structure appears as dense particles in thin transverse and longitudinal sections of silk filaments. Small particles (about 8 nm or less in diameter) occur around or adjacent to the periphery of the filaments. Larger particles (around 10-15 nm in diameter) occur in the form of dendritic arrays in the core region of the filaments. The leading edges of the dendritic arrays are oriented towards the fibre periphery. The particles (microvoids) appear to be either spherical or rod-like in shape and are aligned parallel to the long axis of the filament. A skin/core structure is proposed.     

An electron microscope study of the perfusion-fixed spleen

Summary By preserving the intercellular or plasmatic space, perfusion fixation provides a particularly clear picture of the histological and cytological constitution of the spleen. An analysis of documents obtained with this technique has contributed new information concerning the organization of the intermediate circulation in that organ, and on the topographical relationships and ultrastructure of the various types of cell which are usually grouped under the common denomination of reticulo-endothelial system.Currently available physiological data concerning the flow of blood through the spleen, together with the observed scarcity of sinuses, and of interstices in the latters' walls, to which must be added the narrowness of these interstices, incline us to question the existence of a circulatory system involving the passage of blood from the arterioles into the pulp cords of Billroth, and thence into the sinuses. Thus we are led to conceive of a system whereby about 97% of the blood entering the spleen would flow directly into the sinuses. Phagocytosis of worn blood cells would be carried out by a quantitatively less important side-stream, running parallel to the main stream, but nevertheless sufficiently fast to ensure that every blood cell should be shunted through the spaces of Billroth at least once every 24 hours.The unitary concept of the RES, as it appears from the bulk of existing literature, is based upon the postulated, but never proven, existence of a single cell type capable of developing distinct morphological characteristics, and of carrying out distinct functions, namely phagocytosis and the laying down of reticulin, depending on its situation in the tissue. An ultrastructural study of the various cells encountered in sections of spleen, and a comparison with their counterparts in other organs, seem to rule out this unitarian view.

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Résumé La fixation par perfusion, en conservant l'espace intercellulaire ou plasmatique, donne une image de l'organisation structurale et cytologique de la rate particulièrement claire. L'analyse des documents aínsí obtenus fournit des données nouvelles sur l'organisation de la circulation intermédiaire et sur la position et l'aspect ultrastructural des cellules habituellement réunies sous le nom de système réticulo-endothélial. Les données de physiologie sur le débit splénique, le nombre de sinus observables et le relativement petit nombre et l'étroitesse des fentes sinusiennes font douter de l'existence d'un système circulatoire faisant passer le sang des artérioles dans les cordons de Billroth et de là dans les sinus. Ceci fait envisager un schéma de circulation faisant passer environ 97% du sang directement à travers le sinus. La phagocytose des éléments à détruire est assurée par une circulation parallèle peu importante, mais suffisante pour que chaque élément sanguin ait chaque jour la possibilité de passer par les cordons de Billroth.La théorie du SRE est basée sur l'existence d'une cellule unique, différant seulement par sa position et assurant la macrophagie et la formation de la réticuline. L'étude de ces différents types de cellules au niveau de la rate et leur comparaison au niveau d'autres organes permet d'assurer qu'aucun argument de morphologie ultrastructurale ne peut l'étayer.

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This work was supported by grant No 4237 of the Swiss National Foundation for Scientific Research.

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We wish to thank Mrs. Sidler-Ansermet for her invaluable technical assistance.