Structure of multinucleated smooth muscle cells of the ascidian Halocynthia roretzi

Summary Cells isolated from ascidian smooth muscle were about 1.5–2 mm in length. Each contained 20–40 nucle in proportion to cell length. The cytoplasm was characterized by the presence of an enormous quantity of glycogen particles, tubular elements of sarcoplasmic reticulum coupled to the cell membrane, and conspicuous contractile elements. Thick and thin filaments had diameters of about 14–16 nm and 6–7 nm, respectively. The population density of the thick filaments was much higher (mean 270/m² filament area) than in vertebrate smooth muscles. The ratio of thick to thin filaments was about 16. All the thick filaments were surrounded by a single row of 5–9 thin filaments forming a rosette, and cross-bridges with periodicities of 14.5 and 29 nm were found between them. The contractile apparatus consisted of numerous myofibrils which were arranged nearly along the cell axis and were separated from each other by a network of 10-nm filaments. The myofibrils further consisted of many irregularly arranged sarcomerelike structures, each of which was comprised of a small group of thick and thin filaments with attached dense bodies.     

Electron microscopical observations on the brush border of proximal tubule cells of mammalian kidney

Summary The ultrastructure of the plasma membrane and the core of microvilli of proximal tubule cells has been investigated by electron microscopy using sectioned and negatively stained material. By the technique of negative staining, a particulated coat is disclosed on the outside of the plasma membrane of microvilli of brush borders isolated from rat, rabbit and ox. This coat is composed of 30 to 60 Å particles and is 150 to 300 Å thick and appears to be a distinguishing feature for the luminal plasma membrane (brush border) of proximal tubule cells. The plasma membrane of the basal part of tubule cells is found to be smooth. By thin sectioning, an axial bundle of 50 to 70 Å diameter filaments regularly arranged in an 1+6 configuration, one axially located filament being surrounded by a ring of six, is disclosed. The distance from the ring of filaments to the inner surface of the plasma membrane is 250–300 Å, the diameter of the ring 300 Å and the center-to-center distance between filaments 120 Å. Negative staining also discloses 60 Å filaments in microvilli of isolated brush borders. Broken off, single microvilli (fingerstalls) are observed with thin filaments projecting from their broken ends. Filaments up to 1 in length are seen. At high magnification, the filaments appear beaded and show strong resemblance with actin filaments isolated from skeletal muscle. Based on present evidence, it is postulated that microvilli constituting renal brush borders possess contractile properties, which may play a role in the absorption process operating at the luminal part of the cells.The authors are indebted to Miss Kirsten Sjöberg for skilled technical assistance, and to the Danish State Research Foundation and the Tuborg Foundation for financial support.     

Electron microscopy of the tracheal ciliated mucosa in rat

Summary The structure of the tracheal epithelial cells from rat has been studied by electron microscopy on approximately 200 Å thick sections with a resolution of better than 30 Å.The epithelium is found to be of a simple columnar type composed of ciliated cells, mucus producing (goblet) cells, basal cells and a fourth kind of cell, here called brush cell. A great number of non-ciliated cells has also been encountered. It has been proved that these represent goblet cells in different stages of intracellular synthesis of mucous granules. The ciliated cells have approximately 8–9 cilia per square micron and there are about 270 cilia on each cell, the calculated surface area being 33 square microns. They are covered by a 70 Å thick membrane. The ciliary filaments are arranged in a pattern of 2 separate ones in the center and a ring of 9 peripheral ones, each divided into 2 subfilaments by a wall with same thickness as the filamentous wall itself, this being 60 Å. The peripheral filaments are continuous with the basal corpuscles. The structure of the corpuscles as compared with earlier findings is discussed. A number of 0.05 micron thick and 1 micron long filiform projections emerge from the cell surface. No cuticle is present.The cell membrane facing adjacent cells is 90 Å and separated from their cell membrane by a 105 Å wide space, this space, being expanded towards a level corresponding to the proximal parts of the cell. A structure that represents terminal bar has been encountered. The cytoplasm is loose and composed of 160 Å thick granules. Spaces enclosed by 50 Å thick membranes with attached 160 Å thick granules (-cytomembranes) are rare. The Golgi zone is analyzed and its regular composition of -cytomembranes, granules and vacuoles is confirmed. The mitochondria with a mean width of 0.23 micron differ to their inner structure from the common type in that the triple layered membranes are highly interconnected. Large opaque granules are encountered in the cytoplasm. Ring-shaped, 850 Å wide, structures are present in the nuclear membrane. The goblet cells are not as abundant as the ciliated cells, the ratio being 14. Small filiform projections covered by a 95 Å thick membrane protrude from the cell surface. This membrane is continuous with the cell membrane, the latter with the same dimensions as in the ciliated cells. Terminal bars are present. The cytoplasm is very opaque due to a dense packing of the 165 Å opaque granules, many in clusters of 4–6. The -cytomembranes have the same dimensions as mentioned above for those present in the ciliated cells. The Golgi zone is of regular composition. There is a suggestion that the Golgi vacuoles and the -cytomembranes are involved in the formation of mucus. In the stage of cellular activity with but few mucous granules, there is a great number of large opaque granules, the size varying from 0.4–1 micron. The mitochondria with a mean width of 0.23 micron have an outer triple layered membrane with a total thickness of 180 Å. The central less opaque layer is 70 Å and the opaque layer on either side is 55 Å. The inner membranes are arranged parallel to each other and have a triple layered composition where the central less opaque layer is 65 Å and the opaque layers each 60 Å. The brush cells belong to the non-ciliated cells. They are encountered singly, surrounded by goblet cells. The surface structures are shaped like brushes or clumsy protrusions which emerge from the distal end of the cell, and are covered by a 95 Å thick membrane. There have been no suggestions of the brushes being cilia in a stage of growth, nor is it probable that they represent stereocilia. They most nearly resemble the intestinal brush border extensions and thus might serve as a resorbing structure.The cytoplasm of the brush cells appears of medium opacity between the ciliated cells and goblet cells and is composed of 155 Å opaque granules. The -cytomembranes are very rare. The Golgi zone is diminutive though of regular composition. The mitochondria are abundant and small with a mean width of 0.14 micron. The outer and inner membranes are triple layered with approximately the same dimensions as reported for the mitochondria of the ciliated and goblet cells. The inner membranes are very few, often only one or two are present. Some of the large opaque granules have inside a very regular arrangement of small 60 Å thick opaque granules arranged in a crystallinic pattern. In the cytoplasm 0.5–1 micron long bundles of 30–40 Å wide fibrils are encountered. The nucleolus shows a characteristic structure of concentrically arranged thin membranes. The basal cells are believed to represent lymphocytes or white blood cells. They sometimes rest on the basement membrane, sometimes are encountered in the distal part of the intercellular spaces. They are bordered by a 110 Å thick cell membrane and have a rather opaque cytoplasm characterized by 160 Å thick opaque granules. A very small Golgi zone is present. The mitochondria, the mean width being 0.14 micron, have triple layered outer and inner membranes, where the less opaque central layer is 65–70 Å and the opaque layers 45–50 Å each. The basement membrane has a thickness of 600 Å. No inner structure has been resolved. The basement membrane is separated from adjacent parts of the ciliated, goblet, brush, and basal cells by a 250 Å wide less opaque space. Below the basement membrane is the lamina propria of the trachea, which is composed of collagen and elastin fibers together with fibroblasts, white blood cells and lymphocytes. The relationship between different types of tracheal epithelial cells in rat has been analyzed. There has been found no indication of a transformation of any type of cells observed into a different type of cell. The development of basal cells via supporting cells or intermediate cells to goblet cells or ciliated cells has not been noticed. On the contrary, all cells that in light microscopy could have been considered to be supporting or intermediate cells, we have been able to recognize as brush cells or as goblet cells to a varying degree filled with mucous granules. If the cells did not seem to reach the cell surface it has been found to be due to a diagonal direction of the sectioning. In this connection it should be emphasized that this relationship is valid only in rat where it is known that the epithelium is of a simple columnar type as distinct from the conditions in man, that epithelium being of a pseudostratified columnar type.This paper is based on a report given at the meeting of Deutsche Gesellschaft für Elektronenmikroskopie in Münster, March 28–31, 1955 and at the Scandinavian Electron Microscope Society Meeting in Stockholm, May 13, 1955.     

The ultrastructure of the blood capillaries in the mouse thyroid gland

Summary In the mouse thyroid gland the endothelium lining the blood capillaries is separated from the epithelial cells by a periendothelial space composed of three layers. Two of these are rather dense and localized close to the epithelium and endothelium, respectively. The third layer, interposed between the two dense layers, has a very low density. The dense layers, having each a thickness of 400–500 Å, show at high magnification in some places a lamellated structure. The middle layer varies in thickness and contains sometimes aggregates of a homogeneous material, circular bodies bordered by a membrane and, in single cases, fibrillar structures.The major part of the endothelial wall is very thin, only 200–600 Å. Within these thin parts discontinuities are observed, the endothelial cytoplasm being replaced by a more or less distinct membrane, about 50 Å thick. The dimension of the discontinuities is about 400 Å. The observations made are discussed.     

Ultrastruktur glycerinextrahierter Dünndarmmuskelzellen der Ratte vor und nach Kontraktion

Zusammenfassung Die Tunica muscularis des Dünndarms der Ratte wurde elektronenmikroskopisch vor und nach Glycerinextraktion und nach verschieden lang andauernder ATP-Behandlung untersucht. Vor und nach der Extraktion sind nur 50–80 Å breite F-Actin-Filamente in den glatten Muskelzellen nachzuweisen. Die extrahierten glatten Muskelzellen kontrahieren sich nach Zugabe von ATP. Gleichzeitig treten in der Längsrichtung der Zelle verlaufende 150–200 Å dicke Myosinfilamente auf. Während langanhaltender Inkubation mit ATP trennen sich Actin- und Myosinfilamente zunächst voneinander durch eine Art Gleitmechanismus, da die Actinfilamente noch an der Zellmembran verhaftet bleiben, die Myosinfilamente sich aber verschieben. Dann lösen sich die Actinfilamente von der Zellmembran und Actin- und Myosinfilamente bilden ein dichtes Netzwerk im Zentrum der Zelle. In der Umgebung dieses Netzwerkes verbleiben feine Filamente mit einem Durchmesser von 20–30 Å.

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Ultrastructure of glycerinated small intestine muscle cells of the rat before and after contraction

Summary Tunica muscularis of the rat's small intestine was studied electron microscopically before and after glycerol-extraction and at various times after ATP treatment. Before and after extraction only F-actin-filaments with a diameter of 50–80 Å could be found in smooth muscle cells. Dense bodies disappear during extraction. Glycerinated smooth muscle cells contract when ATP is added. At the same time thick filaments with a diameter of 150–200 Å appear, which probably represent myosin filaments, running longitudinally within the cells. During prolonged ATP treatment actin and myosin filaments first separate from each other by a sort of sliding mechanism because actin filaments are still bound to the cell membrane while myosin filaments move. Then actin filaments are drawn off from the cell membrane and actin and myosin filaments assemble in an intricate network of filaments in the central part of the cell. Around this network fine filaments with a diameter of 20–30 Å remain.

Für die technische Mithilfe danke ich Frau Karla Struwe.     

Intracytoplasmic filaments in pulmonary lymphatic endothelial cells

Summary The cytochemistry and ultrastructure of intracytoplasmic filaments of pulmonary lymphatic endothelial cells of neonatal rabbits were studied by comparison with myofilaments of the peribronchial and pulmonary vascular smooth muscle cells. Two types of endothelial filaments were observed: thin filaments (diameter: 50 Å) which lie close to the abluminal cell membrane; and thick filaments (diameter: 90 Å) which are dispersed throughout the cell cytoplasm.Following heavy meromyosin (HMM) treatment, characteristic arrowhead complexes formed in the thin lymphatic endothelial filaments as well as in the actin filaments of the smooth muscle cells. There was no detectable reaction of HMM with the thick filaments.After incubation with EDTA, the thin filaments were labile, and the thick filaments became the major filamentous component in the endothelial cells. In smooth muscle cells, the actin myofilaments were also labile while the 100 Å filaments were stable.These observations support the hypothesis that the actin-like thin endothelial lymphatic filaments form part of a contractile system, while the thick filaments constitute a plastic cell skeleton. The significance of the contractile system in lymphatic endothelial cells might lie in a mechanism for the active regulation of the endothelial intercellular junctions and gaps and hence the permeability of the lymphatic endothelial cell lining.This study was supported by The Council for Tobacco Research—U.S.A. The authors thank Professor Robert C. Rosan, M.D. (Saint-Louis University—U.S.A.) for expert advice. R. Renwart, B. Emanuel and R. Jullet for technical, G. Pison and St. Ons for photographical and N. Tyberghien for secretarial assistance.     

Ultrastructure of the aortic diverticula of the adult dragonfly Sympetrum danae (Odonata: anisoptera)

Summary The aorta of Sympetrum danae possesses two dorsal diverticula: one in the mesothorax and one in the metathorax. They are very similar in form and position. Each diverticulum has a dorsal valve through which blood is pumped from the wings down into the aorta. The wall of the aortic diverticula consists of two simple cell layers: an outer epidermis-like layer and an inner muscle layer. The nuclei of the muscle cells are situated close to the lumen of the diverticula. The mitochondria are evenly dispersed between the myofibrils and are often paired up on either side of the Z-band. The Z-bands are thick and fragmented. The length of the sarcomeres varies from 3.3 to 6.1 . The A-band length is about 3 . The myofibrils consist of thick (250 Å) and thin (85 Å) filaments. Each thick filament is surrounded by 9–12 thin filaments. The sarcoplasmic reticulum is well developed and separates the myofibrils with one or two layers. The T-tubules are flattened and branch irregularly like a two-dimensional tree between the lamellar myofibrils. Intercalated discs are observed.The peculiarities of the muscle of aortic diverticula in S. danae are discussed in relation to various muscles of other insects and arthropods.     

The fine structure of neuromuscular junctions and contact zones between body wall muscle cells of Ascaris lumbricoides (var. suum)

Summary Neuromuscular junctions and close membrane apposition between body wall muscle cells of Ascaris lumbricoides (var. suum) have been examined with the light and electron microscopes. It was found that the body wall muscle cells send out elongate processes from their basal, myofibril containing portion to terminate on dorsal and ventral nerves. When observed with the aid of the electron microscope the neuromuscular junctions were seen to consist of several muscle cell processes in apposition to a single axon. The intersynaptic cleft was approximately 350–500 Å wide. Both the axolemma and sarcolemma were triple layered membranes which were 75–80 Å thick. Electron dense patches were observed at intervals on the apposed membranes which were due to increased thickness of the inner membrane leaflets of axolemma and sarcolemma. Muscle cell membranes, at the level of the neuromuscular junction, were in close apposition resulting in an apparently five-layered membrane complex which was 170–210 Å thick. The sarcolemmata in these regions were separated by 10–50 Å. Presynaptic axons contained mitochondria, microtubules which were 180–270 Å in diameter, and two, morphologically distinct types and sizes of synaptic vesicles. One was 200–600 Å in diameter, with a single, triple-layered membrane bounding a center of low electron density. The other was 600–1200 Å in diameter, with a single, triple-layered membrane bounding a central, electron dense granule of 500–800 Å size.The functional significances of the close membrane appositions between body wall muscle cells and of the two types of synaptic vesicles found at the neuromuscular junctions of Ascaris lumbricoides were discussed with respect to their possible role in neuromuscular physiology.Supported by U.S.P.H.S. Grant No. NB-01528 and Research Career Development Award No. 9-K3-NB-15255. — The author wishes to express his grateful appreciation for the excellent technical assistance given by Miss Gabrielle Rouiller during the course of this investigation.     

FINE STRUCTURE AND SIZE DISTRIBUTION OF FREE THICK FILAMENTS IN EARLY FIBRILLOGENESIS OF ASCIDIAN TADPOLE

The development and the size distribution of free thick filaments which accumulate in the early stages of myofibril formation in somitic myoblasts of the ascidian tadpole were studied by electron microscopy. Such filaments appeared in the cell cortex but, rather dominantly, the aggregates of these thick filaments and filamentous structures were observed in the interior of the cell. The aggregate consisted of some of the following elements: filamentous structures (20–60 A in diameter); free thick filaments (60–220 A); dense Z-band precursor materials; bundles of thick (140–160 A) and thin (60–70 A) filaments; and ribosomal clusters. The free thick filaments were variable in diameter and showed long lateral projections (300–600 A) and tapered ends.
The variation curve in diameter of the free thick filaments indicates a continuous size distribution, suggesting the continuous growth of these filaments by polymerization of myosin molecules. Free thick filaments thicker than myosin filaments which were found within myofibrils were present; their significance is discussed in relation to myosin filament formation.     

Ultrastructure of the myocardial cell and its membrane systems in the adult fly Calliphora erythrocephala (Insecta: Diptera)

Summary Calliphora erythrocephala has cross-striated cardiac muscle cells with A, I and Z-bands. The diameters of the myosin and actin filaments are 200–250 Å and 85 Å respectively and the length of the myosin filaments (A-band) is approximately 1.5 . Usually 8–10 actin filaments surround each myosin filament.The myocardial cells show a well-developed membrane system and interior couplings. A perforated sheet of SR envelopes the myofibrils at the A-band, dilates into flattened cisternae at both A-I band levels before it merges into a three-dimensional net-work between the actin filaments of the I-bands and between the dense bodies of the discontinuous Z-discs. The T-system consists of broad flattened tubules running between the myofibrils at the A-I band levels forming dyads with the SR-cisternae. Longitudinal connections between the transverse (T-) tubules often occur.It is suggested that this well-developed SR may be an adaptation to facilitate a rapid contraction/relaxation frequency by an effective Ca²⁺ uptake.     

Filaments and rod-shaped tubulated bodies in the endothelia of anterior cerebral arteries in young rats

Summary Endothelia of the anterior cerebral arteries in rats aged 1 to 3 days were studied. Thin (about 50–90 Å) and thick (about 100–110 Å) filaments are present in the endothelia. Numerous spherical- or rod-shaped bodies, measuring approximately 0.07 to 0.3 m in diameter and up to 0.6 m in length occur in the endothelial cells. These bodies contain a tubular structure. The diameter of the individual tubules is about 200 Å. The present observations suggest that spherical- or rod-shaped inclusions are first synthesized in the rough endoplasmic reticulum and thereafter these materials are transported into the Golgi complex for maturation. A small number of the inclusions, however, may originate directly from the rough endoplasmic reticulum and not pass through the Golgi apparatus.A part of this study was demonstrated at the 70th Versammlung der Anatomischen Gesellschaft in Düsseldorf, April, 1–5, 1975The author thanks Mr. Tatsuro Fukushima for preparation of photographs     

Protein filaments-structural components of the phloem exudate

Summary Fine structure and chemical composition of the phloem exudate of Cucurbita maxima and Nicotaiana glauca x suaveolens are investigated. Filamentous structures, several microns in length, are identified as structural components of the exudate by means of negative staining and electron microscopy. Two types of filaments are described: one form measures nearly 40 Å in diameter and shows a beaded appearance with regular spacings of about 50 Å; it is termed elementary filament. The second form has a diameter of about 90 Å and presumably consists of two helically arranged 40 Å subunits.The proteinaceous nature of the filaments is indicated by chemical analysis. The main macromolecular component of the exudate is demonstrated to be protein. Only traces of nucleic acids are detectable, lipids and polysaccharides cannot be found. The identity of the protein filaments with the filamentous structures (slime, P-protein), as revealed in thin sections of mature sieve tubes, is discussed.     

Fine structure of smooth muscle and neuromuscular junctions in the foot of Helix aspersa

Summary The smooth muscle cells in the foot of Helix aspersa are arranged in bundles which interweave to form a complex mesh. In the peripheral cytoplasm of the muscle cells there is a system of interconnected obliquely and longitudinally orientated tubules. The full extent of this system has not been determined; its possible function in relation to Ca⁺⁺ storage and excitation-contraction coupling is discussed. Longitudinal tubules are present among the myofilaments and in association with mitochondria. Distributed throughout the myofilaments are elliptically shaped dense bodies, the fine structure of which resembles an accumulation of thin filaments. Located on the plasma membrane of the muscle cells are dense areas; the fine structure and relationships of these cellular elements resemble desmosomes. They may serve as attachment points for thin, cytoplasmic filaments (not necessarily myofilaments). The muscle cells are innervated by axons which diverge from a coarse, neural plexus (the sole plexus). The axons initially come into close contact with the muscle cells and then pass over their surfaces for up to 35 before being gradually enveloped by flange-like protrusions of the muscle cells. These axons contain either, (i) agranular vesicles (600 Å in diameter), (ii) agranular and very dense granular vesicles (1000 Å in diameter) or (iii) agranular and less dense, granular vesicles (1000 Å in diameter). The possible role of these inclusions as sites of excitatory and inhibitory transmitters is discussed.I wish to thank Professor G. Burnstock for making laboratory facilities available. This work has been supported by the Australian Research Grants Committee.     

The ultrastructure of organelles appearing in spermatids and nutritive cells of Cipangopaludina malleata

Summary The ultrastructure of organelles appearing in the early typical and atypical spermatids, and the nutritive cells of Cipangopaludina malleata has been examined by a Siemens' electron microscope Elmiskop I.Mitochondria appearing in the early typical spermatid have doughnut-like profiles in which the internal ridges appear as triple-layered membranes arranged radially and extending into the interior of the organelle without reaching the other side. Each membrane 40–60 Å in width, separated by a clear interspace 60–90 Å wide, is characterized by a porous structure 20–30 Å in diameter which suggests a filtration apparatus for enzymes.Walls of the flattened saccules consisting the Golgi apparatus are calculated 35–60 Å thick, in which an electron-lucent, porous structure about 30 Å wide has been revealed.The smooth-surfaced endoplasmic reticulum is bordered by a triple-layered membrane consisting of two opaque layers with a less opaque interspace 20–30 Å wide. The outer membrane ca. 15 Å wide presents a more linear appearance than the dotted arrangement of the inner membrane 20–25 Å thick.The plasma membrane is composed of a triple-layered structure where two dense lines 15 Å wide are separated by a layer 20–30 Å thick of less density.The electron micrographs for the present studies were taken with the Siemens electron microscope, model Elmiskop I, at the Anatomical Institute of Kiel University, Germany. The one of the authors, G. Yasuzumi is deeply grateful to Prof. Dr. W. Bargmann and Dr. A. Knoop for the privilege of using this instrument and other equipments in the Laboratory.     

Myosin flares and actin leptomeres as myofibril assembly/disassembly intermediates in sonic muscle fibers

The sonic muscle of type 1 male midshipman fish produces loud and enduring mating calls. Each sonic muscle fiber contains a tubular contractile apparatus with radially arranged myofibrillar plates encased in a desmin-rich cytoskeleton that is anchored to broad Z bands (~1.2 μm wide). Immunomicroscopy has revealed patches of myosin-rich “flares” emanating from the contractile tubes into the peripheral sarcoplasm along the length of the fibers. These flares contain swirls of thick filaments devoid of associated thin filaments. In other regions of the sarcoplasm at the inner surface of the sarcolemma and near Z bands, abundant ladder-like leptomeres occur with rungs every 160 nm. Leptomeres consist of dense arrays of filaments (~4 nm) with a structure that resembles myofibrillar Z band structure. We propose that flares and leptomeres are distinct filamentous arrays representing site-specific processing of myofibrillar components during the assembly and disassembly of the sarcomere. Recent reports that myosin assembles into filamentous aggregates before incorporating into the A band in the skeletal muscles of vertebrates and Caenorhabditis elegans suggest that sonic fibers utilize a similar pathway. Thus, sonic muscle fibers, with their tubular design and abundant sarcoplasmic space, may provide an attractive muscle model to identify myofibrillar intermediates by structural and molecular techniques. This work was supported by the Intramural Research Program of the NIAMS, NIH, HHS (KW).     

Polarity of myofilaments in molluscan smooth muscle

Summary Myofilaments were isolated by gently homogenizing smooth muscle cells isolated from the pedal retractor muscle (PRM) of Mytilus edulis, and observed by electron microscopy. The thick filaments isolated in the presence of ATP (10–20 mM) had projections of myosin heads except near their centre (central bare zone). After extraction of myosin, the paramyosin core of the thick filaments showed a Bear-Selby net or a striated pattern with a main periodicity of 14.5 nm. Both the Bear-Selby net and the striated patterns had a polarity that reversed at the centre of the filament where the patterns were obscured. The thin filaments were attached to dense bodies. Decoration of the thin filaments with heavy meromyosin showed that they have opposite polarity on opposing sides of the dense body. The results indicate that the thick filaments are bipolar and also that the dense bodies are functionally analogous to the Z-disk of the striated muscle.     

The cytoskeleton of Cobaea seed hairs:

The cell wall of Cobaea scandens seed hairs developed in a characteristic sequence, with the deposition of a cellulose thread onto a pectic swelling layer was the final event. The cellulose thread was intracellularly accompanied by a band of 10–18 microtubules. During the formation of the swelling layer the microtubules were homogeneously distributed; they ran circumferentially normal to the cell axis. When cellulose-thread formation started, the microtubules became arranged in a helical band. The density of the microtubules varied during the different phases of development. The highest density was observed before cellulosethread formation and ranged from 6–15 m·m^-2. The length of the microtubules, 20–30 m, was determined by direct measurements, as well as estimated from the total microtubular length in a given area and the counted free ends. With the indirect immunofluorescence technique the microtubules of the band stained inhomogeneously. Those which were located at the edges of the band fluoresced more intensely than those of the central part. Attempts to visualize actin filaments in the hair cells with rhodaminyl-conjugated phalloidin resulted in a homogeneous staining of the area of the microtubular band, indicating that actin filaments may be present in this region. Though, in thin sections and dry-cleaved cells, filamentous structures were observed between the microtubules, caution is expressed that the observed fluorescence was, indeed, due to actin filaments. The role of the filamentous structures is discussed with respect to formation and maintenance of the microtubular band. Microtubules apparently did not cross coated pits which were visualized in the plasma membrane through the dry-cleaving technique.Abbreviations IFT indirect immunofluorescence technique - RP rhodaminyl-conjugated phalloidin - SEM scanning electron microscopy     

The ultrastructural organization of the basement membrane of Bowman's capsule in the rat renal corpuscle

Summary The basement membrane of Bowman's capsule (BCBM) of the rat was studied by means of a modified tissue-preservation technique for transmission electron microscopy, which avoids the usual thorough fixation in OsO₄ and applies tannic acid and uranyl acetate for staining (Sakai et al. 1986). At most sites the BCBM is multilayered, consisting of one to seven dense layers separated by electron-lucent layers. The latter, which can be termed laminae rarae, contain fine filaments which connect the dense layers to each other and the innermost dense layer to the basal cell membrane of the parietal epithelium. The laminae densae are basically composed of fine filaments arranged in an anastomosing pattern. Individual filaments ranging from 5 to 15 nm in diameter, combine to form filament bundles up to 100 nm in thickness and 1 to 2 m in length. Within a dense layer, filaments and filamentous bundles are oriented mainly in the same direction. Often the inner dense layers do not form a continuous sheet, and the filamentous bundles are arranged in anastomosing or spiral patterns to form a ribbon-like structure that we call a microligament. These microligaments are often embedded in basal furrows of the parietal epithelium and are best developed around the vascular pole. Intracellular actin bundles of the parietal cells are regularly associated with these extracellular ribbon-like structures of the basement membrane. In conclusion, the BCBM has an unusual structure: the laminae densae are characterized by their filamentous nature and are arranged in different patterns, i.e. as a multilayered mat and as microligaments.Fellow of the Deutscher Akademischer Austauschdienst     

Elektronenmikroskopische Untersuchungen an Spinnenmuskeln

Zusammenfassung Die Pasern aus den Beinmuskeln der Vogelspinne Dugesiella hentzi sind zwischen 100 und 250 m dick und durch tiefe Einfaltungen des Sarcolemms in Untereinheiten gegliedert. Die meist bandförmigen Myofibrillen liegen darin in radiärer Anordnung. A-Bandbreite und Sarcomerenlänge variieren sehr stark (Extremwerte 2,8 und 5,6 bzw. 3,0 und 7,3 m). Ausrichtung und Anordnung der Myofilamente sind wenig exakt. Auf ein Primärfilament (Durchmesser 230–235 Å) kommen durchschnittlich 4–4,5 Sekundärfilamente (70–80 Å).Das sarcoplasmatische Reticulum (SR) ist extensiv und in Form eines unregelmäßigen Netzes aus schlauchartigen Elementen ausgebildet. Im Bereich des A-Bandes erweitern sich einzelne Schläuche zu Cisternen, die mit den Tubuli des Transversalsystems Dyaden bilden. Die SR-Membran zeigt dabei im Dyadenbereich charakteristische Strukturen: punktförmige Membranverdickungen, die ein Muster von großer Regelmäßigkeit bilden. Lage und Zahl der Dyaden sind sehr variabel (Durchschnitt 3–4 pro Sarcomer).

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An electron microscopical study of spider muscles

Summary Four different leg muscles of the tarantula Dugesiella hentzi were investigated electron microscopically. The fibers measure 100 to 250 m in diameter. They are divided into subunits by deep invaginations of the sarcolemma. The myofibrils have the shape of irregular ribbons which are arranged radially within the fiber subunits. The length of the A band as well as the sarcomer length varies from 2.8 to 5.6 and 3.0 to 7.3 n respectively. The myofilaments do not form very regular patterns. The ratio thick filaments (diameter 230 to 235 Å) to thin filaments (70 to 80 Å) is approximately 1 to 4 or 4.5. The sarcoplasmic reticulum (SE) is extensively developed. It consists of an irregular network of tubular elements surrounding the myofibrils and frequently crossing the Z discs. In the A band region some of the SR tubules widen. These cisternae form dyads with the tubules of the transversal system. In the dyads the membrane of the cisternae shows a characteristic structure: i.e. an exact pattern of small, point-like membrane thickenings. The position and the number of the dyads vary widely. Usually there are 3 to 4 in each sarcomer.

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Mit dankenswerter Unterstützung durch die Deutsche Forschungsgemeinschaft. Fräulein Jördis Behrsing danken wir für ausgezeichnete technische Mithilfe.     

Structural aspects of amitosis: a light and electron microscope study of the isolated macronuclei of Paramecium aurelia and Tetrahymena pyriformis

Thin sections show the macronuclei of Paramecium aurelia and Tetrahymena pyriformis to contain two types of bodies. The smaller, measuring 0.1–0.2 in diameter, have been resolved in the light microscope by first removing the macronuclei from the cells in the presence of Mg⁺⁺, then chelating that divalent cation with EDTA, resulting in expansion of the nuclear material. By staining with methyl green, Azure B, and the Feulgen procedure, the small bodies were shown to contain DNA. In whole mounts these small bodies appear to be joined to one another producing a complex network suspended in which are the larger bodies, or nucleoli. — Macronuclei from both ciliates were isolated in large quantities and purified for spreading on an air-water interface. When the nuclei burst from surface tension forces and are examined with the electron microscope, the DNA containing bodies remain attached to one another by means of 100 Å fibrils. The pattern of attachment is non-linear. Occasionally individual DNA-containing bodies loosen revealing a coil resembling both in shape and dimensions the 250 Å coil characteristic of eucaryotic chromatin. The substructure of the 250 Å coil has not been directly observed. However, the frequent association of pairs of 100 Å fibrils makes it likely that two such fibrils are tightly complexed in the 250 Å coil. The 100 Å fibril, in turn, contains two 20 Å strands, each presumably a DNA double helix. — In Paramecium each small body of the macronucleus contains approximately one chromosome-equivalent of DNA. The fact that these small bodies are joined to form large structured masses of chromatin within the macronucleus indicates that the distribution of genetic material is not random. It is possible that, similar to bacteria, entire genomes segregate as units, thus accounting for successful amitotic division.This work was supported by a Predoctoral Fellowship to the author from the National Institutes of Health, U.S. Public Health Service, and by grant GM-13882 (NIH-USPHS) to Dr. Daniel Mazia.