Dense Strands Research Articles

Ultrastructural study of nucleolar organizers in the quail oocyte during meiotic prophase I

The localization of nucleolar organizers in the quail oocyte was studied during meiotic prophase I. During midpachytene, the oocyte nucleus contains chromocenters formed by association of the heterochromatic centromeric region of the microchromosomes. During late pachytene, the nucleolus always develops in strict relation to a chromosomal structure. The nucleolus may either be directly applied against the surface of a chromocenter or in contact with a specific region of a microchromosome emerging from a chromocenter. This microchromosome presents a juxtanucleolar knob of heterochromatic appearance. Two chromocenters, with their corresponding microchromosomes, may participate in nucleolar genesis, in which case the nucleolar constituents are arranged in a symmetrical manner with respect to the chromosomal structures. Penetration of DNP fibers of nucleolar organizer always takes place in a pale fibrillar zone (fibrillar center) contained within the nucleolus and surrounded by electron dense fibrillar strands. Ultrastructural data suggest that the nucleolar organizer may be situated in the pale fibrillar zone of the nucleolus. Nevertheless, no decisive argument exists excluding either the juxtanucleolar knob, which appears to be formed of facultative heterochromatin, or the DNP fibers of the nucleolonema.

Ultrastructural aspects of chronic oral candidosis

Human biopsy specimens from white patches on the mucosae of tongue and lips which had previously given positive cultures for Candida albicans were examined. The specimens were processed for electron microscopy by conventional techniques. The fine structure of the superficial layer of the epithelium revealed Candida albicans in extra- and intracellular sites. Penetration into the deeper layers was not observed. The fungus had a thick cell wall surrounded by an outer floccular layer. The cytoplasm of the superficial epithelial cells showed degenerative changes and fragmentation of tonofilaments in the immediate vicinity of the fungus. Microabscesses, formed by pools of neutrophils, were common in the upper spinous layer. In places, fungi and bacteria appear to have been phagocytosed by neutrophils and macrophages. The cells of the lower spinous and basal layers showed detachment of desmosomes, marked increase in intracellular glycogen, prominent lysosome-like dense bodies and abundant mitochondria of differing shapes. At the epithelia-connective tissue junction, fine filaments and collagen fibrils were intermingled throughout the basal lamina, forming multiple, highly electron-dense layers. A noticeable change in the connective tissue was the accumulation of numerous mast cells containing differently shaped granules filled with scrolls and dense strands of granular material. The terminal blood vessel exhibited leakage and numerous, concentric duplications of the basal lamina.

An ultrastructural and radioautographic study of the evolution of the interphase nucleus in plant meristematic cells (Allium porrum).

ABSTRACT Radioautography under both light and electron microscopy was exploited to investigate the structural changes of the chromatin reticulum which characterizes the interphase nucleus of a number of plants. Allium porrum meristematic plant cells were used for this purpose. In this species, the telophase chromosomes uncoil into dense strands which, during the G\ period, gradually give rise to a coarse reticulum. There then follows an extensive unravelling of portions of these strands, and high-resolution radioautography reveals that labelling with tritiated thymidine predominantly occurs over zones of the nucleus consisting of diffuse fine fibrillar material. As the S-period progresses, a chromatin reticulum reappears throughout the nuclear cavity, the tortuous strands being approximately 0-25 /μm in diameter. Most of the radioautographic grains still remain over the light nucleoplasmic areas but a number of these are now located on the outermost portion of the dense chromatin profiles. By the end of the S-period, the chromatin strands are slightly thicker (ca. 0-3 μm) and form a looser reticulum. Labelling has decreased noticeably in nuclei of that period, the radioautographic grains being grouped into clusters resting over more or less spherical regions of the chromatin reticulum. Judging from their localization at the surface of the nucleolus or close to the nuclear envelope, these structures correspond to chromocentres. The additional interesting finding that such nuclear structures appear much less compactly organized strongly suggests that chromocentres undergo important conformational modifications during duplication of their DNA.

Electron microscopic study of the central nervous system in mice infected by Omsk hemorrhagic fever (OHF) virus. Virus reproduction in cerebellum neurons

Cerebellar cortex of white mice intracerebrally inoculated with Omsk hemorrhagic fever (OHF) virus suspension was studied electron microscopically. Four or 5 days after inoculation, the mice developed marked encephalitic symptoms, and in the brain high titers of virus were detected. The selection and identification of the infected cells for electron microscopic study has been carried out by means of a light microscope examination of thick reembedded Araldite sections stained with toluidine blue. Infected neurons were found to have a change in the Nissl's substance. The OHF virus particles were discovered in the Golgi complex, in the cisternae of the endoplasmic reticulum of neurons. The dense thickened strands of smooth membranes continuing into rough membranes of endoplasmic reticulum emerged in altered areas of the Nissl's substance. Accumulation of specific microvesicles and aggregates of the OHF virus particles were located in the cisternae delimited by dense strands of smooth membranes and rough membranes of endoplasmic reticulum. It is suggested that specific microvesicles in which virus particles appeared bud off from altered membranes of endoplasmic reticulum of virus-infected neurons.

THE FINE STRUCTURE OF BLASTOCLADIELLA EMERSONII ZOOSPORES

The zoospore of Blastocladiella emersonii has been re‐examined with the electron microscope. The following new findings were made. A double unit‐membrane system surrounds all cell organelles except γ‐bodies, vacuoles and a few fragments of membranes. Lipid granules on one side of the large mitochondrion alternate with vesicles. The kinetosome of the posterior flagellum does not have any central fibrils as previously reported; a small, cylindrical structure is found within its anterior end. An associated centriole is located next to the kinetosome. Three striated rootlets pass from the kinetosome by separate channels through the mitochondrion. There appears to be no connection between the striated rootlets and the mitochondrion. Microtubules originating at the anterior end of the kinetosome pass into the cytoplasm between the mitochondrion and the nuclear cap. Long, dense strands were observed in some nuclei. The axoneme is taken up into the spore during encystment and is found in the freshly encysted spore. No trace of the flagellar sheath has been found in the encysted spore.

The Fine Structure of Blastocladiella emersonii Zoospores

The zoospore of Blastocladiella emersonii has been re-examined with the electron microscope. The following new findings were made. A double unit-membrane system surrounds all cell organelles except γ-bodies, vacuoles and a few fragments of membranes. Lipid granules on one side of the large mitochondrion alternate with vesicles. The kinetosome of the posterior flagellum does not have any central fibrils as previously reported; a small, cylindrical structure is found within its anterior end. An associated centriole is located next to the kinetosome. Three striated rootlets pass from the kinetosome by separate channels through the mitochondrion. There appears to be no connection between the striated rootlets and the mitochondrion. Microtubules originating at the anterior end of the kinetosome pass into the cytoplasm between the mitochondrion and the nuclear cap. Long, dense strands were observed in some nuclei. The axoneme is taken up into the spore during encystment and is found in the freshly encysted spore. No trace of the flagellar sheath has been found in the encysted spore.

Structure and function of the parapodial cirri of the polynoid polychaete, Harmotho�

Structural and physiological studies indicate a sensory and secretory function for parapodial cirri. A cirrus is composed of a neuronal core containing synapses, enveloped by a stroma, an epithelium and a cuticle bearing ciliated papillae enclosing projections of subcuticular somata from which fibers pass into the core. The cuticle contains microvilli and canals associated distally with 470 A granules appearing at all seasons in both sexes. One type of stromal cell produces PAS+ granules, and the nuclei of several types contain dense strands 250 A in diameter. Upon electrical stimulation of the parapodial nerve, a PAS+ secretion appears on the ventral cirrus. The presence of synapses in the core reopens the problem of peripheral sensory confluence in polychaetes.

THE STRUCTURE AND ORGANIZATION OF, AND THE RELATIONSHIP BETWEEN THE ORGANIC MATRIX AND THE INORGANIC CRYSTALS OF EMBRYONIC BOVINE ENAMEL.

Electron microscope and electron diffraction studies of developing embryonic bovine enamel have revealed the organization of the organic matrix and the inorganic crystals. The most recently deposited inorganic crystals located at the ameloblast-enamel junction are thin plates, approximately 1300 A long, 400 A wide, and 19 A thick. During maturation of the enamel, crystal growth occurs primarily by an increase in crystal thickness. Statistical analyses failed to show a significant change in either the width or the length of the crystals during the period of maturation studied. Even in the earliest stages of calcification, the crystals are organized within the prisms so that their long axes (c-axes) are oriented parallel to the long axes of the prisms but randomly distributed about their long axes. With maturation of the enamel, the crystals become more densely packed and more highly oriented within the prisms. The organic matrix in decalcified sections of enamel is strikingly similar in its over-all organization to that of the fully mineralized tissue. When viewed in longitudinal prism profiles, the intraprismatic organic matrix is composed of relatively thin dense lines, approximately 48 A wide, which are relatively parallel to each other and have their fiber axes parallel to the long axes of the prisms within which they are located. Many of these dense lines, which have the appearance of thin filaments, are organized into doublets, the individual 48 A wide filaments of the doublets being separated by approximately 120 A. When observed in oblique prism profiles, the intraprismatic organic matrix is likewise remarkably similar in general orientation and organization to that of the fully mineralized tissue. Moreover, the spaces between adjacent doublets or between single filaments have the appearance of compartments. These compartments, more clearly visualized in cross- or near cross-sectional prism profiles, are oval or near oval in shape. Therefore, the appearance of the intraprismatic organic matrix (in longitudinal, oblique, and cross-sectional prism profiles) indicates that it is organized into tubular sheaths which are oriented with their long axes parallel to the long axes of the prisms in which they are located, but randomly oriented about their own long axes, an orientation again remarkably "blue printing" that of the inorganic crystals. The predominant feature of the walls of the tubular sheaths, when viewed in cross- or near cross-section, is that of continuous sheets, although in many cases closely packed dot-like structures of approximately 48 A were also observed, suggesting that the wall of the sheaths consists of a series of closely packed filaments. The 48 A wide dense lines (filaments) representing the width of the sheath wall were resolved into two dense strands when viewed in longitudinal prism profiles. Each strand was 12 A wide and was separated by a less electron-dense space 17 A wide. The intraprismatic organic matrix is surrounded by a prism sheath which corresponds in mineralized sections to the electron-lucent uncalcified regions separating adjacent prisms. Structurally, the prism sheaths appear to consist of filaments arranged in basket-weave fashion.

The effect of estrogen on the histology of the uterine epithelium of the mouse: II. Changes of pas-reactive structures in the basement membrane and the glandular cell surface

The basement membrane in the uterine epithelium of spayed mice is thick and folded and shows an intense PAS reaction. After estrogen treatment and at estrus it follows a straight course, becomes thinner and is only weakly stained. In early pregnancy the folding and strong PAS reaction reappear. Electron microscopy shows that the dimensions of the basement membrane are unchanged. The variations in staining intensity are therefore due to different degrees of folding in a membrane with constant thickness. This folding depends chiefly on the state of the epithelial cells. A layer of intensely PAS-reactive material appears on the surface after estrogen treatment and at estrus. In the electron micrographs increase in length of the microvilli and deposition of dense strands of an amorphous substance between them are observed. This substance is assumed to represent secretion from the epithelium.

Fine Structure of the Macronucleus of the Gymnostome Ciliate, <i>Chlamydodon pedarius</i>

The fine structure of the macronucleus of the gymnostome ciliate Chlamydodon pedarius at young stage and interphase approaching the division has been investigated by means of the electron microscopy.The macronucleus comprises two different structures: the anterior half is less dense structure which consists of the diffuse filamentous matrix material, and contains one endosome composed of much dense filamentous material in its center; on the contrary the posterior half is a very dense structure, and consists of a reticulum made of very dense strands or cords, very dense nucleolar materials and little less dense matrix material. However, the fine structure changes in the stages of the young macronucleus and of the interphase approaching the division.The nucleoli are distributed beneath the envelope with a short distance. The structure is composed of filamentous material, dense particles of various size, and less dense matrix in the young macronucleus, but in the interphase macronucleus approaching the division it shows very loose and vanishing structure of aggregate of granular texture. They do not have any retaining membrane always.The longitudinal stripes of about 100mμ to 200mμ in width are found in the macronucleus approaching the division. They run parallel to the long axis of the macronucleus, seeming to contribute the elongation of the macronucleus, and are inferred to be preliminary form of the chromonemata in the very early prophase macronucleus.The highly electron-scattering threads of the order of 25mμ to 30mμ in thickness and the electron-dense helices of about 15mμ to 20mμ thick are found in the corresponding places to the DNA containing elements of the macronucleus, and the filament making up them seems to correspond to a nucleoprotein complex. It is possible, consequently, to draw the following conclusions: The individual thread appears as a result of duplication of the filament; the strands in the young macronucleus are fragment of the chromonemata; the cords constituting the network of the posterior half represent the chromonemata in the interphase or very early prophase macronucleus.The structure of an envelope enclosing the macronucleus is composed of double membranes of about 50A thick respectively, separated by a space of about 200A wide, and numerous penetrating pores of about 400A diameter in normal area.