Macromolecular and ultrastructural organization of the mitotic chromosome scaffold

Using electron microscopy (EM), we have examined three structural domains of the mitotic chromosome scaffold of mouse erythroleukemia (MEL) Friend cells with different morphologic organization: centromeric, intermediate, and telomeric. The intermediate, most extensive, domain exhibited a specific fibrogranular structure representing tightly packed granular bodies with diameters between 20 and 60 nm. The chromosome scaffold contained three main components: proteins (81%), RNA (12%), and DNA (7%). The residual DNA extracted from the scaffold represented short fragments, 300 bp on average, belonging to the class of tandemly arranged repetitive DNA. In situ hybridization experiments confirmed its typical centromeric location. Scaffold RNA represented three fractions: a major RNA fraction with an electrophoretic mobility corresponding to that of 5S RNA and two minor fractions with electrophoretic mobilities somewhat lower than that of 18S RNA. Scaffold RNA was localized mainly in the centromeric region. We show that the newly synthesized protein component of the chromosome scaffolds migrates slowly to the chromosomes, reaching a maximum specific radioactivity 12 h from the onset of the chase period.     

Studies on the mitotic chromosome scaffold of Allium sativum

An argentophilic structure is present in the metaphase chromosomes of garlic(Allium sativum),Cytochemical studies indicate that the main component of the structure is non-histone proteins(NHPs).The results of light and electron microscopic observations reveal that the chromosme NHP scaffold is a network which is composed of fibres and granules and distributed throughout the chromosomes.In the NHP network,there are many condensed regions that are connected by redlatively looser regions.The distribution of the condensed regions varies in individual chromosomes.In some of the chromosomes the condensed regions are lognitudinally situsted in the central part of a chromatid while in others these regions appear as coillike transverse bands.At early metaphase.scaffolds of the sister chromatids of a chromosome are linked to each other in the centromeric region,meanwhile,they are connected by scafold materials along the whole length of the chromosome.At late metaphase,however,the connective scaffold materials between the two sister chromatids disappear gradually and the chromatids begin to separate from one another at their ends.but the chromatids are linked together in the centromeric region until anaphase.This connection seems to be related to the special structure of the NHP scaffold formed in the centromeric region.The morphological features and dynamic changes of the chromosome scaffold are discussed.     

The 3-dimensional fine structure of the mitotic spindle in Trypanosoma cruzi

The fine structure of nuclear division in the hemoflagellate Trypanosoma cruzi has been studied with serial sections and three-dimensional reconstructions of each divisional stage. After a preliminary stage in which the chromatin becomes dispersed, there is an equatorial stage defined by the appearance of an arranged set of ten dense plaques located about the equatorial region of the nucleus. At this stage a regular microtubular spindle is formed in the nucleus. Each plaque has a symmetrical structure formed by transverse bands and the bands are formed by tightly packed fibrillar material. The wide faces of the plaques are associated with tangential microtubules coming from the poles while the front and rear edges are free to associate with chromatin. Although structural continuity between chromatin fibers and the material of the plaques is possible, this continuity has not been proved. The equatorial spindle is formed by about 120 microtubules arranged in two sets of about 60 microtubules running from each pole to the dense plaques and divided into discrete bundles which reach a single plaque. The microtubules of each bundle may pass tangential to the wide faces of the plaque and end about 0.2 m beyond it, or they may end at the pole-facing edges of the plaque. No continuous, interpolar microtubules were observed at this stage. At the beginning of the elongational stage the dense plaques split into halves and each set of half-plaques migrates to one pole. During mid-elongational stage the pole-converging microtubules and the polar bulges disappear and microtubules become rearranged between the two sets of half-plaques. During late elongational stages, continuous microtubules run between the two sets of half-plaques and maximum nuclear elongation is attained. Chromatin remains dispersed throughout nuclear division. Two main movements have been observed in these mitotic nuclei: the migration of half-plaques to the poles and the elongation of the nucleus. Both these movements are accompanied by large changes in the architecture of the microtubular spindle and are probably dependent on microtubular function. It is concluded that the dense plaques play a kinetochore-like role and thus T. cruzi would have ten chromosomal units.     

Novel components of human mitotic chromosomes identified by proteomic analysis of the chromosome scaffold fraction

Chromosomal nonhistone proteins have important roles in mitotic chromosome formation and dynamics. In order to identify novel abundant proteins with a potential involvement in these processes, we initiated a proteomic screen of the chromosome scaffold fraction. This screen identified 79 proteins, 30 of which had not previously been described as components of mitotic chromosomes. Furthermore, half of these proteins had no documented function. We analyzed the cell-cycle dependent distribution of three uncharacterized proteins by expressing them as green fluorescent protein (GFP) fusions and showed that they associate with mitotic chromosomes in vivo. One of the proteins, nuclear protein p30, is a novel component of the inner centromere. Over-expression experiments indicated that p30 may have an active role in the formation of centromeric heterochromatin.     

The fine structure of Pseudopedinella pyriforme Carter (Chrysophyceae)

Pseudopedinella pyriforme Carter displays a second non-emergent flagellum and vestigial anterior tentacles. The trailing rhizopodium is without supporting microtubules, and seemingly arises from a posterior vesicle apparently associated with the Golgi apparatus. Stalked pyrenoids, so far unreported for any other member of the Chrysophyceae, are demonstrated. The structure of P. pyriforme is compared with members of the Chrysophyceae and Craspedamonadales and its taxonomic position is discussed.     

Fine structure of chromosome pairing in ten Ascomycetes: meiotic and premeiotic (mitotic) synaptonemal complexes

The meiotic prophase of ten Ascomycetes was studied by both light and electron microscopy. All ten species show a Neurospora type of meiosis. Two Podospora have no distinct synaptonemal complexes but synaptic structures in their nucleolus. The synaptonemal complexes of the eight other species are well differenciated and have uniform dimensions. The banded pattern of the lateral components seems to be a characteristic of Ascomycetes. The most striking observation is the specificity of this banding which can be used as a marker in interspecific crosses. Ascophanus carneus shows two kinds of synaptonemal complexes: normally between the pachytene bivalents and unusually in the ascogenous hyphae mitoses before meiosis and even before caryogamy.     

The fine structure of the spermatozoon of Pennaria tiarella (coelenterata)

Spermatozoa of the hydroid Pennaria tiarella were examined with the electron microscope. The anterior region is characterized by the presence of 30–40 membrane-bounded vesicles which lie anterior to the nucleus. These vesicles are apparently derived from the Golgi apparatus. The nucleus is conical in shape with a protrusion at the anterior end. Posteriorly it is indented by four radially arranged mitochondria. Lying within the fossa formed by the mitochondria are proximal and distal (filament forming) centrioles. The distal centriole is characterized by nine centriole satellite projections which emanate from its matrix. The tubules of the distal centriole are continuous with the alpha filaments of the tail. The tails are typical 9 + 2 flagella with 9 peripheral doublet (or alpha) filaments surrounding two central (or beta) filaments.     

The fine structure of Ameson pulvis (Microspora,Microsporida) and its implications regarding classification and chromosome cycle

Nosema pulvisPerez, 1905, Ameson pulvis (Perez) Sprague, 1977, in muscles of the crabs Carcinus maenas and C. mediterraneus from the coast of France, was observed with the electron microscope. It was found to be structurally similar to the type species A. michaelis (Sprague, 1970). Sprague, 1977, having moniliform sporogonial plasmodia, unikaryotic sporoblasts, and hirsute sporulation stages. It is treated as distinct from A. michaelis because it has slightly smaller spores (by comparison with syntype material of A. michaelis) and appears to have fewer coils in the polar filament. The results require the removal of the genus Ameson from the family Nosematidae Labbé, 1899, where Sprague (1977) had placed it under the erroneous supposition that its sporoblasts are diplokaryotic. Ameson is transferred to family Unikaryonidae Sprague, 1977. Ameson is distinguished from PereziaLéger and Duboscq, 1909, shown by Ormieres et al. to have a similar developmental pattern, by presence of appendages on its sporulation stage. A. nelsoni (Sprague, 1950), the third, and only other species of Ameson, lacks the appendages and is transferred to genus Perezia.     

The fine structure of the pollen wall inStrelitzia reginae (Musaceae)

The pollen wall ofStrelitzia reginae (Musaceae) consists of a nearly unsculptured, very thin, highly reduced, but coherent exine, and a thick intine (with an outer, channeled layer and an inner, largely homogeneous layer). After short, incomplete acetolysis the exine covers the remaining, severely shrinked protoplast as a folded, but unaltered “skin”, while the intine has totally disappeared. After extended acetolysis only the coherent, skin-like exine remains. Thus, the term “exine-less pollen” sometimes used for similar sporoderm structures in other genera ofZingiberales is misleading and should be substituted by the term “skin-like exine”. Surprisingly, the peculiar pollen wall ultrastructure ofStrelitzia and some otherZingiberales is very similar to that of some genera of theLaurales, an example for convergent evolution within the angiosperms.     

The fine structure of the colleterial glands of Hyalophora cecropia (Lepidoptera)

The structure of cells in the colleterial glands of the Cecropia silkmoth was examined. Morphologically and functionally the gland is divided into two regions, a tubular one in which columnar protein-synthesizing cells are located, and an expanded region in which flattened cells with very different structure are most prominent. The fine structure of the latter cells which are presumed to secrete a phenolic glucoside, closely resembles that of cells described in the colleterial glands of orthopterans. The protein-secreting cells have many features normally associated with pancreatic acinar, and other cells of similar function. Among these are extensive rough endoplasmic reticulum, an elaborate Golgi complex, and a modest number of mitochondria. Other features which are less usual in cells of this type are an elaborate secretory apparatus consisting of a cuticular tubule inserted into a microvilli-lined cavity at the apical end of the cell, and large numbers of cytolysomes, myelin figures, and lipid droplets. A chitogenous cell with a very distinct and specific type of ultrastructure is found associated with the secretory cell. This cell type is attached to the cuticular elements of the gland, and the main features of its cytoplasm are extensive bundles of microtubules which presumably serve as supportive elements for the secretory cells.     

The fine structure of the trout blastoderm, Salvelinus fontinalis (Mitchill)

The Speckled Trout blastoderm at the late high blastula stage is characterized by two different cell populations. The “light” blastomeres comprise one cell type while the “dark” and “medium” blastomeres appear to differ from one another only in degree and thus may be considered as the second type. “Dark” and “medium” blastomeres are irregular in shape, are located centrally and deep in the blastoderm, have an abundance of smooth endoplasmic reticulum with associated 520 Å glycogen particles and a single mitochondrial profile. The “light” blastomeres have randomly arranged glycogen particles in minimal quantities in contrast to the “medium” and “dark” blastomeres and in addition exhibit three mitochondrial profiles, which could however represent artifacts. It is postulated that in the Speckled Trout cellular differentiation has commenced by the third day of incubation at 10°C and that this is manifested visually by the appearance of two different cell populations; the more differentiated “dark” and “medium” blastomeres possibly destined to give rise to the hypoblast and the less differentiated “light” blastomeres.     

Silver staining the chromosome scaffold

Cytological silver-staining procedures reveal the presence of a core running along the chromatid axes of isolated HeLa mitotic chromosomes. In this communication we examine the relationship between this core and the nonhistone chromosome scaffolding, isolated and characterized in previous publications from this laboratory. When chromosomes on coverslips were subjected to the steps used for scaffold isolation in vitro and subsequently stained with silver, the characteristic core staining was unaffected. Control experiments suggested that the core does not contain large amounts of DNA. When scaffolds were isolated in vitro, centrifuged onto electron microscope grids, and stained with silver, they were found to stain selectively under conditions where specific core staining was observed in intact chromosomes. These results suggest that the nonhistone scaffolding is the principal target of the silver stain in chromosomes.     

The fine structure of the stomach mucosa of the llama (Llama guanacoe)

Summary The epithelium of the fundic region mucosa of the hind stomach in the Llama guanacoe has been studied using morphological and histochemical methods. Morphology suggests that solute and water absorption may occur in the epithelium of the surface and of the foveolae, although this absorption can not be estimated because of the extensive secretion of the gastric glands. The same cells of the surface and foveolar epithelium show numerous secretory granules. The glands reveal neck cells, chief cells, a large number of oxyntic cells, four types of endocrine cells (A-like, ECL, D and EC), brush cells and wandering cells. PAS and Alcian blue reactions for light microscopy suggest a secretion of neutral and acidic mucosubstances in the surface and foveolar epithelium, of neutral mucosubstances only in the neck cells. Periodic acid-thiocarbohydrazide silver proteinate (PA-TCH-SP) reaction for electron microscopy confirms the presence of neutral mucosubstances within the secretory granules of the surface, foveolar and neck epithelial cells. In all these cells, the reaction product is also evident within sacculi and vesicles of the maturing surface of the Golgi apparatus. A positive PA-TCH-SP reaction also occurs on the membrane (and not on the contents) of the Golgi apparatus (maturing surface) and of the secretory granules of the chief cells as well as on the membrane of the Golgi apparatus and of apical vesicles and tubules of the oxyntic cells. In addition, silver granules slightly enhance the electron density of the contents of the secretory granules in the endocrine cells. Morphological and histochemical findings are discussed and compared with results described by others for monogastric mammals.     

The fine structure of epidermal papillae of Travisia forbesii (Annelida)

The structure of the epidermis of Travisia forbesii was described using light and electron microscopy. The epidermis is a highly modified variant of the normal one-layer polychaete epithelium. It consists of basal epidermal cells and an external layer of closely sited papillae consisting of glandular and supportive epidermal cells, and extensive electron-transparent intercellular spaces. The papillae are embedded in the thick cuticle. Each papilla has a peduncle, which is formed by one cell that penetrates the inner cuticle layer to the basal epidermal cells. A fold of basement membrane forms the core of the peduncle and ends in the base of a papilla. All epidermal cells are connected to each other with apical cell junctions and to the basement membrane with hemidesmosomes, so the epithelium is continuous and uninterrupted. The epidermis has an intra-epidermal neuron plexus. The structure of the papillae is compared with papillae and tubercles of other polychaetes, and the possible functional significance and phylogenetic implications of these structures are discussed.