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Showing papers in "Journal of Cell Science in 1968"


Journal ArticleDOI
S. P. Sorokin1
TL;DR: Reconstruction of the processes of centriolar formation and ciliogenesis based on evidence found in electron micrographs of tissues and organ cultures obtained chiefly from the lungs of foetal rats leads to an interpretation of the centriole as a semi-autonomous organelle whose replicative capacity is separable from the characteristic triplet fibre structure of its wall.
Abstract: This study presents reconstructions of the processes of centriolar formation and ciliogenesis based on evidence found in electron micrographs of tissues and organ cultures obtained chiefly from the lungs of foetal rats. A few observations on living cultures supplement the major findings. In this material, centrioles are generated by two pathways. Those centrioles that are destined to participate in forming the achromatic figure, or to sprout transitory, rudimentary (primary) cilia, arise directly off the walls of pre-existing centrioles. In pulmonary cells of all types this direct pathway operates during interphase. The daughter centrioles are first recognizable as annular structures (procentrioles) which lengthen into cylinders through acropetal deposition of osmiophilic material in the procentriolar walls. Triplet fibres develop in these walls from singlet and doublet fibres that first appear near the procentriolar bases and thereafter extend apically. When little more than half grown, the daughter centrioles are released into the cytoplasm, where they complete their maturation. A parent centriole usually produces one daughter at a time. Exceptionally, up to 8 have been observed to develop simultaneously about 1 parent centriole. Primary cilia arise from directly produced centrioles in differentiating pulmonary cells of all types throughout the foetal period. In the bronchial epithelium they appear before the time when the ciliated border is generated. Fairly late in foetal life, centrioles destined to become kinetosomes in ciliated cells of the epithelium become assembled from masses of fibrogranular material located in the apical cytoplasm. Formation of these centrioles may be under the remote influence of the diplosomal centrioles. More certainly, the precursor material accumulates in close proximity to Golgi elements. Within the fibrogranular areas, osmiophilic granules (400-800A) increase in size and eventually become consolidated into dense spheroidal bodies (deuterosomes), which organize the growth of procentrioles around them. When mature, the newly formed centrioles become aligned in rows beneath the apical plasma membrane. There each centriole produces satellites from its sides, a root from its base, and a cilium from its apex. Early stages in the formation of both primary cilia and those of the ciliated border are similar. In developing cilia of the ciliated border, however, the outer ciliary fibres rapidly reach the tips of the elongating shafts, and a central pair of fibres is formed (9 + 2 arrangement). In primary cilia, development of the fibres seems to lag behind the elongation of the shafts, and only the outer ciliary fibres appear (9 + 0 arrangement). The strengths and weaknesses of the proposed reconstructions of centriolar formation and ciliogenesis are discussed, and the occurrence in other living forms of similar pathways for centriolar formation is noted. Further discussion leads to an interpretation of the centriole as a semi-autonomous organelle whose replicative capacity is separable from the characteristic triplet fibre structure of its wall.

743 citations


Journal ArticleDOI
L. G. Tilney1
TL;DR: It is concluded that the mircotubules are instrumental in the maintenance of the axopodia as well as in supplying the force necessary for their regrowth.
Abstract: When specimens of Actinosphaerium are placed in a solution of colchicine, the axopodia retract and that portion of the birefringent core (axoneme) present in each axopodium disappears. In fixed specimens, it has been shown that the axoneme consists of a highly patterned bundle of microtubules each 220 A in diameter. During colchicine treatment the microtubules, present in the axopodial portion of the axoneme, break down and do not reform until the cells are washed free of the colchicine and allowed to recover. In the basal portion of the axoneme or that portion confined to the cell body, most of the microtubules within the remaining axonemes is altered; in longitudinal section these tubules no longer appear straight but present an undulatory profile. When the specimens are washed free of the colchicine and allowed to recover, axopodia reform and within each numerous microtubules are present. From these observations we conclude that the mircotubules are instrumental in the maintenance of the axopodia as well as in supplying the force necessary for their regrowth. Observations are also presented on the disintegration products of microtubules, on microtubules and cytoplasmic motility, and on the redevelopment of pattern. The latter is discussed at some length, for it appears that the inter-axonemal organization is somewhat modified by the colchicine treatment.

177 citations


Journal ArticleDOI
TL;DR: Fertilization involves fusion of the gamete membranes through the mediation of a specialized structure (the fertilization tubule) and in this respect there are similarities to certain aspects of fertilization in animal phyla.
Abstract: Gametes of C. reinhardi lack the cell wall which vegetative cells possess. Just below the cell apex gametes form a fertilization tubule which is up to 2 µ long and 0.2 µ in diameter; its plasma membrane and that of the apex have slender tubular projections. At the base of the fertilization tubule regularly lies the choanoid body , a collar-shaped cytoplasmic organelle; the plasma membrane overlying the body appears as an electron-dense ring. Gametes possess two ‘free’ basal bodies in addition to the basal bodies of the two flagella. In the initial stage of union the conjugating cells are connected by the fertilization tubule whose plasma membrane is continuous with that of both copulants. At one end of the tubule lies a conspicuous choanoid body, but at the other end is a small structure which possibly is a homologue of the choanoid body. Subsequently, the fertilization tubule shortens and widens until finally no tubule exists and the apical ends of the two protoplasts adjoin. The merging cells then bend like a jack-knife and lateral alignment of the protoplasts occurs. This four-flagellated zygote becomes motile at about the time when the flagellar bases of the former gametes seem to approach each other and when fibrillar elements of the flagellar roots come into contact. In the motile zygote the nuclei do not fuse but remain ensheathed in the cup-shaped plastids of the two gametes. A mating of plus (+) and minus (-) strains cultured, respectively, for high and low starch content suggested that gametes of only the plus (+) mating type contain the choanoid body. Since it appears that the gamete containing the choanoid body also produces the fertilization tubule, it is inferred that gametes of only the plus (+) mating type produce the fertilization tubule. Should further investigation support this inference, it would be established that there is a structural basis for designating the plus (+) mating type as male and the minus (-) type as female. Fertilization involves fusion of the gamete membranes through the mediation of a specialized structure (the fertilization tubule) and in this respect there are similarities to certain aspects of fertilization in animal phyla. The relation of the fertilization tubule to the protoplasmic bridge of other species of Chlamydomonas is discussed.

151 citations


Journal Article
TL;DR: The final stages of Helleborus ontogeny, which culminate in maturation and germination of the grain, have been investigated at the ultrastructural level as discussed by the authors.
Abstract: The final stages of Helleborus pollen-grain ontogeny, which culminate in maturation and germination of the grain, have been investigated at the ultrastructural level. Following the deposition of primary and secondary exine, and during the early stages of intine formation, the microspore passes through a vacuolate phase, in which the cytoplasm appears devoid of most organelles other than the prominent nucleus. The formation of the vacuole results in the displacement of the nucleus to one side of the pollen grain. The vacuole quickly disappears and a number of organelles reappear in the cytoplasm, in particular the dictyosomes and strands of endoplasmic reticulum, with associated grey bodies. Following mitotic division of the pollen grain, the first signs of the generative cell wall appear as a pair of tightly appressed unit membranes in the narrow strand of cytoplasm separating the two newly formed generative and vegetative nuclei. As development proceeds, the space between the two membranes gradually fills with an electron-transparent material similar to the substance found in the numerous dictyosome-derived vesicles which, together with the endoplasmic reticulum, are both closely associated with the developing cell wall. The generative cell wall fuses with the cellulosic intine, which has gradually increased in amount during these stages, and the cell division is complete. The smaller generative cell contains a prominent nucleus and a small amount of cytoplasm devoid of plastids and most other organelles. The larger vegetative cell also contains a prominent nucleus and a large amount of cytoplasm containing amyloplasts, mitochondria, dictyosomes and endoplasmic reticulum, and abundant ribosomes, many of which are in a polysome configuration. The final stages in development are characterized by a progressive decrease in the amount of starch in the vegetative cell and an increase in the size of grey bodies, many of which are invested in multilayered shrouds of endoplasmic reticulum. The generative cell wall disappears and a multivesicular/granular body gradually appears at the periphery of the pollen grain. The granular-vesicular material, which is formed from the dictyosomes and/or the degenerating plastids, is thought to represent metabolic reserves necessary for pollen-tube formation. One or more pollen tubes emerge from the apertural sectors of the pollen grain, and maturation of the grain is complete.

128 citations


Journal ArticleDOI
TL;DR: Physiological cell death and degeneration in the interdigital mesenchyme of the hind foot of the rat foetus have been studied and no evidence was found suggesting that cell death might be initiated by the intracellular release of lysosomal enzymes.
Abstract: Physiological cell death and degeneration in the interdigital mesenchyme of the hind foot of the rat foetus have been studied using classical staining methods and staining methods for enzyme localization. Individual mesenchymal cells die and shrink as the result of some unknown mechanism. Their acid phosphatase and esterase activities are not significantly different from those of viable loose mesenchymal cells. The dead cells are engulfed by viable neighbouring cells which resemble other loose mesenchymal cells in their morphology and in their acid phosphatase and esterase activities. These phagocytes then differentiate and become typical macrophages. Expressions of this process are the altered appearance of their nuclei and the increase in cytoplasm and in acid phosphatase and esterase activities. Many dead cells may be engulfed by a single macrophage and are then digested by its acid hydrolases. No evidence was found suggesting that cell death might be initiated by the intracellular release of lysosomal enzymes.

126 citations


Journal ArticleDOI
TL;DR: In orchd species forming microspores in aggregates, the pollen mitotic division occurs synchronously in all cells of each massula, as do the earlier meiotic divisions, causing synchroneity in these species.
Abstract: In orchd species forming microspores in aggregates, the pollen mitotic division occurs synchronously in all cells of each massula, as do the earlier meiotic divisions. The synchroneity can be traced to the persistence of cytoplasmic connexions between the cells, from the meiotic prophiase until pollen maturation. The mitosis giving the generative nucleus is asymmetrical, and the spindle is truncated at one side, where the microtubules converge towards an amorphuos polar structure lying against the spore wall. The cell plate formed after pollen mitosis is hemispherical, and its curved growth is related to a radial spread of the microtubules of the phragmoplast after the telophase of the division. The plate itself, and the wall derived from it, is identifiable as callose by its fluorescence properties. In the later development of the gametophyte, growth of the callose wall continues until the originally hemispherical generative cell becomes separated from the spore wall. The cell then assumes a spherical shape and moves to the vicinity of the vegetative nucleus, where it remains freely suspended, bathed in the cytoplasm of the vegetative cell but insulated from it by the completely ensheathing callose wall.

126 citations


Journal ArticleDOI
TL;DR: The ontogeny of the tapetum and Ubisch bodies in Helleborus foetidus L has been examined at the ultrastructural level, and their development has been closely linked with that of the sporogenous cell and pollen grains as mentioned in this paper.
Abstract: The ontogeny of the tapetum and Ubisch bodies in Helleborus foetidus L has been examined at the ultrastructural level, and their development has been closely linked with that of the sporogenous cell and pollen grains During development the tapetum passes through successive phases of synthesis, maturity and senescence, ending in complete dissolution During the anabolic phase of growth, precursors of the Ubisch bodies are formed as spheroidal vesicles of medium electron density within the tapetal cytoplasm; they are associated with a zone of radiating ribosomes, which, as development proceeds, can clearly be seen to be situated on strands of endoplasmic reticulum The callose special wall round the microspores and the tapetal cell wall now disintegrate and the pro-Ubisch bodies are extruded through the cell membrance of the tapetal cells, where they remain on the surface of the anther cavity and soon become irregularly coated with sporopollenin Deposition of sporopollenin continues on the Ubisch bodies at the same time as upon the exines of the developing pollen grains In both cases, the later stages of sporopollenin deposition are associated with electron-transparent layers of unit-membrane dimensions appearing in section as white lines of uniform thickness Continuing deposition of sporopollenin leads to the formation of compound or aggregate Ubisch bodies It is conjectured that the sporopollenin is synthesized from the compounds of low molecular weight released into the anther loculus by the breakdown of the callose special wall and the tapetal cell wall The final stages of tapetal autolysis involve the disappearance of all the cell organelles An attempt is made to relate the findings to those described in other recent studies on Ubisch body formation and to combine them in a common ontogenetic pattern

122 citations


Journal ArticleDOI
TL;DR: The final stages of Helleborus pollen-grain ontogeny, which culminate in maturation and germination of the grain, have been investigated at the ultrastructural level, characterized by a progressive decrease in the amount of starch in the vegetative cell and an increase in the size of grey bodies.
Abstract: During the early stages of microsporocyte ontogeny in Helleborus foetidus L. there is protoplasmic continuity between the cells of the tapetum and between the individual sporogenous cells, but not between the two tissues. The plasma canals and plasmodesmata are progressively sealed off by the deposition of thick callose walls, so that by the first meiotic division, each pollen mother cell is isolated from its neighbours and from the surrounding tapetum. Callose is formed by dictyosomes in the individual pollen mother cells. The four meiocytes are separated by the deposition and coalescence of masses of callose forming in the cell plate area. The exine pattern is initiated at the surface of the young microspores while they are still invested with a thick wall of callose. Periclinally arranged endoplasmic reticulum lying just below the microspore cell membrane corresponds with the position of the furrows. The cell membrane in the interfurrow region thickens and becomes highly convoluted. A fibrous layer appears between the outer part of the convolutions and the callose, and locally it becomes less electron-dense at places that become filled with material of moderate electron density corresponding to the probacula; these in turn will become the bacula of the mature exine. In spite of an extensive examination of material prepared by a variety of techniques, no organelle or cytoplasmic component may be consistently associated with the positioning of the first signs of exine patterning.

102 citations


Journal ArticleDOI
TL;DR: It is suggested on the basis of their morphology that the epidermal cells of the vegetative Fucus thallus may be specialized for the absorption of inorganic carbon and sulphate from the outside of the plant and for the secretion of alginic acid, fucoidin and polyphenols.
Abstract: The fine structure of the epidermal cells of the vegetative Fucus thallus has been examined in material fixed with acrolein. These cells are highly polarized, with basal nuclei and chloroplasts, a hypertrophied perinuclear Golgi system, and a much convoluted wall/plasma membrane interface. Much of the intracellular volume is occupied by single membrane-bounded vesicles containing alginic acid, fucoidin and polyphenols. The chloroplasts were examined by light and electron microscopy and shown to contain structured inclusions not previously described in Fucus plastids. It is suggested on the basis of their morphology that the epidermal cells may be specialized for the absorption of inorganic carbon and sulphate from the outside of the plant and for the secretion of alginic acid, fucoidin and polyphenols. The possible role of these cells in the prevention of desiccation and in osmoregulation is discussed.

101 citations


Journal ArticleDOI
TL;DR: The maximum tension developed by the young muscles was found to be attained at an initial muscle length about 10 % greater than their length at maximum limb extension, while the adult muscles developed maximum tension at their length with the limb in the fully extended position.
Abstract: The length of the sarcomeres, the A- and the 1-filaments and their percentage overlap were measured in the fibres of the biceps brachii muscle from mice of different ages. The sarcomere length with the limb in the fully extended position was found to increase from 2.3 µ in the newborn animal to 2.8 µ in the adult. This increase was due to a decrease in the percentage overlap of the filaments and not to any change in the filament lengths. The sarcomeres at the ends of the fibres were found to be shorter than those in the middle of the muscle, at all ages. When the muscles were stretched beyond their resting length, only about the middle 60 % of the sarcomeres in the young muscles increased in length. Length/tension plots were obtained for young and old muscles and the difference in the shape of these plots could be explained as being due to the non-functional terminal sarcomeres of the young muscles. The maximum tension developed by the young muscles was found to be attained at an initial muscle length about 10 % greater than their length at maximum limb extension. The adult muscles developed maximum tension at their length at maximum limb extension.

96 citations


Journal ArticleDOI
TL;DR: By the freeze-etch technique it is possible to investigate crystal-containing bodies, amyloplasts with starch grains and bundles of fibres in addition to mitochondria and Golgi bodies, and distinguishes certain spherical organelles by their characteristic surface appearance.
Abstract: The structure of the inner and outer surfaces of the plasmalemma, the tonoplast and the membranes of the nucleus and endoplasmic reticulum have been investigated. The structure of the plasmalemma probably varies with the metabolic state of the cell, in particular with the synthesis and transport of material for cell-wall formation. The organization of the plasmalemma during the deposition of material in the wall by reverse pinocytosis is shown, and evidence is presented for the possible synthesis of the microfibrillar structure of the wall by synthetic units arranged as particles on and near the plasmalemma surface. A clear indication of substructure along the length of microtubules has been shown, and since views of large surface areas are possible the distribution of the microtubules at the cytoplasmic surface inside the plasmalemma has been revealed; they bound the cell by running around the circumference in one direction only. A definite organization of nuclear pores has been observed and the structure and shape of the pores is described. By the freeze-etch technique it is possible to investigate crystal-containing bodies, amyloplasts with starch grains and bundles of fibres in addition to mitochondria and Golgi bodies. The method also distinguishes certain spherical organelles by their characteristic surface appearance.

Journal ArticleDOI
TL;DR: Electron micrographs of sections through the heterochromatin of erythrocytes from chicken and lamprey reveal alternate equispaced electron-dense and relatively less dense bands lying adjacent and parallel to, and extending considerable distances along, the nuclear envelope, indicating that the heterchromatin contains a well-defined structural unit which can form ordered regions, namely layers in contact with thenuclear envelope.
Abstract: Electron micrographs of sections through the heterochromatin (condensed chromosomes) of erythrocytes from chicken and lamprey reveal alternate equispaced electron-dense and relatively less dense bands lying adjacent and parallel to, and extending considerable distances along, the nuclear envelope. Frequently a triple-structured band (average half-width 183 A), consisting of a less dense band, a dense band (width 130-170 A), and a second less dense band is encountered; sometimes there are many (10-12) bands. The dense bands have a variable structure; they may appear continuous or consist of equispaced granules about 220 A apart, which sometimes have a less dense centre. Similar but shorter dark bands and granules appear throughout the chromatin. Tilting the section in the microscope causes the micrographs to change, bands and granules disappearing and reappearing elsewhere, demonstrating that the images are produced by superposition of structures small in dimensions compared with the thickness of the section. Tilting about an axis normal to the band may resolve it into granules. These data indicate that the heterochromatin contains a well-defined structural unit which can form ordered regions, namely layers in contact with the nuclear envelope. The possibility of errors in interpreting the substructure of the bands due to complexity of superposition effects is stressed. The simplest hypothesis to account for the images is that the units are microtubes of outside diameter 130-170 A, perhaps microhelices formed by coiling DNA with protein, spaced apart equally, perhaps by spacing elements. Microtubes parallel to the optic axis would appear as hollow granules, several one above another in a plane parallel to the axis appearing as a dark band. A similar triple-structured band of average half-width 224 A is found in cells from newt spleen, lymphocytes, polychromatophil erythroblasts, mature erythrocytes, basophilic granulocytes, reticular cells and macrophages. A survey of published micrographs shows a similar triple-structured band of average half-width 212 A to be a common feature of many cell types. A triple-structured band probably gives rise to the sheets of chromatin, now shown to have a similar triple-layered structure, limited on both sides by nuclear envelope, previously found attached to interphase nuclei and mitotic chromosomes in certain polychromatic erythroblasts from the newt. The effects of tilting the section were studied on objects of known geometry, membranes and cytoplasmic microtubules.

Journal ArticleDOI
TL;DR: Evidence is presented which indicates that the crests and fibrous annulus both contract during feeding and act antagonistically to displace the apparently rigid rods, which seems to function as a fairly rigid structure which, acting in conjunction with the suction force, guides the filament into the organism and manipulates it to some extent.
Abstract: The fine structure of the cytopharyngeal basket is described in detail. A circular palisade of cytopharyngeal rods is encircled at certain levels by a sheath and two annuli; crest-shaped structures project outwards from the basket. The rods, crests and sheath are largely composed of cytoplasmic microtubules of approximate diameter 240 A. One of the annuli is mostly composed of densely staining inter-tubular material; the other consists mainly of fine fibres ranging in diameter from 40 to 90 A. Ingestion of algal filaments by Nassula and the sequence of movements of the rods which occurs during feeding are described. Evidence is presented which indicates that the crests and fibrous annulus both contract during feeding and act antagonistically to displace the apparently rigid rods. The initial displacement of the rods is spatially related to the orientation of the algal filament which is to be ingested. It is suggested that certain cilia respond to contact with the filament by transmitting some form of excitation to some of the crests along small bundles of microtubules which interconnect them. The suction force drawing the filament into the organism apparently acts only in the lumen of the basket. The basket seems to function as a fairly rigid structure which, acting in conjunction with the suction force, guides the filament into the organism and manipulates it to some extent. The basket also apparently grips the partly ingested filament to prevent it from slipping out of the organism during pauses in the action of the suction force.

Journal ArticleDOI
TL;DR: The ovaries of newly metamorphosed Xenopus females contain oocytes in all stages of early meiotic prophase and it is proposed that primary replicas of the chromosomal nucleolar organizer undergo a series of post-detachment replications.
Abstract: SUMMARY The ovaries of newly metamorphosed Xenopus females contain oocytes in all stages of early meiotic prophase. In pachytene nuclei extrachromosomal nucleolar DNA appears in the form of a thin cap covering one side of the nucleus. During pachytene this cap of DNA enlarges to occupy half the nucleus. After pachytene the nucleus grows rapidly and the cap of DNA disperses into numerous tiny granules which become scattered throughout the nucleoplasm. Autoradiographs of cells which have been incubated with PHJthymidine, and microspectrophotometric measurements of the Feulgen dye contents of nuclei in various stages of meiosis, show that the extrachromosomal nucleolar DNA is synthesized during the pachytene and immediate post-pachytene stages. Microspectrophotometric comparison of the Feulgen dye contents of post-pachytene nuclei in which the DNA cap has dispersed, and similarly prepared mouse liver nuclei, show that the post-pachytene nuclei have about 30 fifig of extrachromosomal pachytene nucleolar DNA. In PHJthymidine autoradiographs early pachytene nuclei are less heavily labelled than late pachytene and early diplotene nuclei. Consequently it is proposed that primary replicas of the chromosomal nucleolar organizer undergo a series of post-detachment replications.

Journal ArticleDOI
TL;DR: It is shown that on contraction of the body both systems shorten, thicken and remain straight, but that on initial relaxation they behave differently: the km fibres begin to lengthen later than the M fibres, the latter being thrown into sinuous folds.
Abstract: Observations of the microtubular ‘km’ system and filamentous ‘M’ systems in living specimens of Stentor coeruleus show that on contraction of the body both systems shorten, thicken and remain straight, but that on initial relaxation they behave differently: the km fibres begin to lengthen later than the M fibres, the latter being thrown into sinuous folds. Electron microscopy of specimens cooled before fixation appears to confirm this difference in behaviour in relaxing specimens. The M fibres of the stalk region are discrete and prominent bundles, but in the adoral half of the body they are extensively linked together by side branches. There are diffuse filamentous attachments between the M fibres and the kinetosomes. The vertical microtubular stacks which make up the km fibres are each linked with a pair of kinetosomes. Each stack contains about 21 microtubules grouped in a 2 + 19 pattern. Cross-bridges are present between the microtubules of adjacent stacks. The number of stacks in each km fibre cross-section is smaller in extended specimens than in contracted ones, indicating that the stacks slide upon one another as the body changes its length.

Journal ArticleDOI
TL;DR: It is proposed that the ATPase localized on the stacks of lateral plasma membrane may be involved with ion secretion into the intercellular spaces to create the osmotic gradient necessary to extract water from the lumen.
Abstract: Adenosine triphosphatase (ATPase) activity in the rectal papillae of Calliphora has been studied by biochemical and histochemical techniques The microsomal fraction contained a Mg 2+ -activated ATPase with a pH optimum of 80 The enzyme was not stimulated by the addition of Na + plus K + and was insensitive to ouabain Histochemical studies using modifications of the Wachstein-Meisel method showed that at pH 72 this Mg 2+ -activated ATPase was specifically localized on the intracellular surface of the lateral plasma membranes A similar though less intense reaction was obtained with adenosine diphosphate and inosine triphosphate, but not with guanosine triphosphate, uridine triphosphate or β-glycerophosphate as substrates At an acid pH (66-68), very little reaction occurred on the lateral plasma membrane but some reaction product was present in mitochondria and nuclei Very little enzyme activity was found in the flattened rectal epithelium These results are discussed in relation to the available data on transport ATPases and on the structural basis of fluid transport by rectal papillae It is proposed that the ATPase localized on the stacks of lateral plasma membrane may be involved with ion secretion into the intercellular spaces to create the osmotic gradient necessary to extract water from the lumen

Journal ArticleDOI
TL;DR: Staining with fast green before and after treatment with Van Slyke reagent indicates a change from lysine- rich to arginine-rich histone in the maturing spermatid.
Abstract: The testis of Nucella consists of numerous tubules, all directed inwards and joining to form a common testicular duct. In a single tubule the spermatogonia lie round the periphery. Mature sperm line the lumen of the tubule. Cells in the same stage of spermatogenesis are grouped together and all members of a group pass through spermatogenesis in phase. Staining with fast green before and after treatment with Van Slyke reagent indicates a change from lysine-rich to arginine-rich histone in the maturing spermatid. Sperm of Nucella are motile throughout their length. The sperm are thread-like and about 80 µ long. The head is Feulgen-positive and about 40 µ long. The mid-piece lies behind the head and is about 8 µ long. The flagellum runs from the front end of the head to the tip of the tail; in the head it is completely surrounded by the nucleus. The spermatogonia contain two centrioles situated near the nucleus and a conspicuous Golgi complex. There are synaptinemal complexes in spermatocyte nuclei in the synapsis stage. In the early spermatid the centriole pushes a tube through the nucleus. This tube is lined by nuclear membrane and is occupied by the anterior portion of the flagellar shaft. The nucleus elongates and the nucleoprotein condenses into strands arranged helically along the long axis of the nucleus. These strands fuse to form lamellae, which disappear in the mature sperm. Mitochondria aggregate at the base of the early spermatid nucleus and form a loose spiral around the flagellar shaft. The outer mitochondrial membranes fuse. The mid-piece of the mature sperm consists of a large tubular mitochondrion enclosing a portion of the flagellar shaft. At the early spermatid stage a pro-acrosomal granule is formed from a large Golgi complex. From this the acrosome develops; it consists of a cone and an acrosome granule. There are two sets of microtubules associated with the acrosome, one lying within the cone, the other outside the cone and separated from it by a ‘ragged membrane’. The microtubules of the outer set extend backwards along the head for two-thirds of its length. The centriole which gives rise to the flagellar shaft lies at the anterior end of the head and is separated from the acrosome by a thin layer of nucleoprotein and a double layer of nuclear envelope. There is no second centriole or derivative thereof in the mature sperm. In the tail groups of coiled fibres are associated with each pair of the peripheral flagellar fibrils.

Journal ArticleDOI
TL;DR: Standard periodic acid/Schiff techniques have not shown the existence of aldehyde groups in sections of glutaraldehyde-fixed, Araldite-embedded root-tip tissue; peroxidation of such sections resulted in a typical PAS staining pattern.
Abstract: Standard periodic acid/Schiff (PAS) techniques have not shown the existence of aldehyde groups in sections of glutaraldehyde-fixed, Araldite-embedded root-tip tissue; peroxidation of such sections resulted in a typical PAS staining pattern. Permanganate-fixed root tips also gave a weak PAS reaction which was intensified by prior peroxidation of the sections. At the ultrastructural level, silver hexamine was used to detect aldehyde groups produced in polysaccharide by permanganate and/or periodate oxidation. Golgi vesicles and slime material in root-cap cells always reacted strongly; the cell wall proper was less reactive. A marked increase in the stainability of the vesicles was evident, the further removed they were from Golgi bodies. This also occurred in root epidermal cells. In both these types of cells, smallersized vesicles and/or the contents of reticulate Golgi cisternae showed evidence of histochemical staining. In meristematic root tip cells, vesicles closely apposed to Golgi bodies did not stain convincingly, though cell walls stained readily. During cell-plate formation, however, both smaller (possibly Golgi) and larger vesicles (phragmoplasts) stained strongly. The walls of permanganate-fixed sieve-tube cells also stained quite strongly, but callose did not unless the tissue block had been treated with periodate before being embedded. In glutaraldehyde-fixed xylem cells, older wall thickenings reacted very strongly even when the sections had been blocked with iodoacetate and bisulphite (which rendered the rest of the section unreactive). If similar sections of younger xylem cells were peroxidized after such blocking reactions, the primary cell wall and the wall thickenings stained, as did many of the Golgi vesicles. The results are related to other experimental observations, both ultrastructural and histochemical, on plant cells.

Journal ArticleDOI
TL;DR: Its staining reactions suggest absence of nucleic acid, and that it is proteinaceous, and the hypothesis that the stromacentre fibrils consist of linearly aggregated Fraction I protein molecules is discussed.
Abstract: The ‘stromacentre’ is a fibrillar spherulite found in plastids of aldehyde osmium-tetroxide fixed leaves of species in the genus Avena . Fibrils, each up to 0.2 µ x 80-90 A, are associated in bundles, sometimes in hexagonal close packing, and the bundles in turn are aggregated in the spherulite. Individual bundles, or structures resembling them, occur in the plastid stroma in some other plants. In Avena , the stromacentre develops along with the internal membrane system of the plastids. Its staining reactions suggest absence of nucleic acid, and that it is proteinaceous. It is probably present in all mature Avena plastids. Stromacentre fibrils have been negatively stained. They consist of linearly aggregated particles. In side view these measure about 85-90 A square, though the outline of the particles varies according to the orientation of the fibril. Particle outlines and staining patterns within particles are illustrated in photographically reinforced images. Micrographs interpreted as illustrating disaggregation into free particles are presented. These free particles are indistinguishable from numerous others in the preparations, and these in turn are thought to be Fraction I protein molecules. A process somewhat similar to stromacentre formation occurs in etioplasts and chloroplasts in Phaseolus leaves that have been dehydrated by plasmolysis, by wilting, or by high-speed centrifugation. These aggregates are not quite the same as the Avena stromacentre, but negative staining shows that they too are composed of units that are about the same size as Fraction I protein molecules. The hypothesis that the stromacentre fibrils consist of linearly aggregated Fraction I protein molecules is discussed.

Journal ArticleDOI
TL;DR: Chemical and radioautographic studies on sycamore seedling stems have shown an involvement of the Golgi body in cell-wall polysaccharide synthesis from tritiated glucose.
Abstract: Chemical and radioautographic studies on sycamore seedling stems have shown an involvement of the Golgi body in cell-wall polysaccharide synthesis from tritiated glucose. Tritiated phenylalanine is shown to be incorporated only into lignin after short incubation times. The patterns of labelling are compared and discussed for the two precursors.

Journal ArticleDOI
TL;DR: Stalked pyrenoids, which are found in some dinoflagellates, are shown to arise from the inner face of the chloroplasts, to contain a finely granular material and to be frequently surrounded by an electron-transparent area.
Abstract: The chloroplasts of some members of the Dinophyceae are bounded by an envelope consisting of three membranes and having a mean thickness of 230 A°. Within the chloroplast are arranged, in a more or less parallel manner, many lamellae normally composed of three apposed thylakoids, although the number of thylakoids often varies and may reach 30 in a single stack. By study of disintegrated chloroplasts it was found that the thylakoids are circular in shape with a diameter of 0.15-3.6 µ and a mean thickness of 240 A°;. Ribosomes, lipid droplets and DNA areas are present in the chloroplast stroma. No connexions were seen between the chloroplasts and any other organelles, nor did the chloroplasts contain girdle lamellae. Stalked pyrenoids, which are found in some dinoflagellates, are shown to arise from the inner face of the chloroplasts, to contain a finely granular material and to be frequently surrounded by an electron-transparent area. These findings are discussed in relation to the fine structure of the chloroplasts and pyrenoids of other algal classes.

Journal ArticleDOI
TL;DR: One interpretation of these impedance changes is that glutaraldehyde perfusion causes, like asphyxiation, a transport of extracellular material into the intracellular compartment and that during OsO4 perfusion anextracellular space is again created.
Abstract: The conductivity of cerebral cortex drops during perfusion with glutaraldehyde in 5 min to about 60% of the original value, to remain unchanged during the subsequent 10-15 min of perfusion. Circulatory arrest causes a similar drop in the tissue conductivity. Perfusion of asphyxiated tissue with glutaraldehyde does not produce additional major changes in the conductivity. Perfusion of the cortex with an osmium tetroxide solution causes an initial drop in conductivity. However, after about 3 min this trend is reversed and the conductivity increases again to close to the pre-perfusion value. Perfusion of asphyxiated cortex with OsO4 causes a marked increase of the conductivity. So does perfusion with an OsO4 solution of tissue previously treated with glutaraldehyde. One interpretation of these impedance changes is that glutaraldehyde perfusion causes, like asphyxiation, a transport of extracellular material into the intracellular compartment and that during OsO4 perfusion an extracellular space is again created. This possibility is supported by electron micrographs made of this material. Cerebral cortex perfused with glutaraldehyde and post-fixed with OsO4 shows electron-transparent dendritic elements and to a lesser extent pre-synaptic terminals, which seem to be swollen. When the cortex is flooded with a salt solution during glutaraldehyde perfusion the dendrites exhibit ballooning in the surface layer of the cortex, suggesting that the fluid on the cortex participates in the swelling. The tissue elements in the glutaraldehyde-perfused and OsO4 post-fixed cortex are separated by narrow extracellular spaces. The latter may have been produced by the OsO4 perfusion as is suggested by a comparison of micrographs prepared by freeze substitution (which tends to preserve the water distribution) of glutaraldehyde-perfused but not post-fixed cortex with micrographs of cortex treated with OsO4 after the glutaraldehyde perfusion. In accordance with the conductivity changes, the former micrographs showed very little extracellular space, and in many places tight junctions, whereas the latter showed clefts between the tissue elements.

Journal ArticleDOI
TL;DR: Label distribution at first metaphase in spermatocytes stemming from sperMatogonia which incorporated [ 3 H]thymidine during the penultimate intermitotic S shows that random segregation and assortment of labelled and unlabelled chromatids took place at the ultimate sper matogonial mitosis.
Abstract: In male Triturus vulgaris at 16 °C the pre-meiotic synthesis of DNA ( S -phase) takes 9-10 days. The S -phase starts 1-2 days after completion of the ultimate spermatogonial mitosis, and extends into leptotene. Heterochromatic regions, most of which lie close to the centromeres, start and finish replicating about 1 day later than other parts of the chromosomes. Synapsis starts 6 days after the end of pre-meiotic S and is completed 8 days later. Pachytene lasts for 4 or 5 days, diplotene for 1 or 2 days, and first metaphase occurs 20 or 21 days after the end of pre-meiotic S . Label distribution at first metaphase in spermatocytes stemming from spermatogonia which incorporated [ 3 H]thymidine during the penultimate intermitotic S shows that random segregation and assortment of labelled and unlabelled chromatids took place at the ultimate spermatogonial mitosis.


Journal ArticleDOI
TL;DR: The appearances of the cell membrances in the present study suggest that the fracture plane tends to pass along either the outer or the inner surface of the membrane rather than of split the membrane.
Abstract: A general survey of guinea-pig myocardium was undertaken using the freeze-etch technique. Replicas of myocardial cell membranes were obtained. These showed an ordered array of pits or stumps situated at Z levels. The pits are interpreted as the apertures of the transverse tubules (T-tubules) seen from outside the cell, and the stumps as the remnants of the T-tubules remaining attached to the cell membrane after the cell contents have been removed. Pinocytotic vesicles were also present. T-tubules, mitochondria and myofilaments could be seen in replicas of the interior of myocardial cells. Capillary endothelial cells were seen from various aspects; pinocytotic vesicles were their most prominent feature. The appearances of the cell membrances in the present study suggest that the fracture plane tends to pass along either the outer or the inner surface of the membrane rather than of split the membrane.

Journal ArticleDOI
TL;DR: The generative cell wall in the pollen grain of Endymion non-scriptus is formed, as in somatic cells, from a cell plate between the vegetative and generative nuclei.
Abstract: The generative cell wall in the pollen grain of Endymion non-scriptus is formed, as in somatic cells, from a cell plate between the vegetative and generative nuclei. This wall curves around the generative nucleus, and fuses with the intine to enclose the generative cell. The generative cell is subsequently freed from the intine by the constriction of the generative cell wall between the generative nucleus and the intine.

Journal ArticleDOI
TL;DR: Electron-microscopic examination of thin sections of pseudopodia revealed many 1-µ bundles of intimately associated, aligned, 55-75 A microfilaments which probably correspond to the birefringent, refractile fibrils observed in living cells.
Abstract: In Difflugia corona , a free-living amoeboid cell, locomotion is hampered by a heavy shell or test made of sand grains and other debris. Locomotion involves pseudopod extension, attachment to the substratum, and forcible pseudopod retraction which pulls the shelled cell body forward. When observed through a polarizing microscope, the extending pseudopodia appear isotropic or very weakly birefringent. Upon attachment to the substratum a positively birefringent fibrillar array develops rapidly at the attachment point and extends from this region bade to the cell body within the test. These birefringent fibrils extend through and parallel to the long axis of the pseudopod. As the pseudopod retracts, the birefringent fibrillar array disappears, and hyaline blebs, suggestive of syneresis, appear on the pseudopodial surface. The birefringent fibrils correspond in position and approximate diameter (1 µ) to retractile fibrils visible with the Nomarski differential interference microscope. Individual organisms were fixed for electron microscopy at a time when the pseudopodia were firmly attached to the substratum. Electron-microscopic examination of thin sections of pseudopodia revealed many 1-µ bundles of intimately associated, aligned, 55-75 A microfilaments. The orientation and size of the bundles indicate that they probably correspond to the birefringent, refractile fibrils observed in living cells. Microfilaments have also been observed both as randomly oriented and dispersed cytoplasmic components, and as aligned filaments in the ectoplasm adjacent to the plasmalemma. During pseudopod extension with sporadic streaming, birefringent ‘flashes’ have been observed at the front of the pseudopod. These flashes are believed to represent a photo-elastic phenomenon.

Journal ArticleDOI
TL;DR: An electron-microscopic study was made of the mitochondria in normal and denervated rat diaphragm muscles; two types of structurally different muscle fibres were found: one type had a well-defined M-line, while the other did not.
Abstract: An electron-microscopic study was made of the mitochondria in normal and denervated rat diaphragm muscles. Two types of structurally different muscle fibres were found: one type had a well-defined M-line, while the other did not. The mitochondria in normally innervated muscle are regularly arranged at both sides of the Z-line. The mitochondria are very long and branched, and surround the myofibrils at the level of the Z-line. In transverse sections of the muscle fibres the worm-like mitochondria give a very characteristic picture. After denervation the mitochondria are smaller and less regularly arranged. In transverse sections of the muscle fibres the mitochondria have small circular profiles. This contrasts sharply with the normal appearance and makes it possible to distinguish normal from denervated fibres. The mitochondrial changes can be detected less than 24 h after denervation.

Journal ArticleDOI
TL;DR: A detailed study of stereoscopy of tissue sections with the transmission electron microscope has been carried out using different tilt angles and staining techniques, aided with models to clarify problems of depth perception in stereo-pairs of micrographs obtained with transmission electron microscopy.
Abstract: A detailed study of stereoscopy of tissue sections with the transmission electron microscope has been carried out using different tilt angles and staining techniques, aided with models. Pictures with tilt angle difference of 20° or more can be fused to give pairs of micrographs showing stereodepth. Cytoplasmic filaments, glycogen and ribosomal granules give good stereodepth. Oblique membranes, especially when stained with lead or phosphotungstic acid, give good depth in stereo-pairs but membranes running more vertically in the section do not. A theory, based on the ‘transparency factor’ that is a feature of transmission electron microscopy, is given to account for this discrepancy. A learning process is involved in achieving depth perception when viewing stereo-pairs of electron micrographs of tissue sections. Complex structures often cannot be perceived in depth even after prolonged scrutiny through the stereoscope. Where structures overlap in the thickness of the section, they can be ‘separated out’ with stereoscopy and in this way clarified, for depending on which micrograph of a stereo-pair is presented to which eye, a structure can be made to appear either in the front or the back of a section. In general, problems of depth perception in stereo-pairs of micrographs obtained with transmission electron microscopy arise because with our normal everyday vision using reflected light, we look at things, whereas with the electron microscope using a transmission electron beam we look through things.

Journal ArticleDOI
TL;DR: It appears that each T-tubule communicates indirectly with the extracellular space via one or more subsarcolemmal caveolae.
Abstract: A study of the structure of the terminal part of the T-tubule system and the distribution of subsarcolemmal caveolae was undertaken in guinea-pig psoas muscle. The results were correlated with the array of cell surface features as revealed by frozen-etched, shadowed replicas of the cell membrane. Freeze-etching revealed numerous small rounded objects overlying the Z- and I-bands and the A-I junction regions of the sarcomeres. These objects appeared as pits or excrescences, depending on whether the cell membrane was viewed from outside or inside the cell, and were interpreted as apertures in the membrane. Conventional thin sections demonstrated the presence of numerous subsarcolemmal caveolae with a similar distribution to the rounded features seen in the replicas. Such sections also showed that the T-tubules, lying at the A-I juctions, seem to change direction when approaching the cell surface and may occasionally appear to branch in the subsarcolemmal region. The T-tubules often terminated in caveolae. Caveolae were sometimes seen in direct communication with the extracellular space. No simple direct communications of T-tubules with cell surface were observed. After treatment of the muscle with lanthanum during fixation, thin sections revealed apparently continuous dense deposits from the cell surface, through the caveolae to the T-tubule proper. It thus appears that each T-tubule communicates indirectly with the extracellular space via one or more subsarcolemmal caveolae.