A routine method for obtaining high contrast without staining sections

Protocol for preparation of heterogeneous biological samples for 3D electron microscopy: a case study for insects

Modern morphological and structural studies are coming to a new level by incorporating the latest methods of three-dimensional electron microscopy (3D-EM). One of the key problems for the wide usage of these methods is posed by difficulties with sample preparation, since the methods work poorly with heterogeneous (consisting of tissues different in structure and in chemical composition) samples and require expensive equipment and usually much time. We have developed a simple protocol allows preparing heterogeneous biological samples suitable for 3D-EM in a laboratory that has a standard supply of equipment and reagents for electron microscopy. This protocol, combined with focused ion-beam scanning electron microscopy, makes it possible to study 3D ultrastructure of complex biological samples, e.g., whole insect heads, over their entire volume at the cellular and subcellular levels. The protocol provides new opportunities for many areas of study, including connectomics.

Large-volume en-bloc staining for electron microscopy-based connectomics

Large-scale connectomics requires dense staining of neuronal tissue blocks for electron microscopy (EM). Here we report a large-volume dense en-bloc EM staining protocol that overcomes the staining gradients, which so far substantially limited the reconstructable volumes in three-dimensional (3D) EM. Our protocol provides densely reconstructable tissue blocks from mouse neocortex sized at least 1 mm in diameter. By relaxing the constraints on precise topographic sample targeting, it makes the correlated functional and structural analysis of neuronal circuits realistic.

Large-scale dense reconstruction of neuronal circuits (or connectomics) requires methods for large-volume dense en-bloc electron microscopy (EM) staining. Here the authors develop a protocol for staining tissue blocks from mouse neocortex sized at least 1 mm in diameter, enabling correlated functional and structural circuit analyses.

Evolution and development of polarized germ cell cysts: new insights from a polychaete worm, Ophryotrocha labronica

Polarized oogenic cysts are clonal syncytia of germ cells in which some of the sister cells (cystocytes) differentiate not as oocytes, but instead as nurse cells: polyploid cells that support oocyte development. The intricate machinery required to establish and maintain divergent cell fates within a syncytium, and the importance of associated oocyte patterning for subsequent embryonic development, have made polarized cysts valuable subjects of study in developmental and cell biology. Nurse cell/oocyte specification is best understood in insects, particularly Drosophila melanogaster. However, polarized cysts have evolved independently in several other animal phyla. We describe the differentiation of female cystocytes in an annelid worm, the polychaete Ophryotrocha labronica. These worms are remarkable for their elegantly simple cysts, which comprise a single oocyte and nurse cell, making them an appealing complement to insects as subjects of study. To elucidate the process of cystocyte differentiation in O. labronica, we have constructed digital 3D models from electron micrographs of serially sectioned ovarian tissue. These models show that 2-cell cysts arise by fragmentation of larger "parental" cysts, rather than as independent units. The parental cysts vary in size and organization, are produced by asynchronous, indeterminate mitotic divisions of progenitor cystoblasts, and lack fusome-like organizing organelles. All of these characteristics represent key cytological differences from "typical" cyst development in insects like D. melanogaster. In light of such differences and the plasticity of female cyst structure among other animals, we suggest that it is time to reassess common views on the conservation of oogenic cysts and the importance of cysts in animal oogenesis generally.

Insects as model systems in cell biology

For almost 100 years, insects have been favorable "model systems" in biology. Just to mention a few examples: fruit flies in genetics and developmental biology; bugs and caterpillars in hormone research; houseflies, blowflies, and locusts in neurobiology; silk moths in pheromone research; honeybees and crickets in neuroethology. For more than 50 years the electron microscope (EM) has been a valuable tool in analyzing the structure of cells and organs of these creatures. However, progress in specimen preparation was relatively slow compared with mammalian material and, in 1970, it was taken for granted that insects were much more difficult to fix than mammals. Since then, methods have dramatically improved, and satisfactory results can now be obtained routinely with chemical as well as cryofixation. In this chapter we briefly demonstrate what can be achieved with insect material, and help the researcher to find the most appropriate method for her/his systems and scientific questions.

Mode of action of the ecdysone agonist tebufenozide (RH-5992), and an exclusion mechanism to explain resistance to it

Spruce budworm larvae (Choristoneura fumiferana) upon ingesting tebufenozide (RH-5992) stop feeding and go into a precocious, incomplete molt, leading eventually to death. Like 20-hydroxyecdysone (20E), tebufenozide also acts at the receptor level and transactivates the expression of up-regulated genes but, because of its persistence, the down-regulated genes that are normally expressed in the absence of 20E are not expressed. While tebufenozide is lepidopteran-specific, an analog, RH-5849, is effective on dipterans. This is reflected in the respective effects of the two compounds on Cf-203 (C. fumiferana--203), a lepidopteran cell line and Dm-2 (Drosophila melanogaster--2), a dipteran cell line. Cf-203 cells accumulated [14C]tebufenozide and expressed CHR3 (Choristoneura hormone receptor 3), but Dm-2 cells excluded the material and did not express DHR3 (Drosophila hormone receptor 3). Using yeast ABC (ATP binding cassette) transporter mutants, we determined that PDR5 (pleiotropic drug resistance 5) was responsible for the exclusion. We discovered recently that older instars of the white-marked tussock moth (Orgyia leucostigma) are resistant to tebufenozide, perhaps as a result of such an exclusion system. We are currently cloning PDR5 (pleiotropic drug resistance 5), which is an essential step in studying the resistance mechanism.

Ultrastructural Effects of a Non-Steroidal Ecdysone Agonist, RH-5992, on the Sixth Instar Larva of the Spruce Budworm, Choristoneura fumiferana

Force feeding of RH-5992 (Tebufenozide), a non-steroidal ecdysone agonist to newly moulted sixth instar larvae of the spruce budworm, Choristoneura fumiferana, (Lepidoptera: Tortricidae) initiates a precocious, incomplete moult. Within 6h post treatment (pt) the larva stops feeding and remains quiescent. Around 12hpt, the head capsule slips partially revealing an untanned new head capsule that appears wrinkled and poorly formed. By 24hrpt, the head capsule slippage is pronounced and there is a mid-dorsal split of the old cuticle in the thoracic region but there is no ecdysis. The larva remains moribund in this state and ultimately dies of starvation and desiccation. The temporal sequence of the external and internal changes of the integument were studied using both scanning and transmission electron microscopy. Within 3hpt, there is hypertrophy of the Golgi complex indicating synthetic activity and soon after, large, putative ecdysial droplets are seen. Within 24h, a new cuticle that lacks the endocuticular lamellae is formed. The formation of the various cuticular components, the degradation of the old cuticle and changes in the organelles of the epidermal cells of the mesothoracic tergite are described. The difference between the natural moult and the one induced by RH-5992 are explained on the basis of molecular events that take place during the moulting cycle. The persistence of this ecdysone agonist in the tissues permits the expression of all the genes that are up-regulated by the presence of the natural hormone but those that are turned on in the absence of the hormone are not expressed.

Structure and function of podocytes: an update

Glomerular visceral epithelial cells, also termed podocytes, are highly specialized epithelial cells that cover the outer aspect of the glomerular basement membrane. Recent studies point to an important role of podocytes in the physiology and pathophysiology of the glomerulus. This review summarizes the structure-function relationships of podocytes. Following a description of the general morphology of podocytes, the technical problems associated with studying these cells are discussed. A survey of podocyte function forms the center of this review. Finally, selected aspects of podocyte development and cell division are discussed.

Preservation and contrast without osmication or section staining

Conventional treatment of tissues for sectioning and transmission electron microscopy uses aldehyde fixation and osmium tetroxide postfixation. Although the result is aesthetically pleasing, osmication destroys some cell components and reduces the chemical activity of others, such as reaction with antibodies and lectins. We have found that aldehyde fixation followed by uranyl acetate preserves and contrasts most structures and visualizes some that are not easily seen after osmication. Aldehyde/UA treated tissues have enough contrast to be observed without section staining while retaining some of the chemical activity that is lost through osmication. Sections of tissues with good preservation and contrast can be used for immunogold and lectin-gold labelling of at least some molecules.

Successful prediction of immediate effects of undernutrition throughout the brain growth spurt on capillarity and synapse-to-neuron ratio of cerebral cortex in rats

Based on the hypothesis that undernutrition depresses the growth rate of all processes in brain contemporaneous with it to the same extent (Peeling & Smart, 1994), specific predictions were made regarding the effect of undernutrition on two quite different facets of anatomical development within visual cortex. It was predicted that severe undernutrition during the suckling period would leave capillarity unaffected, but would result in a deficit in synapse-to-neuron ratio of similar magnitude to that in brain weight. At birth rat pups were fostered and either well fed to 30 days or undernourished to the same age by underfeeding their foster mothers. Rats were killed at 30 days by perfusion with glutaraldehyde. Visual cortex was dissected out for quantitative histological study. The number of capillary profiles per unit area of section and the area of each profile were assessed with an image analysis system. Quantitative characteristics of the neuron and synapse populations were estimated by light and electron microscopy respectively. Undernutrition resulted in a 21% deficit in brain weight, and a 22% deficit in the number of synapses per neuron. Capillarity, expressed as the fraction of section area occupied by capillary lumen, appeared completely indifferent to nutrition. However, fewer capillary profiles were found per unit area of section in growth-restricted samples, and the profiles which were present were, on average, larger. Neuron density was elevated by 19% in the undernourished group. These findings are in good agreement with the hypothesis.

Fine structure of a developing insect olfactory organ: morphogenesis of the silkmoth antenna

The olfactory organ of the silkmoth Antheraea polyphemus is the feathered antenna which carries about 70,000 olfactory sensilla in the male. It develops within 3 weeks from a leaf-shaped epidermal sac by means of segmental primary and secondary indentations which proceed from the periphery towards the centerline. During the first day post-apolysis, the antennal epidermis differentiates into segmentally arranged, alternating sensillogenic and non-sensillogenic regions. Within the first 2 days post-apolysis, the anlagen of olfactory sensilla arise from electron-dense mother cells in the sensillogenic epidermis. The axons of the developing sensilla begin to form the primary innervation pattern during the second day. The sensilla develop approximately within the first 10 days to their final shape, while the indentations are completed during the same period of time. The indentations are most probably driven by long basal extensions of epidermal cells, the epidermal feet. Primary indentations follow the course of segmentally arranged tracheal bundles and form the segments of the antenna. The secondary indentations follow the course of the primary segmental nerves which are reconstructed by this process. During the remaining time of development, the cuticle of the antenna and the sensory hairs is secreted by the epidermal and the hair-forming cells.

Improved preservation of phospholipid-rich multilamellar bodies in conventionally embedded mammalian lung tissue--an electron spectroscopic study

Different conventional methods of tissue processing were studied to determine the extent to which phospholipid-rich multilamellar bodies of pulmonary alveolar epithelial type II cells of the pig were preserved. Prolonged treatment with half-saturated aqueous uranyl acetate yielded excellent results on the stabilization of the multilamellar substructure, irrespective of whether glutaraldehyde-paraformaldehyde or glutaraldehyde-tannic acid was used as a primary fixative. The lamellar periodicities were observed to be 5.5-6.1 nm. Differences in the phosphorus distribution among several types of lipid bodies of alveolar epithelial type II cells were studied by means of electron spectroscopic imaging and electron energy-loss spectroscopy. Multilamellar bodies gave phosphorus signals which were significantly higher than those obtained from granular regions of composite bodies, whereas homogeneous bodies gave phosphorus signals which were even lower than those obtained from mitochondria, endoplasmic reticulum membranes or ribosomes.

Glycosaminoglycan involved in the cation-induced change of body wall structure of sea cucumber Stichopus japonicus

The body wall of sea cucumber Stichopus japonicus was treated with various concentrations of several cations, and examined for changes in toughness, taking punch force as parameter. Toughness of the body wall tended to decrease with increasing concentration of each cation, but in different modes depending mainly upon the valency of cation: e.g., the body wall completely lost toughness in 0.3 M Na+ or 0.4 M K+, whereas it retained more than half the initial toughness even in 0.4 M Ca2+ or Mg2+. Glycosaminoglycan (GAG) from the body wall was dissolved in water, and examined for viscosity changes as caused by those cations. Specific viscosity (eta sp) decreased from 0.71 (without cation) to 0.47-0.57 in the presence of 0.1 M monovalent and divalent cations. At 0.4 M, monovalent cations reduced eta sp to 0.38-0.46, but divalent cations increased eta sp to 0.56-0.63. Electron microscopy demonstrated that GAG matrix was clearly observed in the absence of cation, but disappeared in 0.4 M NaCl, forming wide free spaces in the body wall. These results all suggested that GAG is closely involved in the change of toughness of sea cucumber body wall.

Structure and development of iridescent butterfly scales: Lattices and laminae

Iridescent butterfly scales are structurally colored, relying upon the interaction of light with detailed architecture to produce their color. In some iridescent scales, the reflective elements are contained within the body of the scale and come in two basic forms, lattices that produce diffraction colors (analogous to those produced by opal), and stacks of laminae that produce thin-film interference colors (analogous to those produced by soap or oil films). Both structures are remarkably complex and precise, yet each is only part of the total edifice built by the cell that makes the scale. To understand better how a cell can produce lattices or thin-film laminae, I studied the development of iridescent scales from two lycaenid butterflies. The presence of diffraction and thin-film scales in the same family (and in some cases on the same individual) suggests that the two types must be developmentally related; yet these results yield no clear explanation as to how. The diffraction lattice appears to be shaped within the boundaries of the scale cell by means of a convoluted series of membranes in which the smooth endoplasmic reticulum plays an important part. The thin-film interference laminae appear to result from the condensation of a network of filaments and tubes secreted outside the boundaries of the cell. This paper outlines the developmental histories of both types of scale and discusses the developmental implications of the mechanisms by which they form.

Heterogeneity of the podocyte membrane in rat kidney as revealed by ethanol dehydration of unosmicated specimens

The ultrastructure of the podocyte membrane was studied by means of transmission electron microscopy of unosmicated tissue samples after acetone or ethanol dehydration and subsequent embedding in a polyester resin. The podocyte membrane in glutaraldehyde (GA)-fixed, acetone-dehydrated samples consisted of a relatively thick, clear layer (about 6 nm) abutted by the dark staining cytoplasm and a dark surface layer. In GA-fixed, ethanol-dehydrated samples a striking intramembranous pattern was observed in the podocyte cell membrane. The luminal podocyte membrane was regularly perforated by gaps about 25 nm wide. In grazing sections these gaps appeared round and were separated by a honeycomb pattern of intact membrane. The abluminal membrane, in contrast, generally maintained its continuity. The clear layer of the podocyte membrane was thinner in ethanol-dehydrated samples than in acetone-dehydrated ones. In tissue samples fixed with GA supplemented by ruthenium red, ethanol dehydration was not associated with cell-membrane perforations. Based on these observations as well as on biochemical data from the literature we suggest that in GA-fixed, unosmicated, acetone-dehydrated samples the structural integrity of the podocyte membrane is well preserved, while ethanol dehydration extracts some specific material from regularly distributed domains in the podocyte cell membrane.

Dendritic membrane from insect olfactory hairs: isolation method and electron microscopic observations

Sensory hairs from antennae of male saturniid moths (Antheraea polyphemus) were separated while deep-frozen by shaking antennal branches with glass beads. The hairs were collected through their differential adhesion to the surface of a petri dish. The yield, determined by the length of the isolated hair fragments, was about 38% of the estimated total hair length per antenna. The dendritic membrane was separated from the hair fragments by centrifugation through Sephadex and further purified by ultracentrifugation in sucrose buffers. Transmission electron microscopy was used to monitor the steps of the hair and membrane isolation and to investigate the membrane pellet. Some membrane vesicles bound cationized ferritin, thus indicating a negatively charged cell surface coat. Negatively stained membrane vesicles exhibited a pattern of repetitive substructures irregularly distributed over the vesicle surface. The units had a diameter of about 3 nm and a maximal density of 30,000/micron2.

The ultrastructure of the tegument of adult Schistosoma haematobium

The ultrastructure of the tegument of Schistosoma haematobium was examined using scanning and transmission electron microscopy. The surface of the male worm is characterized by numerous raised tubercles bearing apically directed spines. The female in contrast to the male is cylindrical and relatively smooth. Details of oral and ventral suckers are given. The use of uranyl acetate as a tertiary fixative and en bloc stain has revealed the heptalaminate nature of the outer membrane. Tegumental mitochondria are shown to be morphologically more complex than those of S. mansoni. Spherical and elliptical inclusion bodies are also described. The ultrastructure of the oesophageal tegument of S. haematobium is described for the first time and corresponds with earlier observations of S. mansoni.

Contacts of pore tubules and sensory dendrites in antennal chemosensilla of a silkmoth: demonstration of a possible pathway for olfactory molecules

Antennal olfactory hairs of Antheraea polyphemus were investigated by means of transmission electron microscopy. Adequate preservation of dendrites and extracellular pore tubules is obtained by mechanical opening of the hair lumen and subsequent chemical fixation. The dendritic membrane has a cell coat. The dendrites contain microfilamentous structures in addition to their cytoplasmatic microtubules. The extracellular pore tubules traverse the hair cuticle and reach into the hair lumen for maximally 350 nm. Their diameter varies between 20 and 40 nm, depending on the preparation method. They consist of an electron-dense wall and an electron-lucent core. The wall has a helical substructure and is covered with a fuzzy coat. Contacts of pore tubules and dendritic membranes occur wherever dendrites are near the inner surface of the hair cuticle. Some of the pore tubules terminate approximately at right angles on the dendritic membrane, others lie against the membrane. The contact seems to be made via the surface coats of the tubules and the membrane. The structure of pore tubules which had been negatively stained with uranyl acetate is similar to the conventionally thin-sectioned material. The observations provide support for earlier assumptions that pore tubules are the pathways by which odor molecules reach the dendritic membrane.

Selective injury to rat liver Kupffer cells caused by beryllium phosphate: an explanation of reticuloendothelial blockade

The i.v. administration of suspensions of beryllium phosphate (5-50 mumol/kg) to rats resulted in the vacuolation of hepatic Kupffer cells within 3 h. After 6 h necrotic Kupffer cells were common throughout the sinusoids of the liver but no changes were detected in the hepatic parenchymal cells during this period. A significant reduction in the numbers of intrasinusoidal cells was observed 14 h after treatment but this population had reverted to normal within 24 h. The administration of colloidal carbon to treated animals at this time did, however, demonstrate a reduction in the complement of functional endocytic cells. These results demonstrate a selective destruction of endocytic cells in the liver by this particulate toxin and the limited response by the organ to this injury. These observations are the most probable explanation for the reticuloendothelial blockade known to be caused in vivo by beryllium phosphate.

The nucleolus during epidermal development in an insect

The fifth stadium of Calpodes has two phases of epidermal cell development corresponding to preparation for intermoult and for moult syntheses. Both phases begin with a period of elevated RNA synthesis and the elaboration of a multilobed nucleolus. The apparent number of nucleoli changes from about two to eight and back to two again within the few hours of elevated RNA synthesis. The nucleolar changes are preceded by elevated titres of haemolymph ecdysteroid. During the two periods of activity, alveoli in the matrix of the nucleoli contain particles believed to be ribosomal precursors. The staining properties of these granules differ according to size in a way that suggests a developmental sequence. Mature granules are about 20 nm in diameter and do not stain with bismuth. They are found at the periphery of the nucleolus, in the nucleoplasm, at the approaches to and within the nucleopores. Perichromatin granules, believed to be m-RNA precursor packages, are up to 60 nm in diameter, do stain with bismuth and are found at the periphery of chromatin, in nucleoplasm and distorted at the approaches to the nuclear pores to fit within the central channel. During these periods of heightened activity the nuclear envelope contains microvesicles that may be free or attached to either nuclear or cytoplasmic surfaces. The structure is appropriate for the microvesicular transnuclear envelope movement of molecules such as the ecdysteroid believed to initiate the nuclear changes.

Apolysis and the turnover of plasma membrane plaques during cuticle formation in an insect

The apical plasma membranes of Calpodes epidermal cells have small fattened areas or plaques with an extra density upon their cytoplasmic face. The plaques are typically at the tips of microvilli. The are present during the deposition of fibrous cuticle and the cuticulin layer. Since the plaques are close (less than 15nm) to the sites where these kinds of cuticle first appear, they are presumed to have a role in their synthesis and/or deposition and orientation. When fifth stage larval cuticle deposition ceases prior to pupation, the plaques are lost as the area of the apical plasma membrane is reduced. The plaques pass from the surface into pinocytosis vesicles and multivesicular bodies where they are presumably digested. The loss of plaques occurs as the blood level of moulting hormone reaches a peak at the critical period after which the prothoracic glands are no longer needed for pupation. Apolysis or separation of the epidermis from the old cuticle is the stage when plaques are absent, the old ones have been lost but the new ones have yet to form. After the critical period, the epidermis prepared for pupation with a phase of elevated RNA synthesis at the end of which plaques and microvilli reform in time to secrete the new cuticulin layer and later the fibrous cuticle of the pharate pupa. There is a new generation of plaques for each moult and succeeding intermoult and each generation is involved in two kinds of cuticle deposition before involution and redifferentiation.

Membrane fusion as a mechanism of simian virus 40 entry into different cellular compartments

Permissive and nonpermissive simian virus 40 (SV40)-infected cells were ultrastructurally analyzed. Viral particles were found in the cytoplasm, rough endoplasmic reticulum, nuclear envelope, lysosomes, and mitochondria. Upon entering the cell the virion obtains a tight membrane envelope. It seems to be either released from the envelope upon fusion with other membranes of the cell or aggregated into tubular membrane specializations upon fusion with other membrane-enveloped particles. Reconstructed morphological sequences and the finding of SV40 in different spaces of the cell suggest that entry of SV40 into the different compartments and eventually into the site of replication is facilitated by its capacity for being enveloped by a variety of membranes (notably the cell membrane and the nuclear membrane) and the sequential fusion and fission of these membranes.

Bovine olfactory and nasal respiratory epithelium surfaces. High-voltage and scanning electron microscopy, and cryo-ultramicrotomy

High-voltage transmission electron microscopy and cryo-ultramicrotomy together with scanning electron microscopy and some conventional transmission electron microscopy of ultrathin sections have been applied to the mucous surfaces of bovine olfactory and respiratory epithelia. Distal segments of olfactory cilia tend to run in parallel and could be followed over distances up to about 30 micrometer using high-voltage electron microscopy. This technique and scanning electron microscopy showed that on average 12--13 of such cilia could be observed per nerve ending. After correction for obscured cilia this number becomes about 17. High-voltage micrographs and micrographs made from sections prepared with a cryo-ultramicrotome showed the presence of electron-lucent pockets inside the olfactory mucus. The latter technique also showed that the mucus itself is not fibrous, but rather a continuum varying in electron density. The mucus layer contains various granular structures. Ciliary and microvillar membranes appear thicker with cryo-ultramicrotomy than when the sections are prepared with conventional techniques. The cores of the axonemal microtubules in olfactory as well as in respiratory cilia are darkly stained with this technique. Vesicles present inside the nerve endings are also darkly stained. Dimensions and some other numerical values of interest in olfaction are presented.

Nucleoprotein localization by bismuth staining

Experiments on isolated mouse liver muclei involving enzyme digestion, the crosslinking of amino groups and alkaline hydrolysis demonstrate that bismuth binds to nucleoproteins through amino and phosphate groups. Analysis of the nucleoproteins extracted with salt and acid solutions in conjunction with bismuth staining after these treatments suggests that: (1) a bismuth amino group interaction occurs on ribonucleo-protein particles, histones and perhaps some non-histone chromosomal proteins, and (2) bismuth phosphate binding is specific for one, or all, of three distinct species of non-histone proteins. These results suggest that histones not tightly bound to DNA through their amino groups are present on interchromatin granules, the presumed transcriptionally active regions of chromatin. Phosphorylated non-histone proteins are also localized at these sites. Staining with heavy metals such as bismuth may be the best method for high resolution localization of nucleoproteins involved with regulating gene activity and maintaining chromatin structure.

Quantitative determination of nuclear pore complexes in cycling cells with differing DNA content

The number of pore complexes per nucleus was determined for a wide variety of cultured cells selected for their variable DNA content over a range of 1-5,6000. The pore number was compared to DNA content, nuclear surface area, and nuclear volume. Values for pore frequency (pores/square micrometer) were relatively constant in the species studied. When the pore to DNA ratio was plotted against the DNA content, there was a remarkable correlation which decreased exponentially for the cells of vertebrae origin. Exceptions were the heteroploid mammalian cells which had the same ratio as the diploid mammalian cells despite higher DNA content. The results are interpreted to mean that neither the nuclear surface, the nuclear volume, nor the DNA content alone determines the pore number of the nucleus, but rather an as yet undetermined combination of different factors. The surface and volume of vertebrate nuclei do not decrease with decreasing DNA content below a given value. The following speculation is suggested to account for the anomalous size changes of the nucleus relative to DNA content in vertebrates. Species with small DNA complements have a relatively large proportion of active chromatin which determines the limits of the physical parameters of the nucleus. The amount of active chromatin maybe the same for at least the vertebrates with low DNA content, At high DNA content, the nuclear parameters may be determined by the relatively high proportion of inactive condensed chromatin which increases the nuclear surface and volume.

The fine structure of developing elastic cartilage

The fine structure of the elastic cartilage of the pinna has been examined in young rabbits aged from 1 day to 1108 days. Changes associated with growth and development are related not only to age but also to the actual situation in the pinna. In the midline, progressive changes are seen from the tip to the base. The changes in the chondroblasts with time are compared with those described in hyaline cartilage. Structures occur that, except for the presence of crystals, are apparently morphologically identical with the matrix vesicles of calcifying cartilage. These matrix vesicles, however, become very prominent with age, and aggregations of them appear to be released into the intercellular tissue from vacuoles at the periphery of the chondroblasts. There is no obvious association with calcification. Occasional single cilia, desmosomes and giant mitochondria are seen. Elastica is present at birth, and eventually every cell is separated from its neighbours by a partial investment of elastica. The quantity of matrix seems to increase with time, and with distance from the tip of the ear. This is accompanied by a marked increase in cell size with time.

Resolution in electron microscope autoradiography. III. Iodine-125, the effect of heavy metal staining, and a reassessment of critical parameters

Resolution for 125I-labeled specimens under electron microscope (EM) autoradiographic conditions was assessed experimentally. With this isotope the size of the silver halide crystal was the most important resolution-limiting factor. Heavy metal staining such as is routinely used in preparing animal tissues for EM autoradiography produced an improvement in resolution of approximately 15-20%. For a 500-1,000-A biological tissue section fixed with OsO4 and stained with uranyl acetate, we obtained resolution (half distance, HD) values of approximately 800 +/- 120 A using Ilford L4 emulsion and 500 +/- 70 A using a Kodak NTE-type emulsion. General aspects of resolution-limiting factors and comparison with 3H and 14C values are discussed.

Time sequence and morphological evaluations of cells fused by polyethylene glycol 6000(1)

Mouse L cells (clone 1D) were fused with polyethylene glycol (PEG). The fusion sequence was determined by using sequential light microscopy of the same group of cells, scanning electron microscopy (SEM),transmission electron microscopy, and freeze-etching. The cells were found to fuse only 1 min after PEG had been washed off at small localized areas. Larger fusion images were found after 3 min. Intramembrane particles were observed to have a tendency to aggregate after PEG treatment, but a direct correlation of this activity with the fusion process could not be made. No pathological changes were noted at longer times after PEG removal, except for the extensive widening of the rough-surfaced endoplasmic reticulum (RER) in some cells. It is proposed that fusion does not occur if apposing cells have many microvilli at the area of apparent contact.

Fibrils attached to the nuclear pore prevent egress of SV40 particles from the infected nucleus

SV40 particles can apparently enter the nucleus intact. However, they do not leave the nucleus despite the high concentration present during the productive phase. We found structural evidence that SV40 virus is prevented from approaching the most likely site of exit, the nuclear pore complex. From these images, it is concluded that the fibrils attached to the nuclear pore complex prevent egress of SV40 particles from the infected nucleus.

Localization of acetylcholinesterase in a crustacean neuromuscular system

1. The localization of acetylcholinesterase (AChE) was studied by light and electron microscopy (LM and EM) at the neuromuscular junctions (NMJs) of the abdominal superficial flexor muscle (SFM) and the claw adductor muscle (CAM) of the crayfish in view of reported pharmacological evidence of cholinergic transmission at the former but not at the latter site. 2. Survey of LM with the copper-thiocholine method disclosed staining for AChE, but not for butyrocholinesterase (BuChE), at numerous NMJs of the SFM; neither enzyme was detected at any NMJs of the CAM. 3. With the bis-(thioacetoxy) aurate (I) method, EM examination showed AChE-staining at NMJs of the SFM to be confined to the postjunctional membrane. 4. The published and present findings suggest a unique situation of more than one excitatory transmitter for the skeletal neuromuscular system in the crustacean studied.

Ultrastructural changes of the canine gastric mucosa after topical application of graded concentrations of ethanol

Changes in the fine structure of the gastric mucosa following exposure to graded concentrations of ethanol were studied in dogs. 300 ml of 12.5, 20, and 40%, vol/vol, were instilled intragastrically for 30 min. Mucosa from the midbody and midantrum along the greater curvature was examined by light and electron microscopy. Ethanol produced a gradation of changes in the surface epithelial cells and in the lamina propria without affecting the parietal cells and chief cells. 12.5% ethanol produced widened and irregular intercellular spaces while 20 and 40% disrupted the apical cell membrane with concomitant exudation of mucin into the gastric lumen. These changes were more severe after 40% ethanol. The tight junction between cells remained intact following exposure to the lower concentrations of ethanol, but focal separation of cell junctions was observed in severely damaged areas. Quantitation of protein, sodium, and potassium concentrations in the gastric contents revealed marked increases following exposure to ethanol which correlated with the concentration. These studies provide additional morphological data on the relationship between structural changes and functional abnormalities induced by agents which break the gastric mucosal barrier.

The beads in the Golgi complex-endoplasmic reticulum region

The region between the rough endoplasmic reticulum (ER) and the Golgi complex has been studied in a variety of insect cell types in an attempt to find a marker for the exit gate or gates from the ER. We have found that the smooth surface of the rough endoplasmic reticulum near Golgi complex transitional elements has beadlike structures arranged in rings at the base of transition vesicles. They occur in all insect cell types and a variety of other organisms. The beads can be seen only after staining in bismuth salts. They are 10-12 nm in diameter and are separated from the membrane and one another by a clear halo giving them a center to center spacing of about 27 nm. The beads are not sensitive to nucleases under conditions which disrupt ribosomes or remove all Feulgen staining material from the nucleus. Under conditions similar to those used to stain tissue, bismuth does not react in vitro with nucleic acids. The component of the beads that stains preferentially with bismuth is therefore probably not nucleic acid.