Ultrastructural localization of DNA in ultrathin tissue sections

A brief history of the Feulgen reaction

One hundred years ago, Robert Feulgen published a landmark paper in which he described the first method to stain DNA in cells and tissues. Although a century has passed since the discovery by Feulgen and Rossenbeck, the chemical reaction still exerts an important influence in current histochemical studies. Its contribution in diverse fields, spanning from biomedicine to plant biology, has paved the way for the most significant studies that constitute our current knowledge. The possibility to specifically explore the DNA in cell nuclei while quantifying its content makes it a contemporary and timeless method. Indeed, many histocytochemical studies following the 1924 paper have led to a deep understanding of genome organization in general as well as several specific mechanisms (e.g. DNA duplication or tumour pathology) that, nowadays, constitute some of the most fundamental pillars in biological investigations. In this review, we discuss the chemistry and application of the Feulgen reaction to both light and electron microscopy.

The Feulgen reaction at the electron microscopy level

The Feulgen reaction has been the first specific method for detecting DNA available at light microscopy since 1924. However, a similar specific method was proposed for electron microscopy only 50 years later. Here, we discuss the problems encountered in finding the electrondense reagent capable of taking advantage of the extremely high resolution offered by electron microscopy as well as some applications of the method.

Correlative Light and Electron Microscopy (CLEM) Analysis of Nuclear Reorganization Induced by Clustered DNA Damage Upon Charged Particle Irradiation

Chromatin architecture plays major roles in gene regulation as well as in the repair of DNA damaged by endogenous or exogenous factors, such as after radiation. Opening up the chromatin might provide the necessary accessibility for the recruitment and binding of repair factors, thus facilitating timely and correct repair. The observed formation of ionizing radiation-induced foci (IRIF) of factors, such as 53BP1, upon induction of DNA double-strand breaks have been recently linked to local chromatin decompaction. Using correlative light and electron microscopy (CLEM) in combination with DNA-specific contrasting for transmission electron microscopy or tomography, we are able to show that at the ultrastructural level, these DNA damage domains reveal a chromatin compaction and organization not distinguishable from regular euchromatin upon irradiation with carbon or iron ions. Low Density Areas (LDAs) at sites of particle-induced DNA damage, as observed after unspecific uranyl acetate (UA)-staining, are thus unlikely to represent pure chromatin decompaction. RNA-specific terbium-citrate (Tb) staining suggests rather a reduced RNA density contributing to the LDA phenotype. Our observations are discussed in the view of liquid-like phase separation as one of the mechanisms of regulating DNA repair.

DNA Labeling at Electron Microscopy

Here, we describe a method for locating DNA on ultrathin sections. This technique is compatible with all usual fixation and embedding procedures and can be combined with cytochemical methods. Ultrathin sections are incubated in a medium containing terminal deoxynucleotidyl transferase (TdT) and various non-isotopic nucleotide analogs. The labeled nucleotides bound to the surface of ultrathin sections are then visualized by an indirect immunogold labeling technique. This high-resolution method provides a powerful tool for pinpointing the precise location of DNA within biological material, even where DNA is present in very low amounts.

Osmium Ammine for Staining DNA in Electron Microscopy

The osmium ammine staining allows the specific detection of DNA in the cell nucleus and represents one of the most used techniques for EM cytochemistry.The procedure is a Feulgen-type reaction, consisting of an acid hydrolysis to obtain free aldehyde groups on DNA followed by their binding to osmium ammine, a Schiff-type reagent. Osmium ammine is polyamminic electron-dense compound commercially available.Here, we describe the staining procedure for ultrathin sections and the different procedures for the preparation of the reagent for acrylic and epoxy sections.

Visualizing the RNA molecule in the bacterially expressed vesicular stomatitis virus nucleoprotein-RNA complex

Packaging of the RNA molecule in viruses is important for the preservation and expression of viral genomic information. The vesicular stomatitis virus (VSV) nucleoproteins are kept associated with its negative-strand RNA during the mRNA synthesis and replication, in contrast to the tobacco mosaic virus whose nucleoproteins are released from RNA. It has been a puzzle how the VSV RNA is packaged to meet the contradicting requirements of protection and the accessibility to the polymerase. We report an 18 A resolution structure of the recombinant nucleoprotein-RNA complex determined by single-particle electron microscopy. In the 3D density map, a ring of density is resolved on the inner surface and the density is proposed to be the RNA. The RNA is located on the inner surface of the decameric complex near the top end. This is dramatically different from the RNA packaging in TMV, but consistent with previously published biochemical findings.

Ultrastructural methods for nucleic acid detection by immunocytology

In the present review are summarized recent developments in immunocytochemical detection of nucleic acids in biological materials at the ultrastructural level. Not only the approaches using antibodies to natural nucleic acids are described but also the techniques involving the use of antibodies raised against various nucleotide analogs incorporated beforehand into nucleic acids. Special emphasis is placed on each method's potential and limitations. These methods, combined or not with molecular biotechnology, are powerful tools for studying the structure and function of nucleic acids. They can be used to investigate the distribution and topological organization of DNA and RNA molecules or of specialized within these molecules in the cells.

Morphological and cytochemical analysis of human cytomegalovirus inoculum: correlation of free particles in inoculum with counterparts in infected cells

Electron microscopic examination of human cytomegalovirus (HCMV) inoculum, as used in the laboratory and generated by infection of human fibroblasts at low multiplicity, led to the distinction of 7 different structures. Complete virions constituted 38% of the inoculum. Non-infectious enveloped particles (NIEP) were also quite numerous (4.7%). Inoculum also contained other enveloped and non-enveloped particles. Dense bodies were the most numerous (50.2%). The Feulgen-like osmium ammine/SO2 reaction applied to ultrathin sections of inoculum suggests that NIEP, considered to be lacking DNA, may contain small and varying amounts of DNA. This DNA was lightly stained and appeared as a filamentous ring in the core structure, extending to the limits of the capsid. A correlation was established between particles identifiable in HCMV-infected cells and their free counterparts in the inoculum, which revealed that all intracytoplasmic particles are present in the inoculum. All of these elements could potentially contribute to virus-induced phenomena associated with HCMV infection of cells in vitro.

Ultrastructural investigation of DNA in megakaryoblastic leukemia by using osmium-ammine-B: comparison with several types of leukemia

Using DNA staining with highly stable osmium-ammine-B, the blasts of AMKL were observed under an electron microscope, in comparison with AML, ALL, CML-MK crisis and TMD. The DNA within the nucleus of the megakaryoblasts was observed as a high electron-density substance and tended to be uniformly dispersed within the nucleus. DNA, associated with nucleoli, could be roughly divided in four types based on the presence or absence of peri-nucleolar clumps and intra-nucleolar clumps. In cases of AMKL, we often observed a type that did not have peri-nucleolar DNA clumps but did have DNA flecks within the nucleoli. By analytical evaluation there were no differences among cells from several subtypes of megakaryocytic leukemia, such as AMKL in children with Down's syndrome, AMKL in children without Down's syndrome, AMKL in adults, and CML-MK crisis. The DNA distribution of TMD blasts, which were self-limited and not malignant, resembled that of AMKL blasts.

Nucleic acid cytochemistry of the nucleus and microtubule-organizing centers in dictyostelium discoideum

We used fluorescence microscopy with DAPI, Hoechst 33258, acridine orange, and ethidium bromide, as well as ultracytochemical regressive staining, the Feulgen-type reaction with osmium-ammine, and the enzyme-gold method to investigate the presence and distribution of DNA and RNA in the nucleoplasm, nucleolus, nucleus-associated body, and spindle pole bodies. We found that the nucleoplasm of interphase nuclei contains mostly DNA dispersed in a fibrillar meshwork, with which some RNA is probably associated as perichromatin granules or fibers. With DNase-gold and RNase-gold the nucleolus, which consists of interspersed fibrillar and granular components, was the most heavily labelled of five cellular compartments analyzed. Accordingly, its fluorescence with acridine orange and ethidium bromide was brightest. In mitotic nuclei the nucleolus was dispersed, filling most of the nuclear volume. Chromosomes were brightly stained by DNA-specific fluorochromes and the osmium-ammine reaction revealed that only the innermost layer of the trilaminar kinetochores contains DNA. Neither the nucleus-associated body of interphase cells nor the spindle pole bodies of mitotic cells contain DNA or RNA.

Ultrastructural detection of DNA within the nucleolus by sensitive molecular immunocytochemistry

This paper describes a new technique for locating DNA on semithin or ultrathin sections of aldehyde-fixed and plastic-embedded cells or tissues. Sections were incubated in a medium containing bromodeoxyuridine (BUdR) triphosphate and terminal deoxynucleotidyltransferase. The labeled nucleotides bound at the surface of the sections were subsequently detected with an anti-BUdR antibody and immunoglobulin-gold complex. On semithin sections, labeled nucleotide detection was achieved by an amplification step with silver enhancement. This technique was applied to a wide variety of biological materials allowing a sensitive detection of DNA-containing structures, even where these are present in very low amounts. Examples of high resolution and sensitive detection include the DNA present in mitochondria, chloroplasts, mycoplasmas, and DNA viruses. Special attention focused on the location of DNA inside the nucleolus. In Ehrlich tumor cell nucleoli, DNA was detected in the fibrillar centers and not in the dense fibrillar component. Identical results were found in the nucleoli of other cell types. These results contradict earlier data but conform with other recent immunocytochemical observations concerning the correlation between structure and function in the nucleolus. This method provides a useful tool for investigations requiring highly precise correlations between a molecular function and a given ultrastructural morphology.

Balbiani ring hnRNP substructure visualized by selective staining and electron spectroscopic imaging

The Balbiani Rings (BR) in the polytene chromosomes of Chironomus salivary glands are intense sites of transcription. The nascent RNPs fold during transcription into 40-50-nm granules, containing in the mature transcript approximately 37-kb RNA. Using a new nucleic acid specific stain, osmium ammine B on Lowicryl sections, in combination with electron energy filtered imaging of sections containing BR granules, we demonstrate a RNA-rich particulate substructure (10-nm particle diameter; 10-12 particles per BR granule). Elemental imaging supports that these particles are enriched in phosphorus. The possible relationship of these RNA-rich particles to ribonucleosomes is discussed, as well as models for their arrangement in the mature BR granules.

DNA cytochemistry in polytene chromosomes: electron contrasting agents for the ultrastructural detection of chromatin DNA after alkaline hydrolysis/methylation-acetylation

Salivary glands from Chironomus tentans larvae were fixed in glutaraldehyde and either subjected to alkaline hydrolysis followed by methylation-acetylation, or dehydrated without these treatments as controls. Ultrathin sections from Durcupan-embedded samples were contrasted by means of uranyl acetate, ruthenium red, indium trichloride, or the complex indium (III)-hematoxylin. Electron microscopic observations revealed a general contrasting pattern in control sections, while after the hydrolytic and blocking procedure only chromatin from polytene chromosomes appeared selectively contrasted. The nucleolus, Balbiani ring granules and puff materials showed weak or no electron opacity. After toluidine blue staining of semithin sections, an orthochromatic blue colour was found in chromatin bands from treated samples. These results indicate that alkaline hydrolysis/methylation-acetylation followed by contrasting with cationic heavy compounds is a valuable procedure to visualize chromatin DNA in polytene chromosomes.

A cytochemical and immunocytochemical study of DNA distribution in spermatid nuclei of mouse, rabbit, and bull

DNA distribution in mouse, rabbit and bull spermatids was analyzed by electron microscopy, after using a Feulgen-like HCl-osmium ammine procedure, and after immunocytochemistry with anti-DNA antibodies. In addition, nucleic acids were visualized with the intercalating dye ethidium bromide and phosphotungstic acid. The parts of DNA displaying a beta helix configuration (possibly A-T rich parts) were identified by epifluorescence microscopy after staining with Hoechst 33258. In all 3 species, young spermatid nuclei were seen to have large areas poor in DNA, as well as DNA-rich areas, which were mostly concentrated into a peripheral layer close to the acrosome and into one or several masses, displaying species-specific locations. These DNA-rich areas were stained with Hoechst 33258. Elongating spermatid nucleic contained homogeneously distributed DNA, and this was evident following both immunocytochemistry and nucleic acid histochemistry in all 3 species. However, the distribution appeared more heterogeneous after the Feulgen-like procedure, and was accompanied by a disappearance of Hoechst-fluorescence. In fully elongated spermatids, all nuclear areas stained with Hoechst 33258, while the 3 other techniques labeled either all or species-specific parts of the condensed chromatin. The reasons for these variable reactions are discussed in terms of technique specificities, DNA configuration and nucleoprotein moiety replacements.

An 'axis-like' material in the centromeric region of metaphase-I chromosomes from mouse spermatocytes

This study reports the persistence of axis-like structures in the centromeric region of both homologues during the metaphase-I and anaphase-I stages of meiotic division of mouse spermatocytes. A novel type of silver 'argentaffin' technique (NH4-Ag) is employed. This technique includes the treatment of glutaraldehyde-fixed tissues with dilute ammonium hydroxide followed by a reduction of aldehyde groups with sodium borohydride. Staining is accomplished with ammoniacal silver nitrate in darkness followed by sulfite washing. The lateral elements of synaptonemal complexes and the single chromosomal axes of diplotene spermatocytes show a prominent reactivity with this technique. The pattern of very small grains over condensed chromatin is uniform and gives only a light opacity to the electron beam. The presence of an axis-like structure is seen in every centromeric end of meiotic chromosomes at metaphase I and anaphase I. The chromatin (heterochromatin) that surrounds the centromeric filament and some material distributed in irregular linear arrays along some of the homologues also showed a higher electron opacity than the bulk of deoxyribonucleoprotein. While the former is related to C+ heterochromatin, the latter could represent dispersed material of diplotene axes. It is suggested that the disposal of axial material is differentially delayed at the centromeric regions. The present evidence supports the hypothesis that axial fragments or lateral-element segments persisting at these regions contribute to the cohesiveness of centromeres of sister chromatids during normal disjunction.

Immunocytochemistry of the cell nucleus

This electron microscopic review addresses in situ immunocytochemistry of the mammalian cell nucleus with special reference to the use of autoantibodies, which are the major source of antinuclear antibodies. The localization of many key nuclear antigens is documented and immunocytochemical data are related to the major functional processes of transcription and processing of RNA and to replication of DNA.

Ultrastructural localization of DNA on ultrathin sections of resin-embedded tissues by the lactoferrin-gold complex

Lactoferrin, a DNA-binding protein, was complexed to colloidal gold and applied on ultrathin sections of resin-embedded plant tissues and bacterial cultures. Optimal results were obtained when lactoferrin was tagged to colloidal gold particles at pH 9.2. Postfixation with osmium tetroxide and embedding with Epon did not prevent the accessibility of the protein towards its corresponding binding sites. In plant nuclei, labeling was observed over the dense chromatin and to a lesser extent over the dispersed chromatin. Nucleolar labeling was preferentially located over the dense fibrillar component. Gold particles were also found to be associated with chloroplasts and mitochondria. In prokaryotic cells, a dispersed labeling was noted over the cytoplasm and, in some cases, the aggregation of few gold particles suggested the presence of packed DNA fibrils. Various control experiments confirmed the specificity of the labeling pattern obtained. Lactoferrin-gold complex appears to be a valuable probe for the intracellular demonstration of DNA molecules in double-fixed and Epon-embedded tissues.

Chromatin arrangement in mouse sperm nuclei: an ultrastructural cytochemical study

The arrangement of mouse sperm nuclei chromatin and, in particular, of DNA has been studied by electron microscopic cytochemistry. It had been previously shown that, after a Feulgen-type reaction using an osmium ammine complex (OAC), the OAC-stained DNA was distributed in a spotted pattern in the nucleus (Biggiogera: Basic Appl Histochem 30:501-504, 1986). The present chapter shows that this pattern is characteristic of mouse spermatozoa from testis to vas deferens, with the exception of some testicular spermatozoa, in which DNA was homogeneously stained. DNase digestion of thin-sectioned nuclei resulted in a distribution of residual material complementary to the pattern of the unstained zones after the OAC reaction. These findings are discussed considering the role of -S-S- crosslinks, characteristics of this extremely condensed chromatin, in limiting the availability of DNA to acid hydrolysis.

Immunoelectron microscope localization of bromodeoxyuridine incorporated into DNA of Ehrlich tumor cell nucleoli

The distribution of DNA within the nucleolus of Ehrlich tumor cells has been investigated by means of a recent immunocytochemical approach involving an electron microscopic detection of incorporated 5-bromodeoxyuridine (BUdR) into DNA by an anti-BUdR monoclonal antibody. An immunogold method has been performed on ultrathin sections of cells embedded in Lowicryl K4M. In the nucleolus, gold particles are essentially found over the perinucleolar chromatin adn over its intranucleolar invaginations which are connected with the fibrillar centers. In addition, a few gold particles are also observed in the fibrillar centers, preferentially toward their peripheral regions. In contrast, the dense fibrillar component is completely devoid of labeling. The results are discussed in the context of other recent findings concerning the functional organization of the nucleolus.

Structural and functional relationships between nuclear bodies and the nucleolus-DNA body complex in the oocyte of Amphiporus lactifloreus

The localization, structure and function of two types of nuclear bodies have been investigated by cytological and cytochemical electron microscopy methods in oocytes from the Hoplonemertean, Amphiporus lactifloreus. Type I nuclear bodies differentiate in contact with the nucleolus-DNA body complex, whereas type II nuclear bodies develop close to the diplotenic chromosomal axes. The structure of type I and type II spherical nuclear bodies, 4-5 micron in width, results from the association of a fibrillar reticulum with some dense included regions. The cytochemical findings following the use of osmium-ammine reaction for DNA and silver reaction for NOR proteins support the hypothesis that type II nuclear bodies, derived from the extranucleolar area, as well as type I nuclear bodies, derived from the nucleolar complex, may be involved in ribosomal biogenesis.

Evidence for the existence of a nucleolar skeleton attached to the pore complex-lamina in human fibroblasts

This work deals with the types of nuclear skeletal structures obtained from human fibroblast nuclei isolated by different procedures. It is confirmed that, in somatic vertebrate cells, the pore complex-lamina is always observed, whereas the presence of internal nucleolar and extranucleolar residual structures depends upon the method of nuclear isolation used. Furthermore, the results reported here argue for the existence of a nucleolar skeleton different from the nucleolar matrix often observed in different cell types by other investigators. The conditions of nuclear isolation which allow us to visualize this nucleolar skeleton without any other internal residual structures are described. The attachment of the nucleolar skeleton to the lamina suggested by the present data is considered in relation to the in situ position of nucleoli near the nuclear envelope.

Shape and fine structure of nucleoids observed on sections of ultrarapidly frozen and cryosubstituted bacteria

Very rapidly frozen cells of Escherichia coli and Bacillus subtilis were substituted at low temperature into acetone with 1% OsO4 and embedded in Epon. They showed ribosome-free spaces filled with globular and fibrillar material of up to 15 nm. The sizes of structures seen do not exclude DNA superstructures such as supercoils, aggregates, and nucleosomes. With the Feulgen analog osmium-ammines stain, DNA was localized within the ribosome-free space. The bulk of DNA, the nucleoid, is therefore a major part of, or identical to, the main ribosome-free space. The ribosome-free space would correspond directly to the light microscopy phase-contrast image of nucleoids in living bacteria. The shape of the ribosome-free space does not reflect intracellular salt concentrations, nor do the Feulgen-positive areas. The previously observed dependency on the salt concentration of the growth medium seems to be due to permeabilization induced by the chemical fixative at room temperature. The ribosome-free space is more cleft in appearance than the nucleoid obtained by fixation with OsO4 but more confined than its very dispersed form found after aldehyde fixation.

Visualization of chromatin distribution in living PTO cells by Hoechst 33342 fluorescent staining

Chromatin distribution was visualized in living cells with the selective DNA fluorochrome Hoechst 33342. This dye was shown to be non-toxic on the rat kangaroo PTO cell line by measuring the labelled cell growth rate. The aim of this work was firstly to visualize chromatin distribution without fixation or dehydration and secondly to demonstrate that quantitative determination of DNA content was possible under these non-toxic labelling conditions. During interphase, condensed, decondensed and thin network chromatin configurations were visualized. In nucleolar regions the fluorochrome revealed well-defined chromocentres. During mitosis, fluorescent chromosome banding was observed in vital conditions and chromocentres on fixed chromosomes. Chromatin segregation was visualized after micronucleation, which induced chromosomal set distribution in individual micronuclei. By this means, we demonstrated that the chromocentres observed in interphase nuclei were part of nuclear organizer region (NOR)-bearing chromosomes. This vital staining of chromatin was shown to be compatible with the quantitative determination of DNA content, both in living PTO cells and in isolated nuclei.

Ruthenium red staining of polyanion containing structures in sections from epoxy-resin embedded tissues

Staining by ruthenium red (0.5 mg/ml in borate buffer at pH = 9.2) has been used for light and electron microscopic visualization of polyanion containing structures in sections from glutaraldehyde-fixed, epoxy-embedded tissues. This staining technique can be applied in a simple and rapid way, showing the reactive cell components with suitable resolution and contrast. Preliminary spectrophotometric studies show the correspondence in absorption characteristics of the dye which is bound to polyanions in situ or in vitro.

The morphological relationship in electron microscopy between NOR-silver proteins and intranucleolar chromatin

The relative distribution of NOR proteins and chromatin fibers in the nucleoli was visualized in human cell line. The chromatin was revealed by a Feulgen-like procedure using osmium-ammine as DNA tracer. This selective staining was combined with NOR-silver staining. We provide morphological evidence for constant overlapping of the silver deposit sites with dispersed intranucleolar chromatin fibers. Silver stained proteins were sometimes observed in contact with the chromatin fibers, suggesting that at least some of the Ag-NOR proteins might be closely connected with the dispersed nucleolar DNA.

A simple method for the use of gallocyanin-chrome alum as an electron strain

A method has been elaborated for the demonstration of DNA in the electron microscope. The method uses glutaraldehyde fixed tissue pieces from which RNA has been removed by incubation with RNase. DNA is stained by gallocyanin-chrome alum in the tissue block. Embedding and cutting is done in the usual manner. The method is based on histochemical observations at the light microscope level which show sufficient specificity and a good stoichiometry of the staining reaction.

Staining of DNA with uranylacetate in hydrolysed ultrathin sections

Rat liver and hepatoma cells fixed with formaldehyde, embedded into Epon and treated on sections with 5 n HCl and then with aqueous uranylacetate show preferential DNase-sensitive reaction. The reaction is highly dependent upon proper fixation, hydrolysis improves its specificity. The binding of the contrast with DNA is of ionic nature. Because of its simplicity, sufficient contrast and resolution the suggested technique is recommended for ultrastructural studies of DNA-containing substrates.

Methods in ultrastructural cytochemistry of the cell nucleus

The electron microscopical study of the cell nucleus as observed in thin sections requires the use of cytochemical methods because of the intricate pattern of the nuclear components. The in situ techniques based on electron staining and enzymatic digestion are reviewed, excluding autoradiography, cytoenzymology and immunocytochemistry. A tentative classification has been adopted according to the chemical nature of the revealed component. Thus, the staining procedures for the nucleoproteins in general, for both nucleic acids, for the proteins, and finally for the deoxyribonucleoproteins and DNA are considered separately. 1--Stains for the nucleoproteins include simple reagents such as the uranyl and lead salts which are largely used in electron microscopy but are of limited specificity. 2--A variety of methods, some of them specific, is available for the simultaneous visualization of DNA and RNA which is based on common properties: basophilia, ability to bind diaminoacridines, presence of hydroxyl groups. However, due to the recent development of specific and preferential methods for each nucleic acid, we feel that among the older methods, only rapid and simple procedures for the detection of both nucleic acids remain of interest. 3--Proteins being ubiquitous, the useful techniques must reveal subsets within the total nuclear proteins. Apart from some endogeneous enzymes, basic proteins -- practically histones -- so far represent the only group for the detection of which reliable methods exist. 4--Several techniques developed recently are available for the specific detection of DNA. In favourable cases, methods derived from the Feulgen reaction allow its visualization at a molecular level. In addition, standard procedures for the preparation of mammalian cells and tissues are described. Each staining method is at least briefly discussed, but emphasis has been placed on a small number of techniques described in detail. They comprise the EDTA regressive stain for the ribonucleoproteins, several reactions of the basic proteins and the Feulgen-like osmium ammine reaction for DNA.

Ultrastructural study of the nuclei in premitotic and repair DNA synthesis following UVB injury

Ultrastructural changes in nuclei synthesizing DNA were studied by cytochemical technique. Guninea pigs ears were UVB irradiated and TdR-H3 was injected intradermally into the irradiated sites 1 hr before biopsy. Areas of the epidermis containing more than 80% of cells in DNA (repair or premitotic) synthesis identified by light microscopic autoradiography were selected and cut at 600 A. The glycolmethacrylate sections were stained with uranyl acetate and lead citrate, and consecutive sections were incubated with 0.01% pronase and 0.5% RNase before staining in order to observe DNA. In cells undergoing DNA repair, the zone of DNA became discontinuous and DNA was scattered throughout the entire karyoplasm as small aggregates and fine filaments. Nuclei in S-phase showed essentially the same change, but quantitatively the disappearance of DNA from the nuclear membrane and distribution in the karyoplasm became much greater. These changes were not seen in specimens treated without cytochemical technique.

Localization of the synaptonemal complex under the light microscope

The use of osmium tetroxide fixation followed by postreatment with p-phenylenediamine gives an opportunity of locating the synaptonemal complex (SC) under the light microscope in mouse testes and Allium cepa anthers. When semi-thin sections from these materials were observed under phase contrast optics or dark field microscopy, fine threads in the pachytene nuclei were clearly visible. Post-staining of semi-thin sections with ammoniacal silver increased the contrast of the SC and allowed for observations using a bright field illumination. Ultrathin sections of osmium tetroxide/p-phenylenediamine treated material showed that, under the electron microscope, this technique stains preferentially elements of the synaptonemal complex, while the surrounding chromatin remains unstained.

Feulgen positive nucleoli in Epon semithin sections: fact or artifact?

Epon semithin sections from glutaraldehyde fixed tissues show stained nucleoli after treatment with the Schiff's reagent. The possibility that aldehyde groups already present could account for this positive staining reaction is briefly discussed.