The apical ectodermal ridge, fibroblast growth factors (FGF-2 and FGF-4) and insulin-like growth factor I (IGF-I) control the migration of epidermal melanoblasts in chicken wing buds

The role of the apical ectodermal ridge and of fibroblast growth factors FGF-2 and FGF-4 and of the insulin-like growth factor I (IGF-I) in the control of the migration of epidermal melanoblasts was investigated using quail-chicken chimeras. Wing buds of a strain of unpigmented chicken were microsurgically modified in several ways (ablation, displacement or implantation of additional apical ectodermal ridges, implantation of grafts devoid of apical ectodermal ridges, ectopic application of growth factors) and received grafts containing quail neural crest cells. The distribution of the epidermal melanoblasts which had differentiated from the quail grafts revealed that both the apical ectodermal ridge and the growth factors invariably caused the migration of epidermal melanoblasts towards them. This leads to the conclusion that the presence of the apical ectodermal ridge is the sufficient condition to direct the migration of epidermal melanoblasts within the avian embryonic wing bud. Furthermore, FGF-2 and IGF-I and to a lesser extent FGF-4 play a decisive role in directing the migration of epidermal melanoblasts within chicken wing buds and are likely to be involved in the molecular cascade by means of which the apical ectodermal ridge controls the migration of epidermal melanoblasts.

Signal enhancement at the electron microscopic level using Nanogold and gold-based autometallography

Immunoelectron microscopy using ultrasmall gold markers is a very sensitive method to detect molecules at high resolution. In order to discriminate the gold particles in the electron microscope, enlargement of gold particles is necessary. So far, mostly silver ions were used for deposition onto the surface of gold grains. In our study, we tested the selective deposition of gold instead of silver ions to enlarge gold particles. This was performed following immunogold detection of DNA at the surface of ultrathin sections embedded in the acrylic resin LR White (postembedding approach). Morphometric analysis of the distribution of DNA in human spermatocytes revealed that the method offers very good specificity and sensitivity and therefore is a good alternative to the use of silver for signal enhancement. This technique was also applied to the detection of ribosomal genes in human testis at the electron microscopic level by in situ hybridization. Ribosomal genes were localized in peri- and intranucleolar chromatin as well as in the dense fibrillar component of nucleoli.

Autosomal-dominant, prelingual, nonprogressive sensorineural hearing loss: localization of the gene (DFNA8) to chromosome 11q by linkage in an Austrian family

A four-generation family suffering from an autosomal-dominant, congenital, nonprogressive, nonsyndromic hearing loss was found in a rural region of Austria. The hearing loss was moderate to severe, a pure tone audiogram showing a U-shaped form with maximum loss at 2, 000 Hz. An initial genome search led to a lod score of 3.01 with markers on chromosome 15. This locus was registered as DFNA8 in the HUGO data base. Further sampling of the family, however, yielded data that reduced the maximal lod score with chromosome 15 markers to 1.81. The genome search was restarted using an ABITM genotyper, which eventually detected several positive two-point lod scores with markers from the long arm of chromosome 11. The highest value was 3. 6, which was seen with the marker D11S934. Haplotype analysis excluded the gene from the chromosomal region proximal from D11S898 and distal to D11S1309. These results place the gene in the region of the hearing loss gene DFNA12. Recent evidence suggests that the somewhat different phenotypes found in these two families are due to two different mutations in the human alpha-tectorine gene (Verhoeven et al., 1998).

Intranuclear anchoring of repetitive DNA sequences: centromeres, telomeres, and ribosomal DNA

Centromeres, telomeres, and ribosomal gene clusters consist of repetitive DNA sequences. To assess their contributions to the spatial organization of the interphase genome, their interactions with the nucleoskeleton were examined in quiescent and activated human lymphocytes. The nucleoskeletons were prepared using "physiological" conditions. The resulting structures were probed for specific DNA sequences of centromeres, telomeres, and ribosomal genes by in situ hybridization; the electroeluted DNA fractions were examined by blot hybridization. In both nonstimulated and stimulated lymphocytes, centromeric alpha-satellite repeats were almost exclusively found in the eluted fraction, while telomeric sequences remained attached to the nucleoskeleton. Ribosomal genes showed a transcription-dependent attachment pattern: in unstimulated lymphocytes, transcriptionally inactive ribosomal genes located outside the nucleolus were eluted completely. When comparing transcription unit and intergenic spacer, significantly more of the intergenic spacer was removed. In activated lymphocytes, considerable but similar amounts of both rDNA fragments were eluted. The results demonstrate that: (a) the various repetitive DNA sequences differ significantly in their intranuclear anchoring, (b) telomeric rather than centromeric DNA sequences form stable attachments to the nucleoskeleton, and (c) different attachment mechanisms might be responsible for the interaction of ribosomal genes with the nucleoskeleton.

Recruitment of bone-marrow-derived cells by skeletal and cardiac muscle in adult dystrophic mdx mice

It is commonly accepted, that regenerative capacity of striated muscle is confined to skeletal muscle by activation of satellite cells that normally reside quiescent between the plasmalemma and the basement membrane of muscle fibers. Muscular dystrophies are characterized by repetitive cycles of de- and regeneration of skeletal muscle fibers and by the frequent involvement of the cardiac muscle. Since during the longstanding course of muscular dystrophies there is a permanent demand of myogenic progenitors we hypothesized that this may necessitate a recruitment of additional myogenic precursors from an undifferentiated, permanently renewed cell pool, such as bone marrow (BM) cells. To this end normal and dystrophic (mdx) female mice received bone marrow transplantation (BMT) from normal congenic male donor mice. After 70 days, histological sections of skeletal and cardiac muscle from BMT mice were probed for the donor-derived Y chromosomes. In normal BMT recipients, no Y chromosome-containing myonuclei were detected, either in skeletal or in cardiac muscle. However, in all samples from dystrophic mdx skeletal muscles Y chromosome-specific signals were detected within muscle fiber nuclei, which additionally were found to express the myoregulatory proteins myogenin and myf-5. Moreover, in the hearts of BMT-mdx mice single cardiomyocytes with donor derived nuclei were identified, indicating, that even cardiac muscle cells are able to regenerate by recruitment of circulating BM-derived progenitors. Our findings suggest that further characterization and identification of the BM cells capable of undergoing myogenic differentiation may have an outstanding impact on therapeutic strategies for diseases of skeletal and cardiac muscle.

Arrangement of individual human ribosomal DNA fragments on stretched DNA fibers

We studied the arrangement of individual human ribosomal (r)DNA repeats by direct visualization of rDNA sequences. We used high resolution fluorescence in situ hybridization on preparations of DNA fibers released from interphase nuclei of HeLa cells. Probes from both the transcription unit and the intergenic spacer were used, and lengths of signals and of the gaps in between were measured and compared to molecular data. We could visualize the repetitive arrangement of individual rDNA sequences at the single gene level. No inversions or deletions were detected. The intergenic spacer was found to be shorter than expected, indicating a length polymorphism.

Mutations in the human alpha-tectorin gene cause autosomal dominant non-syndromic hearing impairment

The tectorial membrane is an extracellular matrix of the inner ear that contacts the stereocilia bundles of specialized sensory hair cells. Sound induces movement of these hair cells relative to the tectorial membrane, deflects the stereocilia, and leads to fluctuations in hair-cell membrane potential, transducing sound into electrical signals. Alpha-tectorin is one of the major non-collagenous components of the tectorial membrane. Recently, the gene encoding mouse alpha-tectorin (Tecta) was mapped to a region of mouse chromosome 9, which shows evolutionary conservation with human chromosome 11q (ref. 3), where linkage was found in two families, one Belgian (DFNA12; ref. 4) and the other, Austrian (DFNA8; unpublished data), with autosomal dominant non-syndromic hearing impairment. We determined the complete sequence and the intron-exon structure of the human TECTA gene. In both families, mutation analysis revealed missense mutations which replace conserved amino-acid residues within the zona pellucida domain of TECTA. These findings indicate that mutations in TECTA are responsible for hearing impairment in these families, and implicate a new type of protein in the pathogenesis of hearing impairment.

Ribosomal gene transcription is organized in foci within nucleolar components

We investigated how only three morphologically distinguished nucleolar components can integrate the many necessary tasks in ribosome biogenesis. For the mapping of ribosomal (r)DNA transcription loci, we combined non-autoradiographic in situ transcription assays with the immunological analysis of the ultrastructural distribution of transcription-associated proteins, i.e., polymerase I, the human polymerase I-specific upstream binding factor, and topoisomerase I. Furthermore, we visualized the nascent transcripts simultaneously with the rDNA in the nucleoli. All tested transcription proteins were found in both the fibrillar center and the dense fibrillar component (DF) of nucleoli in human cells. In the DF the nascent transcripts, detected by bromouridine incorporation, were found colocalized with the transcription proteins only within circumscribed regions. We did not observe colocalization of rDNA with nascent transcripts within the fibrillar centers, which corroborates the view that transcription proteins in this component are rather inactive. Our results suggests that only a minor portion of the DF is involved in transcriptional activity. Transcription appears to be confined to small foci, which exist close to or associated with the DF. Our results are in favor of the view that the DF has different functions which are localized in subcompartments of the DF.

Test tube systems with cutting/recombination operations

We introduce test tube systems based on operations that are closely related to the splicing operation, i.e. we consider the operations of cutting a string at a specific site into two pieces with marking them at the cut ends and of recombining two strings with specifically marked endings. Whereas in the splicing of two strings these strings are cut at specific sites and the cut pieces are recombined immediately in a crosswise way, in CR(cutting/recombination)-schemes cutting can happen independently from recombining the cut pieces. Test tube systems based on these operations of cutting and recombination turn out to have maximal generative power even if only very restricted types of input filters for the test tubes are used for the redistribution of the contents of the test tubes after a period of cuttings and recombinations in the test tubes.

Signal amplification at the ultrastructural level using biotinylated tyramides and immunogold detection

The tyramide amplification technique has recently been developed for signal enhancement in enzyme-linked immunosorbent assays and western blots. This method relies on using labelled tyramides as substrates for peroxidase, resulting in an immobilization of the labelled tyramide residues (tyramide reaction). We succeeded in establishing reliable protocols for the use of the tyramide reaction at the electron microscopic (EM) level. As model systems we chose the visualization of DNA in late spermatocytes, of actin in skeletal muscle, and the visualization of an rDNA probe after DNA-DNA in situ hybridization. We observed a significant increase in signal density after performing the tyramide reaction at the EM level. The tyramide amplification technique at the ultrastructural level therefore appears to be a useful tool to detect even a few epitopes present at the surface of a section as shown after in situ hybridization. It offers advantages over other amplification systems, such as the peroxidase-mediated deposition of diaminobenzidine, because of an increased spatial resolution, whereas specificity and sensitivity are comparable to the conventional immunogold detection method.

Dystrophin expression in heterozygous mdx/+ mice indicates imprinting of X chromosome inactivation by parent-of-origin-, tissue-, strain- and position-dependent factors

Inactivation of one X chromosome (X inactivation) in female mammals results in dosage compensation of X-chromosomally encoded genes between sexes. In the embryo proper of most mammals X inactivation is thought to occur at random with respect to the parental origin of the X chromosome. We determined on the cellular level the expression of the X-chromosomally encoded protein dystrophin in skeletal and cardiac muscle of female mice heterozygous for a null mutation of the dystrophin gene (mdx/+). In all muscles investigated (cardiac, anterior venter of digastric muscle, biceps brachii and tibialis anterior muscle) we found a mosaic expression of dystrophin-expressing versus non-expressing cells and determined their proportion with respect to the parental origin of the X chromosome. In all groups of mdx/+ mice the level and pattern of dystrophin expression were found to be dependent on the parental origin of the mdx mutation. Additionally, the extent of dystrophin expression was clearly dependent on the mouse strains (C57BL/10 and BALB/c) used to produce heterozygous mdx/+ mice. Variable differences and patterns of dystrophin expression in skeletal versus cardiac muscle were found that were strictly dependent on the parental source of the mdx mutation and the strain used to breed mdx/+ mice. Moreover, dystrophin expression was found to be different between the right side and the left side of the body in individual muscles, and this difference was clearly dependent on the parental origin of the X chromosome. Our data provide evidence that in the mouse embryo proper there is a non-random distribution of cells showing inactivation of the paternal versus the maternal X chromosome in skeletal and cardiac muscle, indicating a non-random X-inactivation. Besides gametic imprinting, strain-, tissue and position-dependent factors also appear to bias X inactivation.

The transcription unit of ribosomal genes is attached to the nuclear skeleton

The relationship between various loci of the ribosomal gene repeat and the nucleoskeleton was examined in agarose-embedded HeLa cells. The accessibility of intranucleolar structures to molecular probes was improved by dispersing the granular component of nucleoli, and unattached DNA was removed from permeabilized nuclei under "physiological" conditions by enzymatic digestion and subsequent electroelution. The cells were then hybridized in situ with various human rDNA probes for the transcription unit or for the intergenic spacer. A strong signal was detected with probes for the transcription unit but no signal was seen with probes for the intergenic spacer. These results show that only the transcription unit is strongly attached to the nucle(ol)ar skeleton and imply that rDNA is probably attached to the skeleton primarily via RNA polymerase complexes rather than via sequence-specific attachment sites. Nucleolar fibrillar centers, embedded into the nucle(ol)ar skeleton, provide structural support for these attachments.

Redistribution of ribosomal DNA after blocking of transcription induced by actinomycin D

We report on the effect of different doses and times of incubation of the cytostatic drug actinomycin D (AMD) on nucleolar morphology, rRNA gene transcription and rDNA gene localization using in situ hybridization and the immunocytochemical detection of the human upstream binding factor (UBF) at the electron microscopic level in HeLa cells. Low doses of AMD (0.001 micrograms/ml, 30 min) selectively block rRNA gene transcription but alter neither nucleolar morphology nor the localization of rDNA with respect to the nucleolar components. Treatment with high doses of AMD (0.05 micrograms/ml, 1 h) resulted in a retraction of the rDNA out of the nucleolus in addition to the well-known blocking of rDNA transcription, total nuclear transcription and nucleolar segregation. Under these conditions accumulations of rDNA were found in patches of chromatin at the nucleolar periphery. We conclude that the blocking of rRNA gene transcription and the changes in nucleolar morphology, both induced by AMD at different doses, are independent phenomena.

The RNA polymerase I transcription factor UBF and rDNA are located at the same major sites in both interphase and mitotic pig embryonic kidney (PK) cells

Indirect immunolabeling with anti-UBF antibodies, in situ hybridization with an rDNA probe, and confocal scanning laser microscopy were used to study nucleolar organizer regions (NORs) during the cell cycle in pig embryonic kidney (PK) cells. The chromosomal distribution of the polymerase I transcription factor UBF and rDNA was compared with the number of silver-stained NORs (Ag-NORs) present and nucleolar size. It was shown, both at interphase and mitosis, that the majority of UBF and rDNA signals were located at the same foci and that the amounts of UBF and rDNA at any given site were in a striking positive correlation. At mitosis, only the NORs were labeled; at interphase, the signals for both UBF and rDNA were arranged in necklace-like structures around the nucleoli. No chromosomal NORs without Ag-proteins or UBF were present, indicating that all NORs in PK cells are active at interphase. It was concluded that (1) UBF and rDNA co-localize throughout the cell cycle in PK cells; (2) their association with mitotic NORs is determined by the number of rDNA repeats, rather than by any differential ability of NORs to recruit the transcription factor; and (3) the amount of UBF can be correlated with the size and activity of the nucleoli at interphase.

The metameric pattern of the head mesoderm--does it exist?

It is a long-standing question whether the paraxial head mesoderm of vertebrate embryos is segmentally organized into somites like the trunk or not. On the one hand, no somites are seen in the anterior head mesoderm in vertebrate embryos, on the other hand, such a segmental pattern has been described under the name of somitomeres. In order to investigate the patterning of mesodermal cells in the head of avian embryos we performed scanning electron microscopy, computer assisted reconstructions of the head mesoderm and density analyses of head mesoderm cells. We observed regional differences within the head mesoderm of avian embryos, but we could not see a consistent somitomeric pattern in the head mesoderm. In sum, we consider that the avian head mesoderm is not arranged in a metameric pattern.

Spatial distribution of sex chromosomes and ribosomal genes: a study on human lymphocytes and testicular cells

The location of the sex chromosomes in relation to the rRNA genes in the nuclei of human lymphocytes and testicular cells was examined. Sex chromosomes were found to be located closer to ribosomal genes than would be expected assuming a random arrangement of these chromosomes with respect to rRNA genes. This proximity could be observed irrespective of the transcriptional activity of ribosomal genes indicating that the chromosomal material and not transcriptional activity is responsible for the intranuclear order of these chromosomes.

Emergence of myogenic and endothelial cell lineages in avian embryos

The roles of cell cycles and of cell-cell interactions in the emergence of myogenic and endothelial cell lineages were studied in avian embryos using the quail-chicken marker system. Quail embryos were treated with drugs preventing either DNA replication or the movement of cells. Portions of drug-treated or untreated quail blastoderms were grafted into chicken wing buds. After an incubation for an additional 4 to 10 days, the embryos were analyzed for the presence of quail muscle or quail endothelial cells by the Feulgen reaction and by immunostaining. Both cell lineages differ in the time of their commitment as well as in the conditions necessary for their emergence. Muscle cells did not differentiate from unincubated blastoderms nor did they develop from drug-treated blastoderms. These results corroborate that the commitment of myogenic cells occurs during gastrulation and indicate that this commitment requires both DNA replication and cellular movements allowing cell-cell and/or cell-matrix interactions. Endothelial cells, on the contrary, developed both from drug-treated and from unincubated blastoderms, indicating that their commitment occurs before and independent of gastrulation and does not require DNA replication during gastrulation.

Site of transcription of ribosomal RNA and intranucleolar structure in HeLa cells

Sites of transcription of ribosomal RNA in HeLa cells were visualized by electron microscopy. Cells were either incubated with Br-uridine, or permeabilized and then incubated with BrUTP, before sites containing Br-RNA were immunolabeled with gold particles. Short incubations ensured that most incorporated analogue remained at synthetic sites. Fibrillar centres were unlabelled except at their periphery; label was concentrated over certain regions of the surrounding dense fibrillar component. These results suggest that the dense fibrillar component is the site of rRNA transcription. After dispersing the granular component and the dense fibrillar component by a hypotonic treatment, removal of most chromatin and preparation of resinless sections, fibrillar centres remained fixed to a nucleoskeleton. These structural and functional features are incorporated into a model for rRNA transcription.

Cytochemistry of satellite nucleoli in human lymphocytes

Satellite nucleoli of lymphocytes were studied to provide additional information on the cytochemistry of these nucleoli particularly with respect to the presence of rDNA and RNA polymerase I. According to the results of the in situ hybridization satellite nucleoli contain rDNA similarly as characteristic nucleoli. Immunostaining demonstrated that satellite nucleoli similarly as characteristic nucleoli possess RNA polymerase I in addition to proteins B23, C23 and fibrillarin. RNA of satellite nucleoli was detected in satellite as well as in characteristic nucleoli with buffered toluidine or methylene blue. The cytochemical evidence and morphology of satellite nucleoli strongly supports the supposition that these nucleoli represent solitary small nucleoli containing nucleolus organizer regions which did not participate in the formation of characteristic nucleoli.

The nucleolus

Nucleoli are the sites of biosynthesis of the ribosomal precursors. They contain may copies of the genes for the main rRNAs (18S- and 28 S-rRNA) in the form of tandemly arranged repeats at the chromosomal nucleolar organizer regions (NORs). They also contain the small rRNA (5S-rRNA) that is synthesized outside the nucleolus, specific nucleolar proteins, among them the factors and enzymes necessary for transcription and transcript processing, and the precursor units of the ribosomes. In man as in may vertebrate species, three main components of nucleoli, besides chromatin, can be detected: fibrillar centres (FC), dense fibrillar component (DCF), and granular component (GC). Within a nucleolus the FCs are in many cases situated in its central region. The DFc forms a network of strands surrounding the FCs, but may sometimes reach for out towards the periphery of the nucleolus. The GC is usually situated in the peripheral regions of the nucleolus. In cells with a low level of ribosomal biosynthesis the nucleoli are small, usually with a single FC and little surrounding DFC and GC ("ring-shaped nucleolus"). In active cells the DFC forms a large network enclosing several, sometimes up to hundreds of FCs, and the GC covers a large area in the periphery ("compact nucleoli"). In cells at the onset of a new stimulation, the DFC is very prominent whereas the FCs are few and small, and the GC is also not very extensive ("reticulate nucleoli"). In some special cell types that are very active other arrangements of the structural components are found. In Sertoli cells, for instance, only one nucleolus is found, or occasionally two, each with a single large FC and a distinct area of GC, both areas being engulfed by DFC intermingled with some peripheral GC. Immunocytological and in situ hybridization studies to localize the rRNA genes within the nucleolus have so far led to divergent results. Both fibrillar components, the FCs and the DFC, have been claimed as the most probable candidates. Transcription of rDNA and the subsequent early steps of ribosome biosynthesis are localized in the DFC, whereas later steps (mature rRNA, preribosomes) are localized in the GC. The FCs may also serve as sites for the preparation of the rDNA for transcription, and as a store for certain nucleolar proteins. During mitosis, parts of the nucleolar proteins remain at the NORs. A direct contact between the nucleolus and the nuclear envelope is frequently observed but is not dependent on nucleolar activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of DNA in human Sertoli cell nucleoli

For better understanding of nucleolar architecture, different techniques have been used to localize DNA within the dense fibrillar component (DF) or within the fibrillar centers (FC) by electron microscopy (EM). Since it still remains controversial which components contain DNA, we investigated the distribution of DNA in human Sertoli cells using various approaches. In situ hybridization (ISH) with human total genomic DNA as probe and the use of anti-DNA antibody were followed by immunogold detection. This allowed statistical evaluation of the signal density over individual components. The Feulgen-like osmium-ammine (OA) technique for the selective visualization of DNA was also applied. The anti-DNA antibodies detected DNA in mitochondria, in chromatin, and in the DF of the nucleolus. ISH using human total genomic DNA showed similar labeling patterns. The OA technique revealed DNA filaments in the FC and focal agglomerates of decondensed DNA within the DF. We conclude that (a) EM staining techniques that utilize colloidal gold appear to be less sensitive for DNA detection than the OA method, (b) the DF consists of different domains with different molecular composition, and (c) decondensed DNA is not necessarily confined to one particular nucleolar component.

A study on nucleolar DNA: isolation of DNA from fibrillar components and ultrastructural localization of different DNA probes

The nature and localization of DNA contained in the fibrillar centres and the dense fibrillar component (the fibrillar complex) in the nucleoli, was studied in human LEP cells, Sertoli cells, spermatogonia A and in mitotic chromosomes of stimulated lymphocytes. A novel procedure for isolating the intact fibrillar complex from LEP cells was used; the complex contains DNA that hybridizes to secondary constrictions of mitotic chromosomes and to 28 S rDNA sequences, on Southern blots. Electron microscopic DNA-DNA in situ hybridization was performed, with (a) a probe prepared from DNA extracted from the fibrillar complex of LEP cells, (b) a probe for human total genomic DNA, and (c) a probe for the transcribed part of human rDNA. On the basis of the results obtained we conclude that the ribosomal RNA genes in human Sertoli cells and spermatogonia A are predominantly associated with the dense fibrillar component, including the border region between fibrillar centres and the dense fibrillar component. The ribosomal RNA genes are the main, if not exclusive, DNA type present in the fibrillar complex in the studied cell types.

The uptake of uridine in the nucleolus occurs in the dense fibrillar component. Immunogold localization of incorporated digoxigenin-UTP at the electron microscopic level

A new method was developed to localize the site of transcription within the nucleolus. Incorporation of digoxigenin-labeled UTP in nucleoli of human PHA-stimulated lymphocytes and in equine melanoma cells was visualized with antibodies against digoxigenin and secondary gold-labeled antibodies at the electron-microscopic level. This approach offers much higher spatial resolution than the autoradiographic methods used so far. In both types of cells digoxigenin-UTP was incorporated mainly into the dense fibrillar component; the fibrillar centers, in contrast, did not display significant labeling above background levels. This finding corroborates the view that the dense fibrillar component is the site of RNA transcription in the nucleolus.

Nucleolar silver staining patterns and HLA-DR antigen expression in bronchial epithelial cells in chronic bronchitis

Bronchial epithelial cells obtained by brush biopsy during fiberoptic bronchoscopy performed in 12 patients with chronic bronchitis and 12 healthy control subjects, were investigated for HLA-DR antigen expression and nucleolar silver staining patterns. In all patients with chronic bronchitis the number of bronchial epithelial cells positive to HLA-DR antigen was highly increased (> 90%), whereas in the controls only a few epithelial cells (< 10%) showed a weak HLA-DR antigen expression. Patients with chronic bronchitis showed an increased lymphocytic reaction compared to the control subjects. Both in the patients with chronic bronchitis and in the healthy controls the number of nucleoli was the same. The number of silver stained dots per nucleus was significantly higher in patients with chronic bronchitis than in the control subjects (7.70 +/- 0.87 as against 5.11 +/- 0.52; p < 0.0001). The intensity of the lymphocytic reaction correlated with the HLA-DR antigen expression and the increase in silver staining (Spearman's r = 0.543; p < 0.01). This indicates the influence of inflammation on the activation of epithelial cells derived from the respiratory tract.

Nucleolar morphology and rDNA in situ hybridisation in monocytes

The aim of this study was to correlate morphological changes of nucleoli of non-proliferating monocytes to their functional activity, since nucleolar morphology is currently considered as a diagnostic marker for cell proliferation. Monocytes from healthy donors were fractionated by current counterflow centrifugation and kept in culture for 6 days. Cells were stimulated by the addition of 200 units/ml interferon gamma (IFN gamma). Under this stimulus the monocytes show no proliferation but a strongly augmented expression of type I Fc IgG receptor, human leucocyte antigen DR, human leucocyte antigen DP and human leucocyte antigen DQ. Morphological changes after stimulation included the appearance of multinucleated cells, typical signs of the activation of rRNA synthesis indicated by an increase in nucleolar size, and changes in nucleolar structure such as the appearance of reticulate and compact nucleoli. The number of nucleolus organiser regions (NORs) visualised by in situ hybridisation was compared with the position and number of nucleoli visualised by silver staining in interphase cells. In comparison with control cultures, activated monocytes show a distinct increase in the number of those NORs that take part in the formation of nucleoli. Our results show that, in non-proliferating activated monocytes, the morphology of nucleoli and the increase of NOR activity are similar to those in proliferating cells. NOR activation is therefore an indicator for cellular activity, but is not necessarily correlated with proliferation.

Nucleolar silver staining patterns of lymphocytes in sarcoidosis

Bronchoalveolar lavage lymphocytes from 15 patients with pulmonary sarcoidosis and 8 healthy controls were investigated for nucleolar silver staining patterns and lymphocyte subpopulations. Patients with sarcoidosis had increased numbers of silver stained dots versus controls (2.20 +/- 0.24 versus 1.78 +/- 0.07; p less than 0.001). The number of silver stained dots showed the strongest positive correlation to helper cells (OKT 4+) (r = 0.781; p less than 0.0001). These results may be interpreted as further evidence of lymphocytic activation, especially of helper cells (OKT 4+) in pulmonary sarcoidosis.

Human ribosomal RNA gene repeats are localized in the dense fibrillar component of nucleoli: light and electron microscopic in situ hybridization in human Sertoli cells

The distribution of the human ribosomal gene repeat within human Sertoli cell nucleoli was investigated with the help of DNA-DNA in situ hybridization at the light and electron microscopic level. Probes from both the transcribed part of the gene repeat and the "non-transcribed" spacer were found to hybridize predominantly to the dense fibrillar component of nucleoli. It therefore can be concluded that the dense fibrillar component of nucleoli is the major site of the intranucleolar location of the ribosomal DNA. This holds true not only for the dense fibrillar component adjacent to fibrillar centers, but also for the dense fibrillar component remote from the fibrillar centers.

Nucleoli, nucleolar chromosomes and ribosomal genes in the human spermatocyte

The formation and development of nucleoli and their connections with the nucleolar chromosomes were studied in human spermatocytes using electron microscopy, silver staining of nucleolus organizer regions (NORs), high resolution autoradiography and in situ hybridization in order to localize rRNA genes and their transcription in the different stages of meiotic prophase I. At leptotene, new nucleoli were formed, consisting of a fibrillar centre surrounded by a cap of dense fibrillar component. Following [3H]uridine uptake, label was found only over the dense fibrillar component. In situ hybridization revealed rDNA mainly in the dense fibrillar component and in the chromatin. During zygotene, nucleoli increased in size. The fibrillar centre was connected with the secondary constriction region of the nucleolar bivalent and was partially surrounded by dense fibrillar component. This shell of dense fibrillar component merged into a fibrillo-granular mesh that extended away from the fibrillar centre. Autoradiography following [3H]uridine uptake again showed the label overlaying the dense fibrillar component and the proximal part of the fibrillo-granular strands. With in situ hybridization in both the light and electron microscope, signal was mainly found in the dense fibrillar component. A small quantity of label was observed in the peripheral region of the fibrillar centre and in the adjacent chromatin. From early to late pachytene segregation of nucleolar components occurred, with a reduction in the dense fibrillar component that formed a narrow rim around the fibrillar centre with small extensions along the granular component. [3H]uridine incorporation progressively decreased. In situ hybridization showed signal located mainly in the dense fibrillar component and in the chromatin corresponding to the condensed short arm of the nucleolar bivalent. Our results indicate that the majority of rDNA is located and transcribed in the dense fibrillar component; only a small amount is present in the peripheral part of the fibrillar centre and may be transcribed there. Moreover, from leptotene to zygotene, rDNA unravels from the nucleolar chromosome into the nucleolar dense fibrillar component. From zygotene to late pachytene a progressive return to the condensed acrocentric short arm is observed.

Hyaluronic acid influences the migration of myoblasts within the avian embryonic wing bud

Myoblasts migrate in a proximodistal direction within the avian embryonic wing bud during normal limb development. Since the presence and distribution of hyaluronic acid within the wing bud coincide with the time and with the direction of the migration of myoblasts, we microinjected hyaluronic acid into chicken wing buds that had received grafts containing quail myoblasts. It was found that injected hyaluronic acid has a strong positive effect on the migration of myoblasts: it causes a migration of myoblasts in donor-host combinations in which this is normally not the case, and it can cause migration in a proximal direction, a phenomenon not observed during normal development. From this it may be concluded that hyaluronic acid can influence myoblast migration in vivo. A similar effect could be observed after the microinjection of dextran sulfate, a synthetic compound having similar physicochemical properties. Hyaluronic acid, therefore, may play an important role in the control of the migration of myogenic cells in vivo by its physiocochemical properties.

Distribution and migration of angiogenic cells from grafted avascular intraembryonic mesoderm

The hemangiogenic potencies of initially avascular intra-embryonic mesoderm were studied in chick and quail embryos and in chick-quail chimeras. The prechordal mesoderm, primitive streak and primitive node of quail embryos were heterospecifically grafted into limb buds of chick embryos. Hemangiopoietic quail cells in the host limb were detected by immunohistological staining with the monoclonal anti-MB-1 antibody after 3-6 days of re-incubation. The antibody is specifically directed against quail hemangiopoietic cells and their derivatives. Quail endothelial cells were found in pure quail and in chimeric vessels, inside as well as outside the graft. The main artery of the limb and the vessels inside the graft were connected by chimeric arteries. Proximal to the graft, quail endothelial cells were located predominantly within the lining of the main artery, while distally they were found mainly in the veins and the marginal sinus. The results show that, as early as stage 3 (according to Hamburger and Hamilton 1951, HH) all parts of the avascular intraembryonic mesoderm tested, give rise to endothelial cells. Both mechanisms, angiogenesis and vasculogenesis, contribute to the vascularization of the limb. Immunocytological and scanning electron microscopic studies indicate that centrifugal and centripetal migration of angiogenic cells occurs outside the vessels as well as on the inner surface of the endothelium.

Muscle morphogenesis in the absence of myogenic cells

Experimental evidence indicates, that the myogenic cells themselves are not responsible for the muscle pattern formation. We report on a chance observation that reveals that muscle pattern formation can occur even in the absence of myogenic cells. Epiblastic cells from a quail embryo in the primitive streak stage were implanted into the wing bud of a chick embryo. The grafted quail cells developed into mononucleate, fibroblast-like cells that formed the muscle belly of the extensor medius longus muscle. This showed essentially normal form and topography as revealed by computer-aided 3D-reconstruction. This finding shows, that the formation of muscles does not depend on the presence of myogenic cells.

Differentiation of endothelial cells in avian embryos does not depend on gastrulation

Unincubated quail eggs were treated with Cytochalasin B. By this means, gastrulation of the blastodiscs was inhibited. Fragments of these blastodiscs were grafted into wings buds of chick embryos, and the differentiation fate of graft-derived cells was studied. Results show that only endothelial cells differentiate from the grafts. They were even found outside the graft site in vessels made up of a chimeric endothelium. It can be concluded that determination, differentiation and migration of endothelial cells does not depend on gastrulation.

Transcribed and nontranscribed parts of the human ribosomal gene repeat show a similar pattern of distribution in nucleoli

The distribution pattern of the transcribed and nontranscribed parts of human ribosomal RNA genes were visualized simultaneously in the same cells by nonautoradiographic in situ hybridization. DNA probes labeled with either digoxigenin or biotin were detected in the same cells by different fluorescence systems. The signals from both the transcribed and nontranscribed parts showed a similar distribution pattern. This finding is not compatible with the conclusion, suggested by earlier studies, that the transcribed and nontranscribed parts of the rRNA genes are located at different sites within the nucleoli or in different nucleolar components.

The orientation of the wing mesenchyme influences the direction of the migration of myoblasts in the avian embryonic wing bud

In order to analyze the influence of the orientation of the wing bud mesenchyme on the proximodistal direction of the migration of myoblasts in the avian embryonic wing bud, blocks of wing-bud mesenchyme were cut out and rotated around a dorso-palmar axis through 90 degrees or 180 degrees. Tissues originating from the quail wing bud and containing myoblasts were grafted into the space between the wing mesenchyme and the rotated blocks of mesenchyme proximal to the latter. In all experiments the donor-embryos were older than the acceptor-embryos. From HH stage 24 on, the rotation of the block of mesenchyme inhibited the migration of the myoblasts in a distal direction. We therefore propose that the orientation of the wing bud mesenchyme has an influence on the migratory behavior of myoblasts. This influence could provide "directional information" for the migrating myoblasts, allowing the migration of myoblasts in a distal direction only.

Electron microscopic in situ hybridization and autoradiography: localization and transcription of rDNA in human lymphocyte nucleoli

The distribution of ribosomal DNA (rDNA) in the nucleoli of human lymphocytes was revealed by in situ hybridization with a nonautoradiographic procedure at the electron microscopic level. rDNA is located in the dense fibrillar component of the nucleolus but not in the fibrillar centers. In the same cells the incorporation of tritiated uridine takes place in the dense fibrillar component of the nucleolus as seen by autoradiography followed by gold latensification. From these findings it can be concluded that the transcription of ribosomal DNA takes place in the dense fibrillar component of the nucleolus.

Nucleolus organizer regions in human lymphocytes as studied with premature chromosome condensation

Non-stimulated human lymphocytes from peripheral blood usually contain only one ring-shaped nucleolus. Polyethyleneglycol-mediated cell fusion with mitotic Chinese hamster ovary cells induces premature chromosome condensation in human lymphocytes. Subsequent silver staining reveals that more than one nucleolus organizer region (NOR) is silver-positive and frequently participates in the formation of "satellite associations". It can therefore be concluded that more than one NOR contributes to the ring-shaped nucleoli of lymphocytes in human peripheral blood and that they may be transcriptionally active. During phytohemagglutinin (PHA) stimulation, the number of silver-positive NORs, the number of nucleoli and the number of chromosomes participating in "satellite associations" increase.

Sequential changes in the nucleoli of human spermatogonia with special reference to rDNA location and transcription

The nucleoli of human spermatogonia were studied using electron microscopy, silver staining, radioautography and in situ hybridization. In all types of A spermatogonia, nucleoli were consistently located at the periphery of the nucleus and contained a single fibrillar center associated with the nuclear envelope. In B spermatogonia, nucleoli were centrally located in the nuclei and showed several fibrillar centers or were found to disintegrate. Nucleolar morphology was found to be a good, though not an unequivocal indicator of spermatogonial type. The observed changes in nucleolar morphology reflect the differentiation of spermatogonia: the nucleolar disintegration seen in B spermatogonia corresponds to a pre-leptotene cessation of rDNA transcription. In radioautographs following 3H-uridine uptake, the label was consistently found over the dense fibrillar component, except in the B spermatogonia with disintegrating nucleoli, where no uptake could be detected. In situ hybridization demonstrated that the distribution of rDNA did not correspond to the site of the fibrillar center but to the dense fibrillar component. Compared with radioautographs, this finding clearly established that transcribed units of rDNA were located in the dense fibrillar component. Silver staining was strongly positive in fibrillar centers and in the dense fibrillar component. In Ap spermatogonia the silver deposit was often localized at the edge of the fibrillar threads. The relationships between silver-stained proteins and transcribed and nontranscribed portions of ribosomal genes are reevaluated.

Ribosomal DNA is located and transcribed in the dense fibrillar component of human Sertoli cell nucleoli

The distribution of rDNA and the uptake of tritiated uridine was investigated in nucleoli of human Sertoli cells. The nucleolar components in these cells are spatially arranged in a highly ordered and invariable way and can be recognized in both light and electron microscopy. The pattern of distribution of rDNA and the pattern of uridine uptake in these nucleoli correspond to the distribution of the dense fibrillar component but cannot be correlated to the shape and size of the fibrillar centers in these cells. It therefore can be concluded that the dense fibrillar component, and not the fibrillar centers, is the site of rDNA location and transcription in nucleoli of human Sertoli cells.

On the origin of cells determined to form skeletal muscle in avian embryos

Pieces of quail embryos from various developmental stages ranging from unincubated blastoderms (before the appearance of a primitive streak) to embryos having formed somites were grafted to the wing buds or into the coelomic cavity of chicken embryos. The grafts, which can be identified on a cellular level by virtue of the prominent nucleolus-associated chromatin, present in the quail and absent in the chicken, were screened after suitable periods of reincubation for the presence or absence of skeletal myotubes containing quail nuclei. Grafts having contributed to such skeletal myotubes were considered as having contained determined myogenic cells at the time of the grafting procedure. Determined myogenic cells appeared first in the primitive streak and in the mesodermal cells formed by the invagination (gastrulation) of epiblastic cells through the primitive streak. This is true for both the head process and the paraxial mesoderm. Epiblastic cells never gave rise to skeletal myotubes. Therefore it can be said, that the onset of myogenic determination coincides with gastrulation. It remains, however, to be established, whether these two events are causally related to one another.

[Chimeras in biologic embryology]

Chimeras produced from amphibian, mammalian, and especially avian embryos have provided important insights into vertebrate development. Important contributions have led to new concepts in understanding the development of, for example, the nervous system, the vascular system, and the skeletal muscles. The migration of cells is particularly accessible in chimeras. More important results are to be expected from chimeras in the future, especially by combining this approach with other state-of-the-art techniques.

Structural changes in nucleoli during inhibition of protein- and RNA-biosynthesis

A variety of cells (unstimulated human lymphocytes, phytohemagglutinin-stimulated human lymphocytes, diploid human fibroblasts, human melanoma cells, and Hela cells) were subjected in vitro to inhibition of protein biosynthesis by puromycin. Hela cells were also treated with actinomycin D to inhibit RNA-synthesis. Under puromycin treatment, the fibrillar centers of the nucleoli were smaller in all actively dividing cell types, whereas in small inactive lymphocytes from peripheral blood the inhibition of protein synthesis had no noticeable effect. Nucleoli with nucleolonema changed into compact nucleoli under puromycin treatment. When RNA-synthesis was inhibited, the fibrillar centers remained at an approximately constant volume. These findings indicate that proteins localized in the fibrillar centers are involved in, and are used up during, rDNA-transcription and/or further steps of ribosome biogenesis. The changes in nucleolar architecture after the inhibition of protein synthesis suggest that transcriptional processes become concentrated near sites where proteins have been stored, i.e. the fibrillar centers.

On the position of nucleolus organizer regions (NORs) in interphase nuclei. Studies with a new, non-autoradiographic in situ hybridization method

The distribution of 18S and 28S ribosomal RNA (rRNA), i.e. the chromosomal nucleolus organizer regions (NORs) was visualized in interphases and metaphases of non-stimulated and phytohemagglutinin (PHA)-stimulated human lymphocytes with a recently developed non-autoradiographic in situ hybridization method. This procedure involves mercurated RNA as a probe and a sulfhydryl-trinitrophenyl-mercury binding ligand and FITC-labelled antibodies as detection system. Silver staining was used to visualize nucleoli in interphase. In the secondary constriction of all ten acrocentric chromosomes, varying amounts of rDNA were detected. In the interphase nuclei of most of the non-stimulated human lymphocytes, only one small nucleolus could be seen. The in situ hybridization, however, revealed several agglomerations of rDNA scattered over the whole nuclear area, clearly outnumbering the number of nucleoli in these cells. This means that not all of the NORs are transcriptionally active in non-stimulated lymphocytes and that these inactive NORs lie at a distinct distance from the active ones. With PHA stimulation (transforming the small lymphocytes from peripheral blood into large, lymphoblast-like cells) the number of nucleoli increased slightly, whereas the number of separable rDNA spots decreased. This means that in the course of PHA-induced cellular activation, formerly inactive NORs become transcriptionally active and tend to associate with one another. This indicates the occurrence of movements of the NORs within the nucleus, depending on their transcriptional activity.

On the migration of epidermal melanoblasts in the avian embryonic wing bud

After heterotopic grafting of quail neural crest cells to the wing buds of embryos of an unpigmented chicken strain, epidermal melanocytes of donor origin are found almost exclusively distal from the graft in the host's epidermis. This directed cell migration ceases, if the apical ectodermal ridge (together with a small amount of subridge mesoderm) is removed from the operated wing buds or if impermeable materials are interposed between it and the rest of the wing bud. Under these conditions epidermal melanocytes are found not only distal from but also proximal to the grafts. From this it may be deduced that the apical ectodermal ridge directs the migration of epidermal melanoblasts in the avian embryonic wing bud, possibly by a chemotactic mechanism. The presence or absence of the apical ectodermal ridge had no observable effect on the migratory behaviour of other neural crest derived cell populations (Schwann cells and non-epidermal melanocytes) in the wing bud. This shows that the apical ectodermal ridge specifically influences epidermal melanocytes.

On the differentiation and migration of some non-neuronal neural crest derived cell types

Neural tubes containing premigratory neural crest cells from head and trunk levels as well as somites containing neural crest cells that have migrated away from the neural crest were grafted orthotopically and heterotopically from quail embryos to chicken embryos. Schwann cells and melanocytes of donor origin developed after all grafting procedures. Cartilage developed only from neural crest cells of head levels. No skeletal muscle was ever observed to develop from the neural crest. The development of these different cell types from heterotopically grafted premigratory neural crest cells indicates that the neural crest is not a population of pluripotent undeterminated cells, but that at least some determinated cells are present within it before the onset of emigration of neural crest cells from the neural crest. Different neural-crest-derived cell populations exhibit different migratory behaviour: After heterotopically grafting quail neural crest cells to the wing buds of chicken embryos, Schwann cells and non-epidermal melanocytes were found to have migrated proximally and distally away from the grafts. Epidermal melanocytes of donor origin were found to have migrated in a distal direction essentially.

Ontogeny of avian extrinsic ocular muscles. I. A light- and electron-microscopic study

Light- and electron-microscopic studies were performed on those tissues that are supposed to deliver the anlagen of the extrinsic ocular muscles. Since the blastemata of the ocular muscles can be traced back into the prechordal mesoderm, it can be concluded that this tissue is the source of these muscles. In embryos from stage 8-10 according to Hamburger and Hamilton (HH) cells are found to detach from the lateral border of the prechordal mesoderm. These cells are assumed to give rise to the trochlearis and abducens musculature. In stage-14 embryos the paired premandibular cavity arises within the lateral wings of the prechordal mesenchyme. In 4-day embryos the lateral wall of each premandibular cavity becomes denser forming a premuscular mass, which is subdivided into the anlagen of the oculomotorius muscles in 5-day embryos. The head cavities are not homologous to somites because their structures, origins and sites are very different.

On the fusion of nucleoli in interphase

By comparing unstimulated and phytohaemagglutinine (PHA)-stimulated human lymphocytes from peripheral blood it was found that the fusion of nucleoli in interphase is not merely a passive phenomenon but is strongly correlated to the metabolic activity of cells. The fusion is independent of the cell cycle (DNA-replication cycle).

Changes in nucleolar structure, number and size in cellular activation and inactivation. Observations in human phytohaemagglutinin-treated lymphocytes

Lymphocytes in human peripheral blood are small, relatively inactive cells. The addition of phytohaemagglutinin (PHA) to cultures of these cells causes a marked increase in cellular and nuclear size, DNA-synthesis and metabolic activity, which reaches a maximum three days after the onset of culturing. The cells then undergo an inactivation process over a period of approximately ten days by which time they have reverted to cells resembling small, inactive lymphocytes. Within the first three days, nucleoli increase in size and number, changing from ring-shaped to nucleolonema-exhibiting to compact nucleoli. In the course of the inactivation process the nucleoli decrease in size and change from compact nucleoli directly into ring-shaped nucleoli. Thus activation and inactivation pathways are different. There is an increase in the number of nucleoli during the inactivation phase up to the seventh day in culture, followed by a slight decrease until day 14. This suggests that nucleoli in metabolically active cells have a tendency to fuse, whereas those in inactive cells tend to fragment.