Ring-shaped Nucleoli Research Articles

To the ring-shaped nucleolus seen by microscopy using human lymphocytes of blood donors and chronic lymphocytic leukemia patients.

The present study was undertaken to provide more information on the peripheral RNA containing ring of ringshaped nucleoli (RSNo). Human lymphocytes of blood donors and patients suffering from B chronic lymphocytic leukemia mostly characterized by RSNo represented very convenient cell models for such study. According to the light microscopy the peripheral RNA ring possessed several highly condensed foci. Such regions represented accumulated dense RNA fibrillar components (DFCs) seen by the electron microscopy. In contrary, the incidence of dense granular RNA-containing components (GCs) in surrounding portions of the RNA ring was small. Thus, the structural and morphological organization of the peripheral RNA ring of RSNo apparently reflects sites of micro-segregated foci of DFCs and a small incidence of GCs. That structural organization of the peripheral RNA ring of RSNo appeared to be a prerequisite for further regressive nucleolar changes resulting in the development of micronucleoli in terminal lymphocytes.

RNA Concentration and Content in the Nucleoli and Cytoplasmic Rim in Differentiating Lymphocytes of Patients Suffering from B Chronic Lymphocytic Leukaemia – a Cytochemical Note

Nucleolar RNA optical density (concentration) measurements at the single cell level indicated that differentiation of lymphocytes is accompanied by a slightly decreased nucleolar RNA concentration in contrast to the cytoplasmic rim around the nucleus. On the other hand, the nucleolar size was markedly reduced and the cytoplasmic rim surrounding the nucleus was reduced only weakly. Concerning the calculated rough estimate of the RNA content, the differentiation induced its larger decrease in the nucleoli than in the cytoplasmic rim. These observations indicated that the nucleolar RNA concentration and RNA content together with the nucleolar morphology are more sensitive markers of the differentiation process than the RNA concentration and content in the cytoplasm. Thus, the nucleolar RNA transfer to the cytoplasm in advanced differentiation steps might still be going on regardless of the decreasing or inhibited nucleolar biosynthetic activity. In addition, the presence of ring-shaped nucleoli and micronucleoli characteristic of mature and terminal lymphocytes in some lymphocytic less differentiated steps, i.e., lymphoblasts and prolymphocytes, might indicate the premature differentiation state of such cells.

Cytotoxic effects ofSatureja montanaL. essential oil on oocytes of engorgedRhipicephalus microplusfemale ticks (Acari: Ixodidae)

In addition to pesticidal activity, the capacity of natural compounds to inhibit the reproduction of parasites emerge as an important alternative tick control method. In this context, Satureja spp. stand out due to their recognized pesticidal properties. Among parasites of veterinary importance, the cattle tick, Rhipicephalus microplus, is responsible for great economic losses in livestock and transmission of relevant pathogens. Thus, the aim of this study was to evaluate the effects of different concentrations of the essential oil of Satureja montana L. on the ovary morphology of R. microplus engorged females through histological and histochemical techniques. The most remarkable morphological changes found were: cytoplasmic vacuolation of germ cells, irregular and thicker chorion, irregular oocyte shape, ring-shaped nucleolus, decrease in protein and carbohydrate content in oocytes, in addition to cellular changes in the oviduct and pedicel. All morphological changes were assessed using a semiquantitative method already established in the literature. Ticks exposed to 5.0 μl/ml of essential oil showed the most significant changes when compared to control groups. Thus, the essential oil of S. montana L. damaged the reproductive system of R. microplus, which may impair ticks' offspring production and promote a long-term control of this species.

Sublethal concentrations of acetylcarvacrol strongly impact oocyte development of engorged female cattle ticks Rhipicephalus microplus (Canestrini, 1888) (Acari: Ixodidae)

Rhipicephalus microplus, commonly known as southern cattle tick, causes huge economic losses in the cattle industry. Its infestation affects the production of meat and milk and causes discomfort to hosts. In addition, it is the vector of Babesia spp. and Anaplasma spp. The most frequent method used to control these parasites is through synthetic acaricides. However, their indiscriminate use can be toxic for hosts and environment as well as cause selection of resistant ticks. Plant extracts and essential oils emerge as promising alternatives to manage tick infestation. Carvacrol, an aromatic monoterpene extracted from plants, has recognized antimicrobial, antioxidant, insecticidal, repellent and acaricidal activities. Acetylation of carvacrol is believed to enhance its nematicidal and acaricidal activities and to decrease its toxicity to hosts. Thus, the aim of this study was to evaluate the effect of different concentrations of acetylcarvacrol in the morphology of ovaries of engorged R. microplus ticks. The most remarkable morphological alterations found in the female germ cells were irregular and thicker chorion, decreasing in size and irregular shape of female germ cells (oocytes), cytoplasmatic vacuolization as well as ring-shaped nucleoli. These alterations were analyzed through a semi-quantitative method proposed in this study for ixodid ticks. Treatment group IV, which was exposed to 4.5 μL/mL of carvacrol acetate, showed the most significant alterations, and it was also statistically different when compared to control groups. Therefore, sublethal concentrations of acetylcarvacrol demonstrated to impact the reproductive system of R. microplus by causing several damages in the female germ cells. This would hinder the generation of new individuals, probably contributing for a long-term control of tick infestation.

Ultrastructural and Molecular Analyses Reveal Enhanced Nucleolar Activity in Medicago truncatula Cells Overexpressing the MtTdp2α Gene.

The role of tyrosyl-DNA phosphodiesterase 2 (Tdp2) involved in the repair of 5′-end-blocking DNA lesions is still poorly explored in plants. To gain novel insights, Medicago truncatula suspension cultures overexpressing the MtTdp2α gene (Tdp2α-13C and Tdp2α-28 lines, respectively) and a control (CTRL) line carrying the empty vector were investigated. Transmission electron microscopy (TEM) revealed enlarged nucleoli (up to 44% expansion of the area, compared to CTRL), the presence of nucleolar vacuoles, increased frequency of multinucleolate cells (up to 4.3-fold compared to CTRL) and reduced number of ring-shaped nucleoli in Tdp2α-13C and Tdp2α-28 lines. Ultrastructural data suggesting for enhanced nucleolar activity in MtTdp2α-overexpressing lines were integrated with results from bromouridine incorporation. The latter revealed an increase of labeled transcripts in both Tdp2α-13C and Tdp2α-28 cells, within the nucleolus and in the extra-nucleolar region. MtTdp2α-overexpressing cells showed tolerance to etoposide, a selective inhibitor of DNA topoisomerase II, as evidenced by DNA diffusion assay. TEM analysis revealed etoposide-induced rearrangements within the nucleolus, resembling the nucleolar caps observed in animal cells under transcription impairment. Based on these findings it is evident that MtTdp2α-overexpression enhances nucleolar activity in plant cells.

Ultrastructural Analysis of the Oocytes of Female Rhipicephalus sanguineus (Latreille, 1806) (Acari: Ixodidae) Ticks Subjected to the Action of Azadirachta indica A. Juss (Neem)

The present study provides ultrastructural information about the acaricidal effects of neem (Azadirachta indica A. Juss) on the ovaries of R. sanguineus engorged females. In general, the main damage caused in the oocytes was alteration in the shape of the cell and of the germinal vesicle, ring-shaped nucleolus, cytoplasmic vacuolation, and disorganization of the organelles and of the cell membranes (including the chorion), all of which indicate that these cells could be in the process of death. The results showed that azadirachtin would be an efficient acaricide agent for inhibiting and/or neutralizing the reproduction process of R. sanguineus females, impairing the reproductive ability of this species.

Nucleolar Changes and Fibrillarin Redistribution Following Apatone Treatment of Human Bladder Carcinoma Cells

Ascorbate and menadione (Apatone) in a ratio of 100:1 kills tumor cells by autoschizis. In this study, vitamin-induced changes in nucleolar structure were evaluated as markers of autoschizis. Human bladder carcinoma (T24) cells were overlain with vitamins or with culture medium. Supernatants were removed at 1-hr intervals from 1 to 4 hr, and the cells were washed with PBS and prepared for assay. Apatone produced marked alterations in nucleolar structure including redistribution of nucleolar components, formation of ring-shaped nucleoli, condensation and increase of the proportion of perinucleolar chromatin, and the enlargement of nucleolar fibrillar centers. Immunogold labeling of the nucleolar rRNA revealed a granular localization in treated and sham-treated cells, and immunogold labeling of the rDNA revealed a shift from the fibrillar centers to the condensed perinucleolar chromatin. Fibrillarin staining shifted from the fibrillar centers and adjacent regions to a more homogeneous staining of the entire nucleolus and was consistent with the percentage of autoschizic cells detected by flow cytometry. Because autoschizis entails sequential reactivation of DNase I and DNase II, and because the fibrillarin redistribution following DNase I and Apatone treatment is identical, it appears that the nucleolar and fibrillarin changes are markers of autoschizis.

Verticillium dahliae toxin induced alterations of cytoskeletons and nucleoli in Arabidopsis thaliana suspension cells

In plant cells, cytoskeletons play important roles in response to biotic and abiotic stresses. However, little is known about the dynamics of cytoskeletons when cells are attacked by unphysical stress factors such as elicitors and toxins. We report here that the toxin of Verticillium dahliae (VD toxin) induced changes of microfilaments (MFs) and microtubules (MTs) in Arabidopsis thaliana suspension-cultured cells. When cells were treated with a low concentration of VD toxin, MFs were disrupted ordinally from the cortex to the perinuclear region, and then recovered spontaneously; but the MTs persisted. The MFs in the perinuclear region showed more resistance to VD toxin than the cortical ones. In contrast, when cells were treated with a high concentration of VD toxin, MFs and MTs were disrupted sooner and more severely and did not recover spontaneously. Treatments with high concentrations of VD toxin also induced changes of nucleoli. At the early stages of treatment, a nucleus had a single ring-shaped nucleolus. At the later stages, multiple smaller and more brightly fluorescing nucleoli emerged in a single nucleus. Disrupted MFs could be recovered by removing the VD toxin before the ring-shaped nucleoli appeared. All these results showed that MFs and MTs play important roles in the early defense responses against VD toxin in Arabidopsis suspension cells. The cytoskeletons may be used as sensors and effectors monitoring the defense reactions. The changes of nucleoli induced by VD toxin should be important characteristics of cell death.

Nucleolar asynchrony observed in HL-60 leukemic granulocytic precursors resistant to 5-aminolaevulinic acid-based photodynamic treatment

To provide more information on the 5-aminolaevulinic acid (ALA)-induced photodynamic effect on nucleoli, morphologically expressed nucleolar asynchrony (the presence of ‘active’ large nucleoli with an uniform distribution of RNA and ‘resting’ ring-shaped nucleoli in one and the same nucleus) was studied in cultured HL-60 leukemic granulocytic precursors using a simple cytochemical procedure for the demonstration of RNA. Nucleolar asynchrony was mainly expressed in cells which were apparently resistant to ALA-based photodynamic treatment (PDT) since most of them (about 75%) exhibited this phenomenon.

The silver reaction of nucleolar proteins in the main structural compartments of ring-shaped nucleoli in smear preparations

The present study was undertaken to provide more information on the conditions which result in preferential silver staining of the main nucleolar structural compartments using silver stainable proteins as their markers at the light microscopic level. For this study the mostly used method in cytology and pathology in which the nucleolar silver-positive structures are "developed" with the colloidal developer (Howell and Black, 1980; Ploton et al., 1986) was selected as silver reaction. Ring-shaped nucleoli of mature human lymphocytes represent a convenient model for such a study because they consist of one large fibrillar center, adjacent nucleolar regions with dense fibrillar components and the nucleolar peripheral shell with dense granular components. All these nucleolar compartments are known to possess characteristic silver stainable proteins. The results demonstrated that proteins of the fibrillar center and possibly adjacent nucleolar regions reacted preferentially with silver after a relatively long fixation with formaldehyde or methanol in unwashed specimens before the silver reaction. In contrast, the preferential staining of proteins in the nucleolar peripheral shell with silver was achieved after the fixation with acidified methanol or ethanol as well as after short fixation with formaldehyde vapors. In addition, the commonly used fixation before the silver reaction are not necessary and may be omitted for the visualization of all silver stainable proteins present in the fibrillar center as well as in the adjacent nucleolar regions and the nucleolar peripheral shell. In addition, similar results were achieved for the simultaneous visualization of proteins in the fibrillar center and nucleolar peripheral shell after fixation with ethanol.

Increased functional load on mouse kidney proximal tubule epithelial cells causes changes in nucleolar 3-D architecture.

Ultrastructural 3-D analysis of nucleolar architecture and Ag-NOR protein distribution in mouse kidney-cortex proximal-tubule epithelium has been performed. A principal scheme of structural changes of the nucleolus and organization of its components during the intensification of pre-rRNA synthesis (dynamic model of a nucleolus) based on computer spatial modelling has been advanced. According to the nucleolar composition, three groups of cells, which differ from each other by rRNA synthesis, are defined in normal kidney. Most nephron proximal-section cells (about 52%) are characterized by lower activity of RNA synthesis. Such kind of cells are defined as group I (nucleolar diameter 0.7-1.5 microm) and always contain resting, ring-shaped or close to ring-shaped dense nucleoli, which have 2 or 3 fibrillar centers. Nucleoli of group II cells (about 37%, nucleolar diameter 1.5-2.5 microm) have a higher level of activity, contain 4-7 fibrillar centers, and their structural organization is close to reticulated forms due to the first indications of vacuolar network (identified as prereticulated nucleoli). The most active cells of group III (about 11%, nucleolar diameter 2.5-3.5 microm) include cells with typical reticulated nucleoli with a well expressed vacuolar network and numerous fibrillar centers (18-22). Increased functional load of the epithelium caused by unilateral nephrectomy and diuretic (4-chlor-H [2-furylmethyl] 5-sulphamyl-antranic acid) injection changed the proportion of the different cell groups: group I decreased (about 25%), whereas groups II and III increased (about 8% and 17%, respectively). The increase of nucleolar activity first causes a deformation of the individual fibrillar centers as well as complication and growth of their surface. Further, a progressive fragmentation of the fibrillar centers and the growth of their total volume is observed. The complication and growth of the total volume of Ag-positive zones is another indication of the nucleolar activation. The vacuolar system develops by a gradual fusion of small isolated cavities into a united vacuolar network. Nucleoli with 2-7 fibrillar centers are considered to be intermediate forms reflecting successive stages of its activation or inactivation: from the resting ring-shaped nucleolus via transient stages of increasing functional activity to the active reticulated nucleoli and vice versa. The observed differences in the nucleolar ultrastructure are regarded as evidence of the functional heterogeneity of cell populations within one functional segment of nephron.

Development of nucleolar apparatus in the chick primitive erythroid cells.

The primitive erythroid line cells of chick embryos were studied during embryonic days 2-14 by means of a cytochemical method to investigate the appearance and frequency of the main nucleolar types. The populations of erythroblasts and erythrocytes were classified according to the presence of functionally dominant nucleoli in their nuclei. In the course of primitive erythroid cell differentiation and maturation, compact nucleoli and nucleoli with nucleolonemas (both supposed to be RNA biosynthetically active) were gradually replaced by ring-shaped nucleoli and finally by micronucleoli reflecting the reversible and irreversible inhibition of RNA synthesis, respectively. The occurrence of the main nucleolar types and their values in primitive erythroid cells of the developing chick depend not only on the maturation stage of the blood cells, but also on the developmental stage of the chick embryo. In comparison with the definitive erythroid line of the post-hatching chick and hen, the cells of the chick embryonic primitive erythroid line possess relatively high values of "active" nucleolar types. These are still present in advanced maturation stages, and occur also as definitive erythroid lines of lower vertebrates.

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)

Ultrastructural changes of tracheal epithelial cells of chicks experimentally infected withCryptosporidiumsp.

The ultrastructural changes in the tracheal epithelial cells of chicks with experimental cryptosporidiosis were examined. There was deciliation and formation of compound cilia whereas the microvilli showed Y-shaped formation, atrophy or blunting, fusion and branching. The basal bodies and striated rootlets were irregular and sometimes wanting while there was an increase in centrioles beneath the cilia. Nuclear atypia was common whereas the nucleoli were hypertrophic with reticular nucleolonema. Double nucleoli were mostly reticular and rarely ring-shaped or of thin-shelled type. There was an increase in well-developed rough endoplasmic reticulum and Golgi complex, ribosomes, coated vesicles, smooth-membraned vacuoles, mitochondria and mitochondrial-RER associations. The smooth and rough endoplasmic reticula had dilated cisternae. Nuclear bodies, triple nucleoli, nucleolar margination and segregation, ring-shaped and thin-shelled nucleoli, cytoplasmic imagination, autolysosomes, multivesicular bodies, electron-dense fibres and vesicular rough endoplasmic reticulum were rare. Cryptosporidium sp. attached even to the goblet cells, which confirmed that the microvilli played an important role in the attachment process of the parasite. The surface changes of the cilia and microvilli may denote either an adaptation or compensatory response to cryptosporidiosis. The major changes, however, indicated that the tracheal epithelial cells have the same capacity to increase their protein synthesis as the chicken bursae, in response to the parasitism.

Ultrastructural Distribution of DNA within the Ring-Shaped Nucleolus of Human Resting T Lymphocytes

The precise distribution of DNA within the resting human T lymphocyte nucleolus has been investigated, at the ultrastructural level, by cytochemical and molecular immunocytochemical techniques. The nucleolus is partially enveloped by a layer of condensed chromatin which, at several places, penetrates into the nucleolar body until in close contact with the fibrillar centers. Morphometric analysis reveals that 32% of the fibrillar center surface is essentially occupied by condensed chromatin. Using the in situ terminal deoxynucleotidyl transferase-immunogold procedure for detecting DNA, we further show that evident label is exclusively found over the condensed chromatin and over the fibrillar centers, whereas no significant label is detected over the dense fibrillar component of the nucleolus.

Ultrastructural changes of chick bursal epithelial cells experimentally infected with Cryptosporidium sp.

The ultrastructural changes in the bursal epithelial cells of chickens infected with Cryptosporidium sp. were investigated. The principal features were as follows: deep invaginations or indentations of the nucleus with separate lobulations in some instances, nucleolar hypertrophy with formation of reticular or open nucleolonema, increase or swelling of mitochondria, increase in coated vesicles, ribosomes and well-developed rough endoplasmic reticulum (RER), increase in mitochondrial-RER association, appearance of small, smooth-membraned vacuoles in the apical cytoplasm, and formation of electron dense fibres. In addition, nuclear bodies, thin-shelled or ring-shaped nucleoli, nucleolar segregation, nucleolar margination, ribo-some-lamella complex, microtubuloreticular complex, disaggregation of polysomes and vesiculation of RER were rarely observed. Most of the ultrastructural changes observed here seemed to be related to a heightened metabolic activity, namely active or increased protein synthesis, in response to the parasitism.

Morphological effects of cadmium on proximal tubular cells in rats.

Twenty-four male rats of the Wistar strain divided into four groups were injected s.c. with a dose of 0.8, 1.5, and 3.0 mg Cd/kg body wt as CdCl2 in saline, and saline alone to the control rats, three times a week for 3 wk. Cadmium levels of whole kidney homogenate, supernatant (cytosol), precipitate, and metallothionein (MT) fraction were measured. Histological changes of the renal proximal tubules were investigated by optical and electron microscopy. In the kidneys, Cd levels were increased with the increment of Cd dosage; 80-90% of Cd was contained in cytosol, and 55-75% was in MT fraction. Non-MT-Cd reached a maximum in the 1.5 mg Cd group, whereas that of the 3.0 mg Cd group showed some decline. With increasing Cd doses, the size of nuclei and nucleoli in the cells of proximal tubule showed significant enlargement and also an increase in the number of nucleoli on light microscopy. At higher doses, chromatin condensation of the tubular nuclei and vacuolar degeneration of the tubular cells were evident. On electron microscopy, perichromatin granules of the proximal tubular nuclei were increased in number, especially in the rats of Cd 0.8 mg and 1.5 mg/kg groups. As the Cd doses increased, ring-shaped nucleoli were increased in number and nucleolar segregation was observed more clearly. Moreover, in the 3.0 mg/kg Cd group, nuclear indentation and nucleoli containing compact dense granules were observed. In the cytoplasm, there was an increase of lysosomes, myelin bodies, ring-shaped mitochondria, and vesiculation; ultimate changes were degeneration and cell necrosis. The injured cells were heterogenously distributed in each nephron and this heterogeneity was attributed in the difference in Cd content and cell cycle in each cell of the nephron.

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.

Primary culture of axolotl spinal cord ependymal cells

In order to examine the role of ependymal cells in the spinal cord regeneration of urodele amphibians, procedures were established to identify and culture these cells. Cell isolation and culture conditions were determined for ependymal cells from larval and adult axolotls ( Ambystoma mexicanum). Dissociated cells prepared from intact spinal cords were cultured on fibronectin- or laminin-coated dishes. Dissociated cells attached more rapidly to fibronectin, but attached and spread on both fibronectin and laminin. Essentially pure populations of ependymal cells were obtained by removing 2 week old ependymal outgrowth from lesion sites of adult spinal cords. These ependymal outgrowths attached and grew only on fibronectin-coated dishes. Growth and trophic factors were tested to formulate a medium that would support ependymal cell proliferation. The necessary peptide hormones were PDGF, EGF, and insulin. TGF-beta 1 affected the organization of cell outgrowth. Initially, longterm culture required the presence of high levels of axolotl serum. Addition of purified bovine hemaglobin in the culture medium reduced the serum requirement. Outgrowth from expiants was subcultured by transferring groups of cells. Intrinsic markers were used to identify ependymal cells in culture. The ependymal cells have characteristic ring-shaped nucleoli in both intact axolotl spinal cords and in culture. Indirect immunofluorescence examination of intermediate filaments showed that ependymal cells were glial fibrillary acidic protein (GFAP) negative and vimentin positive in culture. Identification of dividing cells was made using 3H-thymidine incorporation and autoradiography, and by the presence of mitotic figures in the cultured cells

Development of the nucleolus in early goat embryos.

In vivo nucleologenesis was studied in goat embryos from the pronuclear stage to the blastocyst stage by light and electron microscopy. Ultrastructural changes of the nucleoli were characterized by the following progression: homogeneous electron-dense fibrillar primary nucleoli in the pronucleus; small, dense fibrillar masses dispersed in clusters of chromatin at the two-cell stage; ring-shaped nucleoli made up of a fibrillar center surrounded by a layer of dense fibrillar components at the four-cell stage; reticulated nucleoli composed of a three-dimensional network of fibrillar components surrounded by small amounts of granular components at the eight-cell stage; fully developed compact-type nucleoli consisting of several fibrillar centers each surrounded by a layer of fibrillar components and abundant granular components in morulae and blastocysts. Moreover, it was concluded that activation of rRNA transcription, as evidenced by specific silver nitrate staining of the nucleolus organizer regions of metaphase chromosomes, occurs at the two- to four-cell stage and that the morphological changes accompanied a substantial increase in nucleolar transcriptional activity up to the blastocyst stage. This study provides evidence that a structure-function relationship exists during nucleologenesis in goat embryos.