Showing papers on "Nucleolus published in 1983"
TL;DR: Silver staining during mitosis closely paralleled the distribution of protein C23, supporting previous conclusions thatprotein C23 is a silver staining nucleolus organizer region (NOR) protein.
225 citations
TL;DR: It is the opinion that at the present state most of the observations on NORs and nucleoli can be interpreted so as to achieve a general understanding of the nucleolus.
Abstract: In the last few years numerous papers have been published on the nucleolus and its relation to the nucleolus organizer region (NOR). Several, even very recent, publications still reveal differences in the interpretation of these results. It is, however, our opinion that at the present state most of the observations on NORs and nucleoli can be interpreted so as to achieve a general understanding of the nucleolus. For this purpose we shall summarize some morphological results and problems with particular reference to the example of resting and stimulated lymphocytes. This may supplement recent reviews covering molecular aspects (Perry 1981; Moss and Birnstiel 1982), the cytological visualization of nucleolar genetic activity (Miller 1981; Scheer et al. 1982), the amplification of ribosomal genes (Macgregor 1982), problems of nucleolus formation in plants (Flavell and Martini 1982; de la Torre and Gimenez-Martin 1982), as well as reviews dealing mostly with the functional morphology of nucleoli in vertebrate and also human cells (Goessens and Lepoint 1979; Jordan and McGovern 1981; Bouteille et al. 1982; Stahl 1982). A few notes on the history of the early work on nucleoli are given in the beginning, supplementing the introduction to the review by Miller (1981), since we feel that the very careful observations of the early cytologists should be remembered and reread, if not for their face value, then at least as an example of exact descriptive morphology. Needless to say, this short review article can give only a small selection of the many papers on our topic and will be far from complete.
127 citations
TL;DR: A rabbit antiserum has been prepared using as antigen the 110,000- dalton mammalian heat-shock protein, which was purified for injection by two-dimensional PAGE of heat-shocked Chinese hamster ovary cells and shows association with nucleoli which may have implications in the regulatory aspects of the heat- shock response.
Abstract: A rabbit antiserum has been prepared using as antigen the 110,000-dalton mammalian heat-shock protein. This protein was purified for injection by two-dimensional PAGE of heat-shocked Chinese hamster ovary cells. Characterization by immunoautoradiography and immunoprecipitation reveals that the antiserum is specific for the 110,000-dalton protein. Both techniques also reveal that the protein against which the antiserum is directed is induced by heat shock. Indirect immunofluorescence shows that the antigen is primarily localized at or near the nucleolus in cultured cells and numerous murine tissues. Treatment of cultured cells with deoxyribonuclease destroys the organization of staining within the nucleus while ribonuclease appears to completely release the antigen from the nucleus. A binding of the antiserum to cytoplasmic structures is also observed by immunofluorescence. This association with nucleoli may have implications in the regulatory aspects of the heat-shock response.
105 citations
92 citations
TL;DR: Most cells in lymph node smears of lymphocytic non-Hodgkin's malignant lymphoma (NHML) developed a bright nucleolar fluorescence with HMTNA antibodies and PCNA was detected in nucleoli of a limited number of cells which apparently represent the proliferating cell population in these lymphomas.
Abstract: Lymphoma (Lymphocytic non-Hodgkin's malignant lymphoma) and leukemic (chronic lymphocytic, acute and chronic myeloid, myelomonocytic leukemia) cells were studied by indirect immunofluorescence to evaluate the presence of proliferating cell nuclear antigen (PCNA) and human malignant tumor nuclear antigen (HMTNA) in their nucleoli. Most cells in lymph node smears of lymphocytic non-Hodgkin's malignant lymphoma (NHML) developed a bright nucleolar fluorescence with HMTNA antibodies. PCNA was detected in nucleoli of a limited number of cells which apparently represent the proliferating cell population in these lymphomas. Similarly, in the bone marrow smears of patients with chronic lymphocytic leukemia most cells possessed a nucleolar fluorescence for HMTNA and PCNA was present in nucleoli of a limited number of cells. In the bone marrow smears of patients with myeloid or myelomonocytic leukemias most blastic or monocytoid cells also developed a bright nucleolar flurescence with HMTNA antibodies and PCNA was present only in a small percentage of these cells. Leukemic cells with PCNA in their nucleoli like thekhuntigen might represent a proliferating cell population in late G1 — early S phase.
64 citations
TL;DR: The earliest responses of cellular RNA metabolism to adenovirus infection were, however, the rapid and apparently coordinate reductions in the levels of newly synthesized 28S rRNA and cellular mRNA sequences entering the cytoplasm.
Abstract: The production of cytoplasmic and nucleolar rRNA species was examined in HeLa cells infected with high multiplicities of adenovirus type 5. Both 28S and 18S rRNA newly synthesized in infected cells ceased to enter the cytoplasm as reported previously (N. Ledinko, Virology 49: 79-89, 1972; H. J. Raskas, D. C. Thomas, and M. Green, Virology 40: 893-902, 1970). However, the effects on 28S cytoplasmic rRNA were observed considerably earlier in the infectious cycle than those on 18S rRNA. The inhibition of cellular protein synthesis and of the appearance in the cytoplasm of labeled cellular mRNA sequences (G. A. Beltz and S. J. Flint, J. Mol. Biol. 131: 353-373, 1979) were also monitored in infected cultures. During the later periods of an infectious cycle, from 18 h after infection, nucleolar rRNA synthesis and processing and exit of 18S rRNA from the nucleus were inhibited, probably reflecting the failure of infected cells to synthesize normal quantities of ribosomal proteins. The earliest responses of cellular RNA metabolism to adenovirus infection were, however, the rapid and apparently coordinate reductions in the levels of newly synthesized 28S rRNA and cellular mRNA sequences entering the cytoplasm.
60 citations
Journal Article•
TL;DR: A proliferating cell nuclear antigen (PCNA) was identified with autoantibodies from a patient with systemic lupus erythematosus and very weak or negative nucleolar fluorescence was observed in several normal human tissues including liver, kidney, prostate, and cheek cells.
Abstract: A proliferating cell nuclear antigen (PCNA) was identified with autoantibodies from a patient with systemic lupus erythematosus. Specific antibodies were purified by affinity chromatography in which Novikoff hepatoma nucleolar proteins were conjugated to Sepharose-4B. The purified anti-PCNA antibodies produced bright nucleolar fluorescence in tumor cells as shown by indirect immunofluorescence. PCNA was found in nucleoli of human cell lines, HeLa, Hep-2, and Namalwa, and a solid human renal and a prostate carcinoma. Both strong and weak nucleolar fluorescence areas were found in the renal and prostate carcinoma indicating that there are varying degrees of proliferation among tumor cells. Two human colon carcinoma cell lines, omega (an aggressive, fast-growing clone of human colon carcinoma cell line HCT 116) and CBS [a slow-growing human colon carcinoma cell line (group 3)], with different growth rates were compared. The fast-growing colon carcinoma cells, omega, exhibited a higher percentage of nucleolar fluorescence (28.5%) than that of the slow growing colon cells (13.6%). By enzyme-linked immunosorbent assays, the omega cell extract had a higher PCNA antigen content (2.8-fold) than that of the CBS cell extract which, in turn, was higher than that of human liver extract. PCNA was also found in a human fetal lung fibroblast cell line (IMR-90). Very weak or negative nucleolar fluorescence was observed in several normal human tissues including liver, kidney, prostate, and cheek cells. Nucleolar fluorescence was also observed in rat Novikoff hepatoma cells. Although normal rat livers do not have PCNA nucleolar fluorescence, nuclear and nucleolar fluorescence were observed at 18 hr after partial hepatectomy.
54 citations
TL;DR: In this paper, the structure of ribosomal chromatin in differentiating cells was examined using essentially the same methods, and the combined results suggest that whereas regions that are not transcribed are packaged with proteins that protect them from nuclease digestion, actively transcribing riboomal genes are associated with few macromolecular constituents apart from those required for transcription and its regulation.
52 citations
TL;DR: A 100 kDa nucleolar protein which is transitorely associated with preribosomes in the nucleoli of Chinese hamster ovary cells has been found to be specifically cleaved by a thiol protease.
Abstract: A 100 kDa nucleolar protein which is transitorely associated with preribosomes in the nucleoli of Chinese hamster ovary cells has been found to be specifically cleaved by a thiol protease During an ‘in vitro’ incubation of nucleoli, the 100 kDa protein is processed into eight different proteins which are detected by immunoreaction with a serum raised against the 100 kDa protein qualitative and quantitative variations in the maturation products of the 100 kDa protein are obtained by ‘in vitro’ incubation of the 60S and 80S preribosomes The 100 kDa protein has been purified to homogeneity with the protease activity still associated The properties of the enzyme are described and its role in the maturation of preribosomes is discussed
52 citations
TL;DR: It is found that, in sections selectively stained for DNA, the intranucleolar chromatin composed of extended, nonnucleosomal DNA filaments formed roundish agglomerates with a spatial distribution which was superimposable on that of the fibrillar centers and the RNP fibrillsar component around them and on sites of the silver reaction in samples selectivelystained for Ag-NOR proteins.
49 citations
TL;DR: The effects of phosphorylation on the function of proteins are not completely understood although some data are available as mentioned in this paper, such as the level of ribosomal DNA transcription and specific proteolytic cleavage.
Journal Article•
TL;DR: These immunolocalization findings support the conclusion based on fractionation studies that the cylindershaped particles and their protein constituents occur both in the nucleoplasm and the cytoplasm of a broad range of cell types.
TL;DR: Aflatoxin B1, after metabolic activation either in vivo or in vitro, binds preferentially to the physiologically active regions of rat liver nucleolar chromatin, and that this binding specificity is largely lost after the removal of chromosomal proteins from the nucleoli.
Abstract: The literature on various isolated carcinogen-DNA adducts indicates clearly that the binding of chemical carcinogens to DNA is highly specific. Since DNA in eukaryotic cells is complexed with chromosomal proteins and organized into transcriptionally active and inactive chromatin, chemical carcinogens also might show binding specificities at the chromatin level. Using Escherichia coli RNA polymerase and the endogenous engaged RNA polymerase I as specific probes to monitor respectively the physiologically inactive and active nucleolar chromatin template function, this paper reports that aflatoxin B1, after metabolic activation either in vivo or in vitro, binds preferentially to the physiologically active regions of rat liver nucleolar chromatin, and that this binding specificity is largely lost after the removal of chromosomal proteins from the nucleoli.
TL;DR: The chapter discusses the morphology of viral transcription and provides an example to demonstrate the kind of information provided by the use of the loosening procedure in combination with conventional fixation and the spreading technique.
Abstract: Publisher Summary This chapter discusses the morphology of transcription at the cellular and molecular levels. The visualization of RNA synthesis at the molecular level is possible by the use of suitable cell systems displaying a high degree of transcription, such as oocytes of amphibian or of insects, spermatocytes, and embryos of insects or nuclei of Acetabularia and other algae. The chapter emphasizes the extensive variation among cells in the patterns of transcription that are observed despite the use of almost identical experimental conditions. The localization of the sites of transcription and RNA processing within the cell nucleus have been well documented, and it is now possible to correlate the appearance of in situ ribonucleoprotein (RNP) structures—such as perichromatin granules and fibrils—to variations of RNA metabolism. Numerous electron microscopic studies of eukaryotic cells have shown that ribosomal RNA synthesis is localized in the fibrillar component of the nucleoli. The chapter discusses the morphology of viral transcription and provides an example to demonstrate the kind of information provided by the use of the loosening procedure in combination with conventional fixation and the spreading technique.
TL;DR: It is concluded that the markedly lower rate of ribosome biogenesis in brain is specified mainly at the level of transcription of rRNA genes.
Abstract: The kinetics of in vivo labeling of cellular free UMP and nucleolar, nucleoplasmic, and cytoplasmic rRNA with [14C]orotate in rat brain and liver were investigated. Evaluation of the experimental data shows: (a) The rate of nucleolar precursors of ribosomal RNA (pre-rRNA) synthesis and the deduced rate of ribosome formation in brain is about fivefold lower than in liver and corresponds to 220-260 ribosomes/min/nucleus. (b) The lower rate of in vivo pre-rRNA synthesis is correlated with a lower activity of RNA polymerase I in isolated brain nuclei. (c) The half-lives of nucleolar rRNA in brain and liver are 210 and 60 min, respectively, thus showing a slower rate of processing of pre-rRNA in brain nucleoli. (d) The nucleo-cytoplasmic transport of ribosomes in brain is also markedly slower than in liver and reflects the lower rates of synthesis and processing of pre-rRNA. (e) Cytoplasmic ribosomes in brain and liver turn over with half-lives of about 6 and 4 days, respectively. It is concluded that the markedly lower rate of ribosome biogenesis in brain is specified mainly at the level of transcription of rRNA genes.
TL;DR: Nucleolar ultrastrure was studied in fully grown human oocytes obtained from multilaminar preantral follicles and from follicles at different stages of antrum formation to better understand the nature and functional significance of homogeneous dense nucleoli found in oocytes from large antral follicle.
Abstract: Nucleolar ultrastrure was studied in fully grown human oocytes obtained from multilaminar preantral follicles and from follicles at different stages of antrum formation. Selective staining for ribonucleoproteins and 3H-uridine labeling were used in attempt to better understand the nature and functional significance of homogeneous dense nucleoli found in oocytes from large antral follicles.
There was an apparent increase in the radio of nucleonema to nucleolar interstices, accompanied by a gradual degranulation of the nucleolonema during early stages of antrum fromation. The process of nucleolar homogenization continued in oocytes from medium-size antralfollicles, island of more tightly packed fibrils being hybothesized to represent persisting active transcription units. Entirely filamentous and homogeneous nucleoli were typical for oocytes from large antral follicles. They were demonstrated to ribonucleoprotein filaments embedded in pale- staining matrix. They were demonstrated to contain newly synthesized RNA after a 30-min pulse with 3H-uridine. The described nucleolar transformations are interpreted as acorrelate of nucleolar transition from a site of RNA synthesis into a site of RNA Storage during in human oocyte preovulatory development.
TL;DR: Serial section analysis has demonstrated that ring-shaped nucleoli of mature human lymphocytes are spherical structures consisting of a peripheral ribonucleoprotein shell that surrounds one large fibrillar center.
Abstract: Serial section analysis has demonstrated that ring-shaped nucleoli of mature human lymphocytes are spherical structures consisting of a peripheral ribonucleoprotein shell that surrounds one large fibrillar center. The shell exhibits usually one or, less frequently, two openings. The fibrillar center is in contact with the nucleoplasm and perinucleolar condensed chromatin, which frequently appears as a pedicle-like structure. Several chromocenters are associated with the ring-shaped nucleolus.
TL;DR: Morphological evidence suggests that the karyosome may be material that formerly was part of a fibrillar centre that lies on the nucleolus of certain plant species, particularly those with a relatively low nuclear DNA content and an areticulate nuclear structure.
Abstract: The karyosome is a spherical body up to 1 μm in diameter that lies on the nucleolus of certain plant species, particularly those with a relatively low nuclear DNA content and an areticulate nuclear structure. It can be seen in the light microscope after impregnation with silver; in the electron microscope its structure consists of fibrillo-granular material. Nucleoli of cells in root apices may bear 0, 1, or 2 karyosomes. The frequency with which these numbers of karyosomes are observed depends on the location of the cells within the apex. In roots ofPisum sativum andZea mays the nucleoli of both slowly-dividing and young differentiating cells bear karyosomes more frequently than the nucleoli of rapidly-dividing cells. The karyosome seems to adopt a preferred location on the nucleolus, lying most frequently on the nucleolar surfaces directed towards the apex or base of the root. The origin and functional significance of the karyosome are discussed. Morphological evidence suggests that it may be material that formerly was part of a fibrillar centre.
TL;DR: The results raise the possibility that during these stages of the reactivation process chick rRNA genes are transcribed by heterologous RNA polymerase I molecules of rat origin.
Abstract: Transcriptionally inactive chick erythrocyte nuclei were reactivated by Sendai virus-induced fusion of erythrocytes with rat L6J1 myoblasts. We used antibodies to trace the appearance of a specific protein engaged in transcription of a defined class of genes, those coding for rRNA, during reactivation. Using immunofluorescence microscopy, we found increasing amounts of rat RNA polymerase I to appear, during a certain period of time after fusion, in the reforming nucleoli of the chick nuclei. Amounts of rat RNA polymerase I sufficient to be detected by immunofluorescence microscopy had accumulated in the newly developed chick nucleoli 72-190 h after fusion was initiated. This time interval coincides with the time when chick rRNA synthesis can first be detected. The results raise the possibility that during these stages of the reactivation process chick rRNA genes are transcribed by heterologous RNA polymerase I molecules of rat origin.
TL;DR: Evidence is obtained for the sequential addition of proteins during the formation of the small ribosomal subunit in Friend erythroleukemia cells.
Abstract: Nucleolar ‘80-S’ and ‘40-S’ preribosomes (containining 45-S and 21-S pre-rRNA, respectively), as well as cytoplasmic ribosomes, were isolated from Friend erythroleukemia cells. The presence of structural ribosomal proteins in the isolated particles was studied by using antisera against individual rat liver small ribosomal subunit proteins. The analysis is based on the established crossreactivity between rat and mouse ribosomes [F. Noll and H. Bielka (1970) Mol. Gen. Genet. 106, 106–113]. The identification of the proteins was achieved by two independent immunological techniques: the passive haemagglutination test and the enzyme immunoassay of electrophoretically fractionated proteins, blotted on nitrocellulose. All 17 proteins tested are present in cytoplasmic ribosomes. A large number of proteins (S3a, S6, S7, S8, S11, S13, S14, S18, S20, S23/24 and S25) are present in the ‘80-S’ pre-ribosome. Only two proteins (S3 and S21) are added during the formation of the ‘40-S’ preribosome in the nucleolus. Four proteins (S2, S19, S26 and S29) are added at later, possibly extranucleolar, stages of ribosome formation. The results obtained provide evidence for the sequential addition of proteins during the formation of the small ribosomal subunit in Friend erythroleukemia cells.
TL;DR: Observations suggest that the micronucleoli, already visible at the leptotene stage, are the morphological expression of an early transcription of amplified rDNA, suggesting that it has a specific function somehow related to nucleolar proteins.
Abstract: Meiotic nuclei preparations obtained from human fetal ovaries were studied with the silver-NOR technique. At leptotene, the NOR’s were located at the periphery of the nucleoli. The mean number of NOR’
TL;DR: Using nuclear volume as an indicator of the age of the nucleus since mitosis, the numbers of DBs and NABs were related to the nuclear growth cycle.
Abstract: After impregnating root meristems with silver nitrate two types of small (< 1 Μm diameter) body can be seen in the nuclei. These have been termed “dense body” (DB) and “nucleolus-associated body” (NAB). The number of these bodies within a nucleus varies from species to species, but in general DBs are relatively numerous and lie in the nucleoplasm, while the NAB is usually solitary and lies on the surface of the nucleolus. Using nuclear volume as an indicator of the age of the nucleus since mitosis, the numbers of DBs and NABs were related to the nuclear growth cycle. In the meristem ofPisum sativum andZea mays DBs are characteristically present in early interphase; in some regions they persist in the nucleoplasm until the next mitosis, in other regions they disappear during interphase. DBs are probably pieces of the pellicle of ribonucleoprotein that coats mitotic chromosomes which have not coalesced (as does the remainder of the pellicle) to form the nucleolus at the start of interphase. NABs grow out from the nucleolar surface during the later stages of interphase. At the end of interphase there is on average 1 NAB per nucleolus.
TL;DR: Endogenous phosphorylation of pp 105 studied with purified mouse nucleoli showed optimal activity at isotonicity, pH 8.7, in the presence of 10 mM magnesium.
TL;DR: Cultured human fibroblasts were exposed to 5-azacytidine which inhibits methylation of newly synthesized DNA and a significant increase in the mean number of Ag-stained active nucleolus organizers has been observed, suggesting that DNA methylation is involved in modulation of human rRNA synthesis.
TL;DR: Comparisons between groups using one-way analysis of variance and multivariate statistical analysis support the conclusion that there are significant differences in the nuclear and nucleolar characteristics of these groups.
01 Jan 1983
TL;DR: Both methods indicate that there is a very low of poly(A)+ RNA synthesis throughout prophase, and these events are considered in terms of the juncture at which they occur in the plant life-cycle.
Abstract: SUMMARY Levels of poly(A) + RNA have been investigated at each stage of male meiosis in LJlium (var. Firecracker). Two methods were employed in this work: in one extracts from labelled meiocytes were passed through oligo(dT) columns, while in the other the specific probe [ 3 H]poly(U) was hybridized in situ with resin-embedded sections of pollen mother cells. The label contained in the eluate from the oligo(dT) columns was measured by liquid scintillation, and the quantity of [ 3 H]poly(U) hybridized was determined by statistical analysis of light microscopic autoradiographs. Both techniques revealed a dramatic decline in detectable poly(A) + RNA during prophase. Lowest levels are reached in the pachytene stage, following which a gradual restoration of this species of RNA takes place in both nucleus and cytoplasm. The data presented here provide no clear indication as to whether this fall in RNA levels is caused by the action of novel enzymes specific to the meiotic prophase, by a cessation of synthesis and the activity of normal turnover processes, or by a combination of the two. Although there is some evidence from the [ 3 H]poly(U) hybridization study that a small peak of poly(A) + RNA synthesis may take place in leptotene, both methods indicate that there is a very low level of poly(A) + RNA synthesis throughout prophase. The presence of poly(A) + RNA was not detected in either the accessory nucleoli or the cytoplasmic nucleoloids that characterize the nucleus and cytoplasm of these cells. These events are considered in terms of the juncture at which they occur in the plant life-cycle.
TL;DR: The effects of different inhibitors of the transcriptional processes on the accumulation of perichromatin granules suggest that these granules represent a particular subpopulation of hnRNP.
TL;DR: In this article, levels of poly(A)+ RNA have been investigated at each stage of male meiosis in Lilium (var. Firecracker) and two methods were employed in this work: in one extracts from labelled meiocytes were passed through oligo(dT) columns, while in the other the specific probe [3H]poly(U) was hybridized in situ with resinembedded sections of pollen mother cells.
Abstract: Levels of poly(A)+ RNA have been investigated at each stage of male meiosis in Lilium (var. Firecracker). Two methods were employed in this work: in one extracts from labelled meiocytes were passed through oligo(dT) columns, while in the other the specific probe [3H]poly(U) was hybridized in situ with resin-embedded sections of pollen mother cells. The label contained in the eluate from the oligo(dT) columns was measured by liquid scintillation, and the quantity of [3H]-poly(U) hybridized was determined by statistical analysis of light microscopic autoradiographs. Both techniques revealed a dramatic decline in detectable poly(A)+ RNA during prophase. Lowest levels are reached in the pachytene stage, following which a gradual restoration of this species of RNA takes place in both nucleus and cytoplasm. The data presented here provide no clear indication as to whether this fall in RNA levels is caused by the action of novel enzymes specific to the meiotic prophase, by a cessation of synthesis and the activity of normal turnover processes, or by a combination of the two. Although there is some evidence from the [3H]poly(U) hybridization study that a small peak of poly(A)+ RNA synthesis may take place in leptotene, both methods indicate that there is a very low of poly(A)+ RNA synthesis throughout prophase. The presence of poly(A)+ RNA was not detected in either the accessory nucleoli or the cytoplasmic nucleoloids that characterize the nucleus and cytoplasm of these cells. These events are considered in terms of the juncture at which they occur in the plant life-cycle.
TL;DR: Microsporogenesis of Allium flavum was investigated by light microscopy using a silver impregnation technique and a heteromorphism with regard to the presence of a NOR allowed the determination of the crossing-over frequency in the chromosome arm concerned.
Abstract: Microsporogenesis of Allium flavum was investigated by light microscopy using a silver impregnation technique. Ag-positive structures were present at the nucleolus organizing regions (NORs) in all ...
TL;DR: The results indicate that red blood cell-mediated microinjection of antibodies can be used to study nucleolar and nuclear antigens and that preimmune immunoglobulins remained in the cytoplasm 36 h after microin injection.
Abstract: To determine whether red blood cell-mediated microinjection of antibodies can be used to study nuclear protein localization and function, we microinjected antibodies that have been shown to react specifically with nucleolar acidic phosphoprotein C23 into Walker 256 cells. The intracellular distribution of microinjected anti-C23 antibodies and preimmune immunoglobulins were determined by immunofluorescence. At 3 h after microinjection, affinity-purified anti-C23 antibodies were localized in the cytoplasm and nucleolus. At 17 h after microinjection, the affinity-purified antibody was localized to those nucleolar structures previously shown to contain protein C23. Furthermore, the antibody remained localized in the nucleolus for at least 36 h after microinjection. In contrast to the results obtained with specific antibodies, preimmune immunoglobulins remained in the cytoplasm 36 h after microinjection. These results indicate that red blood cell-mediated microinjection of antibodies can be used to study nucleolar and nuclear antigens.