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Sertoli cells produce special microenvironment for developing germ cells; therefore it is assumed that they play primary role in the onset and control of spermatogenesis. In this connection we extended our previous study on the ultrastructure of Sertoli cells in different domestic and wild animals with special regard to nucleolus. Sertoli cells of domestic and wild ruminants possess the typical vesicular nucleolus except for fallow deer, in this species no vesicular nucleolus occurs in Sertoli cells even during the rut. In roe.buck, another wild ruminant with seasonal spermatogenesis, cyclic changes were found in the nucleolus of Sertoli cells. If no spermatogenesis is present, the Sertoli cells have a reticular nucleolus. Membranous vesicles appear in the nucleolus of Sertoli cells of roe-buck at the onset of spermatogenesis 1-2 months before rut. In domestic ruminants with continuous spermatogenesis the vesicular nucleolus in Sertoli cells is present permanently. During postnatal development of bull and ram the vesicular nucleolus appears in Sertoli cells just before the onset of spermatogenesis. In experimental cryptorchidism of bulls a vesicular nucleolus is found in the Sertoli cells. Our observations and experiments support a hypothesis that Sertoli cells have primary role at the onset and the maintenance of spermatogenesis.
In the present review on the organization of the mammalian cell nucleolus, we report and discuss data obtained during the past 10 years by means of cytochemical and immunocytochemical ultrastructural techniques. Particular emphasis is placed on the following topics: location of the nucleolus organizer regions in interphasic nucleolar components, structure of nucleolar chromatin in situ, and the structure-function relationship of the nucleolar components. The cytochemical and immunocytochemical results are compared and the concordant data are stressed for each topic.
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An auto-antibody from human serum of patients with the autoimmune disease scleroderma was used to localize the nucleolus in meristematic cells of onion and soybean roots using indirect immunofluorescence microscopy. Similar lots of antiserum recognized a single 34 kD, nucleolar protein, fibrillarin, in a variety of animal cells (Ochs et al. 1984, 1985). In both plants, antibody linked fluorescence is associated with the one to several nucleoli present in the interphase nucleus. The fluorescence becomes diffuse around condensing prophase chromosomes and becomes more diffused at metaphase with slightly more intense fluorescence surrounding the chromosomes. At anaphase-telophase the fluorescence is localized in dense areas within the chromosomes, presumably representing prenucleolar bodies which will form the interphase nucleoli of the daughter nuclei. This antiserum provides a new, valuable tool for the study of the nucleolus and the highly conversed nucleolar antigen(s) that it recognizes.
We have investigated the fine spatial distribution of RNA and rRNA within the Ehrlich tumor cell nucleolus by in situ hybridization with a biotin-labeled probe and by two new strategies, the polyadenylate nucleotidyl transferase-immunogold technique and immuno-labeling with anti-RNA antibodies. Besides the presence, as expected, of RNA and rRNA in the granular component and the dense fibrillar component, we show, for the first time, significant label over all the fibrillar centers of the nucleoli. When RNA and DNA were detected simultaneously on the same sections, only the fibrillar centers were positive for both. These results throw light on the controversial subject of the precise location of transcribing rRNA genes within the nucleolus. The fibrillar centers, and not the dense fibrillar component, should thus be the site of rRNA synthesis.
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Nucleolar transcription in isolated onion cell nuclei was visualized, after Br-UTP incorporation, under the conventional fluorescence microscope, the confocal microscope, and the transmission electron microscope. The confocal microscopy study of transcription was combined with immunodetection of fibrillarin, a component of the RNP complex involved in the early processing of pre-rRNA. Superposition of transcription and fibrillarin images from the same optical section showed some small "black holes" in the nucleolus, around which a lateral and radial differentiation of labeling was observed: laterally, zones corresponding to transcription labeling alternated with zones of fibrillarin labeling; radially, areas of transcription gradually became areas of colocalization of transcription and fibrillarin, and, further outward, of fibrillarin alone, which occupied the major part of the labeled nucleolar area. Three-dimensional reconstruction of the nucleolar transcription labeling, from confocal optical sections, showed clusters of foci arranged around an area of low or no labeling. Thin labeled extensions, connecting single foci, were observed. Visualization of transcription at the ultrastructural level identified the black holes as fibrillar centers, in view of their size and the absence of labeling in them. In fact, most of the labeling was observed in discrete areas of the dense fibrillar component, near fibrillar centers, including the transition area between these two components. This observation was supported by a quantitative study. Otherwise, the outline of fibrillar centers did not appear entirely surrounded by particles, and a minor proportion of particles was detected dispersed throughout the dense fibrillar component. As a complementary study, the transcription factor upstream binding factor (UBF) and the protein NopA64, a plant nucleolin homologue, were immunolocalized. Small foci of UBF localization alone and other foci in which the two protein markers overlapped were observed. The outer areas of the nucleolus showed the exclusive presence of NopA64. Under the electron microscope, UBF labeling, quantitatively assessed, appeared as clusters of particles, most of them surrounding fibrillar centers. A graphic model is presented to give a molecular interpretation of these data.
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The development and aging of the nucleolus and nucleus in layer V pyramidal cells in the hamster cerebrum were studied by light and electron microscopy. The nucleoli appeared in the newborn as occasional fibrillar masses adjacent to peripherally placed bodies of chromatin. By maturity, a single, generally central, nucleolus proper with nucleolus-associated chromatin was present. Nucleolar microbodies were observed at 10, 15, 20 and 480 days, but not in the newborn, 5-or 90-day animal. An intranucleolar body was not observed at the electron at the electron-microscopy level in these pyramidal cell nucleoli at any age in this series, in contrast to the situation in large motor neurons of the facial nucleus. The nucleus progressed from an irregular shape at birth to an oval shape at maturity. At 10 days, incipient invaginations of the nuclear membrane appeared; these subsequently increased in depth and frequency in the adult. The above changes, particularly in the nucleoli, are correlated in time with changes involving the endoplasmic reticulum. The correlations may indicate different periods of metabolic activity in the hamster pyramidal neurons. Four such periods can be differentiated on the basis of cytomorphic changes which may be correlated to reported development of function. The sequence of these changes, peculiar to the developing and aging hamster pyramidal neuron, differs from that seen in large spinal and cranial motor neurons. It appears that some features of nuclear immaturity, which are lost in larger neuronal types, are retained in the adult pyramidal neuron.
The nucleolus of the human Sertoli cell displays a spontaneous segregation of its components and has only one or 2 large fibrillar centers. The 3-dimensional reconstruction and quantitative analysis of its components was undertaken using a Quantimet 900 image analysis system in order to define the spatial relationships between the dense fibrillar component and the fibrillar center and especially to investigate whether threads of dense fibrillar component exist independently, without being linked to a fibrillar center. Our 3D reconstructions demonstrated that the dense fibrillar threads or sheets were never independent of fibrillar centers. These structures belonged to a continuous network that joined the layer of dense fibrils surrounding the fibrillar center. When the nucleolus contained 2 different-sized fibrillar centers, quantitative analysis showed that there was a proportional relationship between the volume of the dense fibrillar component and the volume of the fibrillar center. These data, compared with those previously obtained by means of autoradiographic techniques, suggest that the rDNA-containing chromatin passes through the fibrillar center and unwinds from there into the dense fibrillar component.
The intranucleolar distribution of phosphoproteins B23 and C23 was visualized simultaneously by post-embedding immunoelectron microscopy in HeLa cell nucleoli, using specific antibodies. The data show that proteins B23 and C23 co-localize to the same nucleolar compartments, i.e., the dense fibrillar component and the granular component. Neither of the two antibodies is significantly associated with the fibrillar centers in these cells, although the fibrillar centers appear positive after silver staining. These findings suggest that other unidentified components must be responsible for the silver staining observed in the fibrillar centers of interphase nucleoli. The results are discussed in the light of previously reported data obtained by preembedding immunolabeling techniques and by silver staining, which both suggested a localization of protein C23 inside the fibrillar centers.
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