Nucleolus

About: Nucleolus is a research topic. Over the lifetime, 5873 publications have been published within this topic receiving 232435 citations. The topic is also known as: GO:0005730 & cell nucleolus.

Electron microscopic examination of the sites of nuclear rna synthesis during amphibian embryogenesis.

Abstract: The site of H3-uridine incorporation and the fate of labeled RNA during early embryo-genesis of the newt Triturus pyrrhogaster were studied with electron microscopic autoradiography. Isolated ectodermal and mesodermal tissues from the embryos were treated in H3-uridine for 3 hours and cultured in cold solution for various periods before fixation with OsO4 and embedding in Epon. At the blastula stage, the only structural component of the nucleus seen in electron micrographs is a mass of chromatin fibrils. At the early gastrula stage, the primary nucleoli originate as small dense fibrous bodies within the chromatin material. These dense fibrous nucleoli enlarge during successive developmental stages by the acquisition of granular components 150 A in diameter, which form a layer around them. Simultaneously larger granules (300 to 500 A) appear in the chromatin, and they fill the interchromatin spaces by the tail bud stage. Autoradiographic examination has demonstrated that nuclear RNA synthesis takes place in both the nucleolus and the chromatin, with the former consistently showing more label per unit area than the latter. When changes in the distribution pattern of radioactivity were studied 3 to 24 hours after immersion in isotope at each developmental stage, the following results were obtained. Labeled RNA is first localized in the fibrous region of the nucleolus and in the peripheral region of chromatin material. After longer culture in non-radioactive medium, labeled materials also appear in the granular region of the nucleolus and in the interchromatin areas. Further incubation gives labeling in cytoplasm.

Nucleolar fine structure and RNA synthesis in bovine oocytes from antral follicles

Abstract: Nucleolar ultrastructural changes occurring in vivo in bovine oocytes during follicular growth were analyzed by electron microscopy. The rates of in vitro incorporation of 3H-uridine by oocytes of the same size class were evaluated by autoradiography. One to two large fibrillogranular, vacuolated nucleoli were present in oocytes from small to medium antral follicles 0.5–3 mm in diameter. These oocytes showed intense hnRNA and rRNA synthesis. The homogeneous, agranular nucleoli in oocytes from follicles 3–4 mm in diameter were composed of a compact fibrillar material. This morphological change was accompanied by an impairment of nucleolar transcriptional activity as well as by a decrease in hnRNA synthesis.

The Maternal Nucleolus Is Essential for Early Embryonic Development in Mammals

Abstract: With fertilization, the paternal and maternal contributions to the zygote are not equal. The oocyte and spermatozoon are equipped with complementary arsenals of cellular structures and molecules necessary for the creation of a developmentally competent embryo. We show that the nucleolus is exclusively of maternal origin. The maternal nucleolus is not necessary for oocyte maturation; however, it is necessary for the formation of pronuclear nucleoli after fertilization or parthenogenetic activation and is essential for further embryonic development. In addition, the nucleolus in the embryo produced by somatic cell nuclear transfer originates from the oocyte, demonstrating that the maternal nucleolus supports successful embryonic development.

Nucleolar and perichromosomal RNA synthesis during meiotic prophase in the mouse testis.

Abstract: The transcriptional activity during meiotic prophase in the mouse testis is studied with light microscopy and high-resolution autoradiographic techniques using [3H]uridine as a labeled precursor. In the present study, two types of RNA synthesis are detected during meiotic prophase: an extranucleolar RNA synthesis of perichromosomal localization and a nucleolar RNA synthetic activity. In some of the autosomes and close to the basal knobs, the activity of the nucleolar organizers is evidenced by the incorporation of [3H]uridine into nucleolar masses from zygotene on and at earlier labeling times. The evolution of nucleoli and the formation of a nucleolus attached to the sex pair are described during the different meiotic stages. Perichromosomal labeling, from leptotene on, reaches a maximum during middle pachytene and falls progressively to a low level at longer incorporation times. Sertoli's cell, the most active RNA synthetic cell in the seminiferous epithelium, rises to a maximum of labeling and drops at earlier times compared with the meiotic prophase cells. The condensed sex chromosomes show some scattered silver grains especially at middle pachytene. The axial chromosome cores and synaptonemal complexes are devoid of silver grains during the meiotic prophase. The observations suggest that a control mechanism operates during meiotic prophase to regulate transcriptional activity in the sex chromosomes and to provide differential RNA synthesis in autosomal bivalents at various stages of prophase and within certain segments of the chromosomes.

RNA synthesis during male meiosis and spermiogenesis

Abstract: 1. Using 3H-uridine, the course of RNA synthesis has been followed autoradiographically during all stages of male meiosis and spermiogenesis in the locusts Schistocerca gregaria and Cyrtacanthacris tartarica and the grasshopper Chorthippus brunneus. Using 3H-thymidine, premeiotic DNA synthesis was followed in Cyrtacanthacris tartarica. 2. RNA synthesis is actively carried out by all autosomes throughout first meiotic prophase, up to and including diakinesis, and at second prophase. No RNA synthesis occurs at the contracted stages of first or second metaphase or first or second anaphase. 3. RNA is again synthesized by young spermatids, but such synthesis ceases by the time that differentiation begins. No label was detected in the nuclei of differentiating spermatids, even after 8 hours' incubation with 3H-uridine. 4. Morphological and functional comparisons suggest that orthopteran prophase chromosomes at male meiosis, and probably all prophase chromosomes, are lampbrush in nature, though to varying degrees. 5. The X univalent, allocyclic in its appearance and staining properties, is apparently completely inactive throughout the whole of meiosis. No RNA or DNA precursor could be successfully demonstrated to be incorporated by the X at any stage of male meiosis. 6. Nucleoli are present at interphase and early prophase but are usually absent during later prophase stages in this material. They do not label heavily in advance of, or at the same time as, the commencement of the labelling of the rest of the chromatin: they only accumulate greater densities of label after several hours' incubation with the labelled precursor. 7. No independent cytoplasmic synthesis of RNA could be detected after up to 2 hours' in vitro incubation in labelled saline at 30° C, though nuclei are heavily labelled after only 1/2 hour. It is some 2–4 hours before nuclear synthesized RNA appears to pass to the cytoplasm in detectable amounts.