Schwarzacher H G, Wachtler F
Histologisch-Embryologisches Institut, Universität Wien, Austria.
Anat Embryol (Berl). 1993 Dec;188(6):515-36. doi: 10.1007/BF00187008.
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)
核仁是核糖体前体生物合成的场所。它们含有多个主要核糖体RNA（18S - 和28S - rRNA）基因的拷贝，这些基因以串联重复的形式存在于染色体核仁组织区（NORs）。它们还含有在核仁外合成的小核糖体RNA（5S - rRNA）、特定的核仁蛋白，其中包括转录和转录加工所需的因子和酶，以及核糖体的前体单位。在人类以及许多脊椎动物物种中，除染色质外，核仁的三个主要组成部分可被检测到：纤维中心（FC）、致密纤维成分（DCF）和颗粒成分（GC）。在一个核仁内，FC在许多情况下位于其中心区域。DCF形成围绕FC的丝状网络，但有时可能延伸至核仁周边。GC通常位于核仁的周边区域。在核糖体生物合成水平较低的细胞中，核仁较小，通常只有一个FC，周围的DCF和GC较少（“环形核仁”）。在活跃细胞中，DCF形成一个包围多个（有时多达数百个）FC的大网络，GC覆盖周边的大片区域（“致密核仁”）。在新刺激开始时的细胞中，DCF非常突出，而FC较少且小，GC也不太广泛（“网状核仁”）。在一些非常活跃的特殊细胞类型中，发现了结构成分的其他排列方式。例如，在支持细胞中，只发现一个核仁，偶尔有两个，每个都有一个大的FC和一个明显的GC区域，这两个区域都被与一些周边GC混合的DCF包围。迄今为止，用于在核仁内定位rRNA基因的免疫细胞化学和原位杂交研究得出了不同的结果。两种纤维成分，FC和DCF，都被认为是最有可能的候选者。rDNA的转录以及核糖体生物合成的后续早期步骤定位于DCF，而后期步骤（成熟rRNA、前核糖体）定位于GC。FC也可能作为转录前rDNA的准备位点，以及某些核仁蛋白的储存位点。在有丝分裂期间，部分核仁蛋白保留在NORs处。经常观察到核仁与核膜之间有直接接触，但这不依赖于核仁活性。（摘要截于400字）
