Alterations of three-dimensional structure of nucleolar organizers during cell division and differentiation

利用光镜和电镜对莴苣根尖细胞核仁层进行了研究，重点研究了其三维结构组织。它的轮廓非常复杂，风干标本的厚度变化很大。它通常呈盘绕状或蜿蜒状。纵向连续超薄切片首先显示了短核仁小体片段的堆叠，然后是一对节状框架，中间有空间，然后是短核仁小体片段的堆叠。另一方面，随着开口方向的改变，横截面上出现了“c”形的段。核仁线束瘤形态测定显示核仁线束瘤分为两类；平均厚度分别为510 nm和1.19 nm。综上所述，薄核仁胞体可以卷曲或折叠形成厚核仁胞体。含dna结构的纤维中心与薄核仁紧密相连，沿核仁线状体呈间歇性排列。在低氧条件下，研究了洋葱切下的茎尖核内DNA的定位。银浸渍技术在连续超薄切片上的应用表明，核仁分裂成两个区域，致密纤维成分（DFC）的中心核心和颗粒成分（GC）的外围，并在DEC核心内形成蜿蜒的消色差通道。电镜显示，消色通道的发生是由纤维中心（fc）的发育和/或融合引起的，纤维中心被认为含有暂时静止的核糖体rna基因（rDNA）。与细胞质相比，DFC的抗dna抗体的标记性明显强，标记特异性在整个ec中是均匀的。有趣的是，缺氧条件下的标本中ec及其周围区域的标记特异性与正常条件下生长的标本几乎相同。在低氧条件下，随着FCs的发育，FCs中形成了直径约35 um （35 nm）的颗粒。它们根本没有被抗DNA抗体和RNA -金复合物标记，表明它们既不含DNA也不含RNA。总之，当细胞暴露于缺氧条件下时，含有可能参与RNA转录的核仁内DNA的DFC可能会逐渐坍塌，变成FCs，同时释放失活的rDNA，形成染色质样团簇和Sonic核仁蛋白，形成35 inn颗粒。少

The nucleoloneina of Raphanus sativus root-tip cells was studied with light and electron microscopy especially focusing on its three-dimensional structural organization. Its countour was very intricate and thc thickness showed a large variation in air-dried specimens. It usually appeared to coil or meander. Serial ultrathin sectioning of thc nucleolonema iii its longitudinal orientation first revealed the stack of short nucleolonema fragments, then a pair of knobbly frames with space in-between, and again the stack of the short nucleolonema segments. On the other hand, transversal sections revealed 'c'-shaped segments with the change of their openings' orientation. Morphometry of the nucleolonema indicated two categories of the nucleolonema ; each had an average thickness of 510 nm and 1.19mum, respectively. Taken together, the thin nucleolonema may coil or fold to form the thick one. The fibrillar centers including DNA-containing structures were closely associated with the thin nucleo … More lonem a and intermittently arranged along the nucleolonema.Localization of intranucleolar DNA under hypoxic conditions was examined in excised loot tips of Allium cepa. Application of a silver impregnation technique to serial ultrathin sections indicated that the nucleolus segregated into two regions, the central core of the dense fibrillar component (DFC) and the periphery of the granular component (GC), and developed achromatic channels meandering within the DEC core. Electron microscopy showed that the Qccurrence of the achromatic channel was caused by development and/or fusion of the fibrillar centers (FCs), which are believed to contain the temporarily resting ribosomal-RNA genes (rDNA). The label of the DFC with anti-DNA antibodies was significantly strong compared with that of the cytosol and the labeling specificity was uniform throughout the DEC.Interestingly, the labeling specificity of the ECs and their surrounding region in the specimens exposed to hypoxic condition was almost the same as those grown under a normal condition. Concurrently with the development of the FCs under hypoxic conditions, the granules about 35 um in diameter (35 nm granules) were formed in the FCs. They were not labeled at all with anti-DNA antibodies and RNase-gold complexes, indicating that they contain neither DNA nor RNA.Altogether, the DFC containing the intranucleolar DNA probably engaged in the RNA transcription may gradually collapses to turn into the FCs and simultaneously releases the inactivated rDNA as clusters of chromatin-likemasses and Sonic nucleolar proteins as the 35 inn granules, when the cells are exposed to hypoxic conditions. Less