The spatial relationships of acrocentric chromosomes were studied during prophase I of meiosis in human oocytes and spermatocytes by using cytogenetic techniques, electron microscopy, and in situ hybridization. Ultrastructural investigations revealed an ordered arrangement of nucleolar bivalents at the zygotene and pachytene stages. The end of the bivalent corresponding to the cytological satellite was consistently attached to the nuclear envelope. The fibrillar center of the nucleolus always contained rDNA chromatin fibers emanating from the secondary constriction region. Association of ribosomal genes from two bivalents in the same fibrillar center was frequently observed. Ultrastructural studies demonstrated the close proximity of chromatids in the short arm region of the involved nonhomologous acrocentrics. A breakage/reunion model based on our data can explain the formation of all observed types of Robertsonian translocations: monocentrics and dicentrics with or without rDNA.Robertsonian translocations are the most frequent form of chromosome rearrangement in man, their incidence being about 1% in the general population. They result from different modes of exchange occurring within the centromeric region of two acrocentric chromosomes, thus producing a metacentric or submetacentric element (1). In man, the chromosomal segment above the centromere is divided into three parts: the proximal short arm, the secondary constriction or stalk, which is the nucleolus organizer region (NOR), and the distal cytological satellite. The five pairs of acrocentric chromosomes share multiple copies of rRNA genes variably dispersed among the NORs (2, 3). Nonhomologous chromosomes are involved in 90% of Robertsonian translocations, and the most frequently observed are the 13;14 and the 14;21 translocations (4). Most translocations between heterologs are dicentric and devoid of Ag-positive NOR (4-8).The formation of Robertsonian translocations has been ascribed to chromatid exchange after a break between acrocentric chromosomes associated with the same nucleolus, during spermatogonial and oogonial mitosis (9). It has also been suggested that Robertsonian rearrangements result from an orderly, nonrandom process during meiotic pairing and exchange (10). Indeed, nonhomologous acrocentric chromosomes are often associated with the same nucleolus at meiotic prophase I in human spermatocytes and oocytes (11)(12)(13)(14). The nonrandom involvement of chromosomes in translocations can also be explained by an accidental meiotic recombination between partially homologous sites on nonhomologs. If the pairing is "end-to-end," crossing-over results in two recombinants, one acentric and the other dicentric (15). The two centromeres are so close to one another that they may act as a single centromere (16).Electron microscope studies provide more precise information about the relationship between the nucleolus and the acrocentric chromosomes. A connecting region of the nucleolus was reported in close association with the nucleolar chro...

Use of specific stains permits analysis of the frequency of nucleolus-associated heterochromatin in chromosomes 1 and 9 from human fibroblasts. In 81 per cent of interphase nuclei the heterochromatic segment of both No. 1 chromosomes is associated with the nucleolus, while in 19 per cent only one heterochromatic segment shows such an association with the other occupying a random position in the nucleoplasm. The nucleolar association of chromosome 9 heterochromatin is less constant: in 42.3 per cent of the nuclei both segments are associated with the nucleolus, in 39 per cent of the nuclei only one heterochromatic segment presents such an association, and in 18.7 per cent neither of the two heterochromatic segments is in nucleolar association. In 6 per cent of the cells, one or two chromosome 9 heterochromatic segments are in contact with the nuclear membrane. In situ hybridization using tritium-labeled 28S and 18S RNA shows that in the interphase nucleus the acrocentric short arms, carriers of ribosomal cistrons, are associated with the nucleolus. These observations demonstrate the complexity of the nucleolus-associated chromatin which, in addition to segments of chromosomes 1, 9, 13, 14, 15, 21, 22, may include the Y chromosome. They also confirm that the nucleolus constitutes one of the orientation points determining the relative localization of chromosomes in the interphase nucleus.

No abstract

The formation and development of nucleoli and their connections with the nucleolar chromosomes were studied in human spermatocytes using electron microscopy, silver staining of nucleolus organizer regions (NORs), high resolution autoradiography and in situ hybridization in order to localize rRNA genes and their transcription in the different stages of meiotic prophase I. At leptotene, new nucleoli were formed, consisting of a fibrillar centre surrounded by a cap of dense fibrillar component. Following [3H]uridine uptake, label was found only over the dense fibrillar component. In situ hybridization revealed rDNA mainly in the dense fibrillar component and in the chromatin. During zygotene, nucleoli increased in size. The fibrillar centre was connected with the secondary constriction region of the nucleolar bivalent and was partially surrounded by dense fibrillar component. This shell of dense fibrillar component merged into a fibrillo-granular mesh that extended away from the fibrillar centre. Autoradiography following [3H]uridine uptake again showed the label overlaying the dense fibrillar component and the proximal part of the fibrillo-granular strands. With in situ hybridization in both the light and electron microscope, signal was mainly found in the dense fibrillar component. A small quantity of label was observed in the peripheral region of the fibrillar centre and in the adjacent chromatin. From early to late pachytene segregation of nucleolar components occurred, with a reduction in the dense fibrillar component that formed a narrow rim around the fibrillar centre with small extensions along the granular component. [3H]uridine incorporation progressively decreased. In situ hybridization showed signal located mainly in the dense fibrillar component and in the chromatin corresponding to the condensed short arm of the nucleolar bivalent. Our results indicate that the majority of rDNA is located and transcribed in the dense fibrillar component; only a small amount is present in the peripheral part of the fibrillar centre and may be transcribed there. Moreover, from leptotene to zygotene, rDNA unravels from the nucleolar chromosome into the nucleolar dense fibrillar component. From zygotene to late pachytene a progressive return to the condensed acrocentric short arm is observed.

No abstract