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PubMed1983

Nucleoli, micronucleoli, and nucleolus-like structures in human oocytes at meiotic prophase I studied by the silver-NOR technique.

Meiotic nuclei preparations obtained from human fetal ovaries were studied with the silver-NOR technique. At leptotene, the NOR's were located at the periphery of the nucleoli. The mean number of NOR's per nucleus was 6.4. Moreover, 2-12 micronucleoli, each containing an Ag-positive zone, were observed. At pachytene, each nucleolus was divided into two zones, one of which was Ag-positive. The mean number of Ag-positive zones was 2.1 per nucleus. The subterminal region of the short arm of nucleolar bivalents was in contact with the argyrophilic nucleolar zone. In most cells, micronucleoli were dispersed between the bivalents. A single rounded body which stained heavily with silver was consistently observed. At early diplotene, the NOR's separated, appearing again as distinct rounded Ag-positive structures embedded in the peripheral part of the nucleoli. The silver-stained round body was still visible. Eight to twelve micronucleoli were observed. At a more advanced stage of diplotene, the number of micronucleoli increased, varying from 20 to 50 per nucleus. These observations suggest that the micronucleoli, already visible at the leptotene stage, are the morphological expression of an early transcription of amplified rDNA. The biological significance of the Ag-positive round body remains obscure, but the consistency of its appearance suggests that it has a specific function somehow related to nucleolar proteins.

Cell Nucleolus↗
PubMed1984

On the nature and extent of XY pairing at meiotic prophase in man.

Evidence is presented that pairing between the human X and Y chromosomes could be more extensive at early pachytene than has previously been supposed and could involve even the entire euchromatic portion of the Y chromosome. Following desynapsis over the major part of the X and Y axes, a small paired segment of Xp and Yp remains into late pachytene. Association between the distal tips of Xq and Yq can also be observed in about one half of the spermatocytes examined. A hypothesis linking meiotic pairing to early replicating sites along the chromosomes is proposed.

Adult↗
PubMed2006

A link between meiotic prophase progression and crossover control.

During meiosis, most organisms ensure that homologous chromosomes undergo at least one exchange of DNA, or crossover, to link chromosomes together and accomplish proper segregation. How each chromosome receives a minimum of one crossover is unknown. During early meiosis in Caenorhabditis elegans and many other species, chromosomes adopt a polarized organization within the nucleus, which normally disappears upon completion of homolog synapsis. Mutations that impair synapsis even between a single pair of chromosomes in C. elegans delay this nuclear reorganization. We quantified this delay by developing a classification scheme for discrete stages of meiosis. Immunofluorescence localization of RAD-51 protein revealed that delayed meiotic cells also contained persistent recombination intermediates. Through genetic analysis, we found that this cytological delay in meiotic progression requires double-strand breaks and the function of the crossover-promoting heteroduplex HIM-14 (Msh4) and MSH-5. Failure of X chromosome synapsis also resulted in impaired crossover control on autosomes, which may result from greater numbers and persistence of recombination intermediates in the delayed nuclei. We conclude that maturation of recombination events on chromosomes promotes meiotic progression, and is coupled to the regulation of crossover number and placement. Our results have broad implications for the interpretation of meiotic mutants, as we have shown that asynapsis of a single chromosome pair can exert global effects on meiotic progression and recombination frequency.

Animals↗