Association of ribosomal genes in the fibrillar center of the nucleolus: a factor influencing translocation and nondisjunction in the human meiotic oocyte.

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Abstract
Prophase I of meiosis was studied in human oocyte obtained from 16- to 24-wk-old fetuses. EM and Ag staining showed that, at pachytene, the ribosomal genes belonging to several chromosomes are gathered in the same nucleolar fibrillar center, where they are embedded in an argyrophilic protein. The nucleolus showed spontaneous segregation of its components due to temporary inactivation of the ribosomal genes. The fibrillar center, separated from the other nucleolar components, was penetrated at midpachytene by chromatin fibers containing rDNA emanating from 1-3 nucleolar bivalents. The ribosomal genes from 4-12 chromatids are temporarily juxtaposed inside the same structure. Such a structural arrangement is completely different from that observed in the pachytene-stage mouse oocyte, where 2 independent and active nucleoli, each displaying its own fibrillar center, were formed on the bivalents containing paired ribosomal genes. These different structural patterns are correlated with the high frequency of nondisjunction in the human oocyte and the relative infrequency of such in the mouse oocyte. The pattern observed in the human oocyte may be a cause of translocations.