Structural basis for Robertsonian translocations in man: association of ribosomal genes in the nucleolar fibrillar center in meiotic spermatocytes and oocytes.

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.