Association of ribosomal genes in the human oocyte at meiotic prophase: cytogenetic consequences

Abstract

The human acrocentric chromosomes, which contain a nucleolus organizer region (NOR) in their short arm, are often involved in numerical and structural anomalies. Trisomies of acrocentric chromosomes represent almost 40 % of all human lethal trisomies (Boue et al., 1975). In Down syndrome, the extra chromosome 21 originates in most cases as a result of nondis junction during the first meiotic division of the oocyte (Mattei et al., 1979). The incidence of the Robertsonian translocations, which are the most frequent form of chromosome rearrangement in man, is about 10/00 in the general population. In 90 % of Robertsonian translocations, nonhomologous acrocentrics are involved, the most frequently observed being the t(13;14) and t(14;21). These facts have led Hartung et al. (1979), Mirre et al. (1980) and Stahl et al. (1983) to investigate whether a structural arrangement exists at prophase I of human meiosis that might favor the failure of disjunction of acrocentrics and the formation of Robertsonian translocations. Using NOR-silver techniques and electron microscopy they showed that NORs belonging to several chromosomes are associated at pachytene within the fibrillar centre of the nucleolus. Mirre et al. (1980) observed the penetration into the fibrillar centre of chromatin fibers emanating from 1 to 3 bivalents and postulated that the ribosomal genes from 4 to 12 chromatids were temporarily juxtaposed inside the same nucleolar component.