[Cytogenetic of nucleolar organizer regions (NOR) of human chromosomes: identification of four morphofunctional variants of NOR, their inter-individual and inter-chromosomal distribution]

Cytogenetic characters of the nucleolus organizer regions (NORs) located on the short arms of five acrocentric chromosomes were studied in chromosome preparations obtained from cultured blood cells of 17 donors. In situ hybridization to a 3H-labeled probe was used to estimate the relative copy number of ribosomal genes (RGs) in all NORs of chromosomes identified by G-banding in each sample. The relative amount of potentially active RGs (0 to 4 arbitrary units) in each NOR was estimated from the size of AgNOR selectively stained with silver nitrate. Linear regression analysis revealed clusters of silent RGs (CSRGs) in 24 out of 170 NORs (14%). Based on the presence or absence of active and inactive RG clusters, NORs of human chromosomes were classified into four morphological functional variants (MFVs): (1) Ag-/CSRG-, (2) Ag-/CSRG+, (3) Ag+/CSRG-, and (4) Ag+/CSRG+. These variants were observed in 7.65%, 2.35%, 11.8%, and 78.2% of 170 analyzed NORs, respectively. NORs with CSRGs (MFV 2 and 3) were absent in 5 out of 17 donors. One, two, and three NORs with CSRGs were observed in four donors each. Analysis of the chromosome distribution of NOR MFVs showed that their frequencies remained almost the same in group-D and group-G acrocentric chromosomes. Although the tested samples were small (34 chromosomes for each pair), two observations were made with regard to individual chromosomes. First, almost half MFV-1 NORs (6 out of 13) were detected on chromosome 15. Second, the frequency of CSRGs was higher in chromosome 21 (29%) than in the other chromosomes (10%).