Argyrophilic nucleolar organizer region (Ag-NOR) analysis was performed on bone marrow biopsies from 90 patients with multiple myeloma (MM) at presentation. The pattern of AgNOR expression and its relationship to histological features were evaluated. The mean AgNOR number per plasma cell was directly correlated with the degree of MM differentiation (3.18 for G1, 4.36 for G2, 6.13 for G3; P less than 0.0001), with the percentage of bone marrow plasma cells (BMPC%) (3.06 for BMPC% less than or equal to 20, 4.28 for BMPC% 21-50, 5.14 for BMPC% greater than 50; P less than 0.0001), with the pattern of medullary involvement (3.63 for interstitial, 4.44 for nodular, 5.17 for diffuse involvement; P less than 0.001) and with medullary fibrosis (5.23 for cases with fibrosis, 4.29 for cases without fibrosis; P less than 0.05). The plasma cells of G1 MM showed 2-3 large AgNORs, tightly grouped in a central nuclear cluster; those of G2 MM showed a central nuclear cluster composed of 4-5 medium-size dots and/or two clusters of 2-3 dots; the G3 MM plasma cells showed many small dots scattered in the nucleolus or dispersed in the nucleus. Our results indicate the diagnostic value of AgNOR analysis in MM and suggest the use of this method for identifying clones of atypical plasma cells with different proliferative activity in bone marrow biopsies. It allows simultaneous evaluation of the morphology and kinetics of MM cells in routinely fixed, decalcified, paraffin-embedded material.
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