NUCLEOLAR ORGANIZER REGIONS IN A MODEL OF CELL HYPERACTIVITY AND REGRESSION

Nucleolar organizer regions stained with colloidal silver techniques ( AgNOR) evidence sites of active rRNA transcription. It has been proved that AgNOR undergo a rise in number and variations in size and shape in conditions which traditionally involve enhanced cell proliferation and rRNA transcription. AgNOR have been described as a marker of malig nant transformation in multiple entities. Our laboratory has previousl y described their value as markers of radioinduced damage. The finding , at light microscopy level, that silver staining persisted at later p ost-irradiation times when cells are characteristically inactive, prom pted the present study to correlate findings at light microscopy level with the ultrastructural analysis of nucleoli and their AgNOR in a mo del of irradiated skin. We herein attempt to explain the biological si gnificance of AgNOR variations in the different phases of radioinduced response (which involves cellular hyperactivity followed by regressiv e features). Ten Wistar rats were submitted to local irradiation of th e left leg (the shielded right leg was used as control) with 50 Gy x r ays and killed 1-5 days post- irradiation. Silver staining was perform ed on ultrathin sections. In the basal layer of control epithelium sil ver affinity was established for fibrillar centers (FC) and fibrillar dense components (DFC). During the phase of radioinduced hyperplasia ( 1-3 days post-exposure) basal cells exhibit large reticular nucleoli, with irregular contours and silver staining on DFC. In the regressive phase (4-5 days post-irradiation) silver staining persists despite the halt in transcriptional activity, associated to homogeneous and compa ct nucleoli. These findings suggest caution in the interpretation of s ilver staining patterns.