Localization of rDNA transcription sites in nucleoli of human Sertoli cells: an EM quantitative autoradiographic study using 3H-uridine

The sites of rDNA transcription within human Sertoli cell nucleoli have been localized using EM autoradiography after a 45-min pulse of 3H-uridine and an exposure time of 6 months. Two successive quantitative image analyses, one derived from the 50% probability circle method and the other from the cross-fire method, allowed us to estimate the radioactivity incorporated within each nucleolus compartment. This study demonstrated that rDNA transcription occurred mainly at the border between fibrillar centers and dense fibrillar components and to a lesser extent within the dense fibrillar component. The other Sertoli cell nucleoli compartments did not incorporate 3H-uridine and therefore were not involved in rDNA transcription.