Locating transcribed and non-transcribed rDNA spacer sequences within the nucleolus by in situ hybridization and immunoelectron microscopy.

Immunoelectron microscopy and in situ hybridization have been used to investigate the precise location of transcribed and non-transcribed rDNA spacer sequences. Whereas a 5'-external transcribed spacer sequence is predominantly visualized in the fibrillar centers of nucleoli, a non-transcribed spacer sequence is preferentially detected in the interstices, in close contact with the fibrillar centers and which interrupt the surrounding dense fibrillar component. Occasionally these two spacers are also observed in clumps of dense nucleolus-associated chromatin. These observations provide insights into the organization of ribosomal repeats within the nucleolus.