Structure of landmark loop ribonucleoprotein matrices in Pleurodeles waltlii lampbrush chromosomes visualized by scanning electron microscopy

Landmark loop ribonucleoprotein (RNP) matrices of Pleurodeles waltlii lampbrush chromosomes were systematically examined by scanning electron microscopy (SEM). The results, which corroborated similar studies by electron microscopy (EM), showed that RNP transcripts in normal loops, and RNP matrices in granular, globular and dense loops, are composed of one basic structure: an RNP particle with a diameter of 30 nm. SEM observations also clarified the spatial arrangement of this particle in the RNP matrices of all the loop types examined. The specific morphology of normal, granular, globular and dense loop RNP matrices depended on the degree of compaction of the transcription products; this compaction resulted both from the packaging of RNP transcripts and the progressive coiling of the loop axis.