Ultrastructural cytochemistry of the chromosomes of growing oocytes of Perinereis cultrifera (Nereidae, Polychaeta, Annelida) after fixation in situ

In situ cytochemical study of oocytes of Perinereis demonstrated that the so-called ’nucleoplasmic granulations’ (NG) are in fact sections of diplotene chromosomes active in transcription. Indeed, with the osmium-ammine Feulgen-like reaction, a first type of NG, the fibrillar clumps, were shown to contain nucleosomal DNA. Therefore, fibrillar clumps represent chromatin of bivalents. These chromatin clumps frequently exhibit a ring-like configuration due to the presence at their center of a non DNA-containing core. The latter could correspond, in section, to the proteinaceous chromosomal axis as suggested after in situ silver staining. The second type of NG, the dense aggregates, were stained with the EDTA regressive procedure preferential for RNA, indicating that they are transcription products. Evidence is presented that these RNP are of chromosomal origin. Similarities between the nuclear RNP-rich aggregates of oocytes of Perinereis and the loop matrix of lampbrush chromosomes are considered.