Three-dimensional organisation of RNA-processing machinery in avian growing oocyte nucleus

Chromosome architecture and assorted nuclear compartments play an essential role in RNA transcription and processing. Growing vertebrate oocytes represent an advantageous model to study the principles of nuclear structure and function. In this article, the data on three-dimensional (3D) organisation of intact and non-deformed oocyte nuclei (germinal vesicles) in four species of birds (domestic chicken, Japanese quail, rock pigeon and chaffinch) obtained by confocal laser scanning microscopy are presented. The nucleus of the growing avian oocyte has an unusual structure of RNA processing machinery. Germinal vesicles from any of the avian species studied, with the exception of the rock pigeon, are characterized by absence of extrachromosomal RNA-enriched nuclear bodies including Cajal bodies, histone locus bodies and interchromatin granule clusters. The absence of Cajal bodies and histone locus bodies in chicken oocytes correlated with the inactivation of nucleolus organizer and clustered histone genes. Splicing factors such as SR-protein SC35 accumulated in chromosome-associated domains that were classified as complex loops (terminal giant and lumpy loops). Formation of such depot in avian oocyte nuclei is supposed to be nucleated by transcripts of non-coding tandem repeats. The results obtained strongly support a model of RNA-mediated nuclear domains formation.