Three-dimensional electron microscopy of the internal nucleolus-associated chromatin and of the nucleolar vacuoles during early germination of Sinapis alba.
Spatial relationships between the internal nucleolus-associated chromatin (NAC) and the numerous nucleolar vacuoles that appear during early germination have been studied in nucleoli of quiescent (non-germinated) and early germinating embryos of Sinapis using serial sections. In quiescent non-vacuolated nucleoli, the transcriptionally inactive internal NAC is a short strand about 900 nm thick that in cross-section appears as heterogeneous fibrillar centres (FCs). At 4 and 6 h after germination one or several large networks of interconnected nucleolar vacuoles develop around the dispersing internal NAC. Clumps of dense chromatin are still present within the nucleolar vacuoles and are probably unfolding into deoxyribonucleoprotein (DNP) fibres (about 110 nm thick), which rapidly intrude within the nucleolar body and form thin chromatin threads. At 24 h after germination the internal NAC is more dispersed and forms, for its greatest part, a long thread (about 240 nm in diameter) wrapped up with a few dense fibrillar component, the whole forming the first outline of a nucleolonema. In cross-section most of the internal NAC appears as homogeneous FCs but short portions remain more condensed and appear as heterogeneous FCs always associated with a nucleolar vacuole. From 48 h the internal NAC is a longer thinner strand (about 160 nm in diameter), probably continuous and surrounded entirely by a homogeneous muff of dense fibrillar component, the whole forming a typical nucleolonema (about 950 nm thick) meandering throughout the nucleolus. Small amounts of the internal NAC still remain undispersed in the form of heterogeneous FCs associated with a nucleolar vacuole. The repeated association of nucleolar vacuoles and dispersing internal NAC suggests that they could play a role in chromatin dispersion and, or, activation by creating a favourable microenvironment.