Structure
Three major components of the nucleolus are recognized: the fibrillar centers (FC), the dense fibrillar components (DFC), and granular components (GC). The DFC or pars fibrosa consists of newly transcribed rRNA bound to ribosomal proteins, while the GC, called pars granulosa, contains rRNA bound to ribosomal proteins that are beginning to assemble into ribosomes.
However, it has been proposed that this particular organization is only observed in higher eukaryotes and that it evolved from a bipartite organization with the transition from anamniotes to amniotes. Reflecting the substantial increase in the DNA intergenic region, an original fibrillar component would have separated into the FC and the DFC.
Another structure identified within many nucleoli (particularly in plants) is a clear area in the center of the structure referred to as a nucleolar vacuole. Nucleoli of various plant species have been shown to have very high concentrations of iron in contrast to human and animal cell nucleoli.
Read more about this topic: Nucleolus
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