The nucleoli rim is a potential fourth subcompartment of the nucleoli, but its function is still unknown. The proteins localizing here might be associated to the perinucleolar heterochromatin, where they could aid the tethering of the chromatin to the nucleoli. A large fraction of the proteins that localize to the rim of nucleoli also localize to the perichromosomal layer of mitotic chromosomes.
The nucleoli rim staining can be seen as a thin circle around the nucleolus. The staining could be more or less distinct dependening on the cell line.
Read more about the proteome of the nucleoli rim as a substructure of nucleoli.