Atypical CLL is characterized by the presence of prolymphocytes like this one. These cells are unusual in that they have single prominent nucleoli in their nuclei. That's the light colored circle amongst the dark marron colored middle.
The nucleolus is a prominent nuclear structure best known for its central role in the manufacture of ribosomes. When a cell enters division, the nucleoli are taken apart but at the end of cell division they begin to reassemble around clusters of ribosomal RNA genes, called nucleolar organizer regions.
The nucleolus, which is not bound by a membrane, can be divided into three sub-regions: the fibrillar centre, the dense fibrillar component and the granular component. In addition to manufacturing ribosomes, other functions include control of the cell-cycle, cellular stress responses and synthesis of small RNAs and RNA-protein complexes. Over 4,500 proteins are believed to localize to the nucleolus in human cells and these proteins are estimated to represent over 80 % of the total nucleolar proteome.
The nucleolus appears to be highly conserved throughout all life forms, since approximately 90 % of human nucleolar proteins have homologues in yeast. The size and number of nucleoli in human cells vary according to the cell type and the proliferation rate of the cell. Some inherited diseases such as Diamond-Blackfan anemia, dyskeratosis ongenita and Shwachman-Diamond syndrome, all associated with increased risk of cancer, are caused by mutated nucleolar proteins. Consistent with the nucleolus having a significant influence on cell proliferation and growth, disturbances and alterations in its structure and function have been identified in a number of types of cancer but nothing is known about nucleolar function in CLL