INCREASED FUNCTIONAL LOAD ON MOUSE KIDNEY PROXIMAL TUBULE EPITHELIAL-CELLS CAUSES CHANGES IN NUCLEOLAR 3-D ARCHITECTURE

Ultrastructural 3-D analysis of nucleolar architecture and Ag-NOR prot ein distribution in mouse kidney-cortex proximal-tubule epithelium has been performed. A principal scheme of structural changes of the nucle olus and organization of its components during the intensification of pre-rRNA synthesis (dynamic model of a nucleolus) based on computer sp atial modelling has been advanced. According to the nucleolar composit ion, three groups of cells, which differ from each other by rRNA synth esis, are defined in normal kidney. Most nephron proximal-section cell s (about 52%) are characterized by lower activity of RNA synthesis. Su ch kind of cells are defined as group I(nucleolar diameter 0.7-1.5 mu m) and always contain resting, ring-shaped or close to ring-shaped den se nucleoli, which have 2 or 3 fibrillar centers. Nucleoli of group II cells (about 37%, nucleolar diameter 1.5-2.5 mu m) have a higher leve l of activity, contain 4-7 fibrillar centers, and their structural org anization is close to reticulated forms due to the first indications o f vacuolar network (identified as prereticulated nucleoli). The most a ctive cells of group III (about 11%, nucleolar diameter 2.5-3.5 Crm) i nclude cells with typical reticulated nucleoli with a well expressed v acuolar network and numerous fibrillar centers (18-22). Increased func tional load of the epithelium caused by unilateral nephrectomy and diu retic (4-chlor-H [2-furylmethyl] 5-sulphamyl-antranic acid) injection changed the proportion of the different cell groups: group I decreased (about 25%), whereas groups II and III increased (about 8% and 17%, r espectively). The increase of nucleolar activity first causes a deform ation of the individual fibrillar centers as well as complication and growth of their surface. Further, a progressive fragmentation of the f ibrillar centers and the growth of their total volume is observed. The complication and growth of the total volume of Ag-positive zones is a nother indication of the nucleolar activation. The vacuolar system dev elops by a gradual fusion of small isolated cavities into a united vac uolar network. Nucleoli with 2-7 fibrillar centers are considered to b e intermediate forms reflecting successive stages of its activation or inactivation: from the resting ring-shaped nucleolus via transient st ages of increasing functional activity to the active reticulated nucle oli and vice versa. The observed differences in the nucleolar ultrastr ucture are regarded as evidence of the functional heterogeneity of cel l populations within one functional segment of nephron.