[The 3-dimensional organization of the nucleolus and nucleolus-organizer regions of differentiated cells. IV. The structural and functional heterogeneity of the nucleoli in the epithelium of the proximal nephron in the mouse].

By means of stereological and morphometrical analysis, the ultrastructure of nucleoli in epitheliocytes of mouse kidney cortex proximal tubuli has been studied. In accordance to the nucleolar composition, three main groups of nephrocytes with different levels of rRNA and protein synthesis were defined. Functional heterogeneity of proximal tubuli epithelium was established by correlation between different variants of ultrastructural organization of nucleoli and the total RNA synthesis activity, determined by 3H-uridine incorporation intensity. It has been shown that a greater part of cells (about 52%) in the nephron proximal section, which is characterized by slow RNA synthesis, causing a low functional activity of these cells, presumably represents a reparative cellular reserve. Such cells, defined as the 1st group cells, have resting, ring-shaped nucleoli with one fibrillar centre, and nucleoli similar to the ring-shaped ones but containing 2-3 fibrillar centres. Nucleoli of the 2nd group of nephrocytes (about 37%), most actively incorporating labeled precursor, contain 4-6 fibrillar centres. Their structural organization is closer to the reticular type of nucleoli. The 3rd most actively labeled group of nephrocytes includes cells with typical reticulated nucleoli. The number of fibrillar centres in the reticulated nucleoli is much higher (18-22) than in the 1st and 2nd groups of nephrocytes. Structural and functional polymorphism of nephrocytes was revealed not only in the proximal part of one nephron. During the increase in functional activity of nephrocytes, caused by unilateral nephrectomy, the quantitative correlation between cells related to these different groups was seen to change. The number of cells of the 1st group decreased by 24%, whereas that in the 2nd and 3rd groups increased by 9 and 15%, respectively. Nucleoli with 2-3 fibrillar centres are considered as transitional forms between the inactive ring-shaped nucleoli and the active reticulated nucleoli. Differences in the ultrastructure of nucleoli may be considered as an evidence of functional heterogeneity of nephrocytes within the proximal segment of nephron.