3-DIMENSIONAL ORGANIZATION OF NUCLEOLI AND OF NUCLEOLUS ORGANIZING REGIONS IN DIFFERENTIATED CELLS .2. RETICULATED, VACUOLIZED AND NUCLEOLONEMAL NUCLEOLI OF NORMAL AND REGENERATING MOUSE HEPATOCYTES

By means of stereological and morphological analysis, the dynamics of nucleolar structural changes in stimulated mouse hepatocytes has been studied. Reticulated and vacuolized nucleoli typical for normally functionated hepatocytes were shown to convert into the nucleolonemal nucleoli, first noticed 18 hours following operation. The process of activation of mouse hepatocyte nucleoli involves two steps. At the first step (within 1-18 hours following operation) a progressive growth of nucleoli volume, due to a simultaneous reduction of vacuolar sizes and growth of RNP-component occurs. Such changes were observed in both groups or nucleoli being present in the intact mouse liver, because at this particular step a considerable decrease in the number of vacuolized nucleoli took place. Besides, in the stimulated hepatocyte nucleoli the number and total volume of fibrillar centers increase. However, in spite of their considerable changes, the nucleoli preserve the marks characteristic of the reticulated type. The second step of activation, noted 18 hours after stimulation of hepatocytes, includes more fundamental structural reconstructions. As the result, reticulated nucleoli obtain the nucleolonemal structure. At the same time, on the background of a further decrease in the individual volume and an essential growth of the total volume in the fibrillar centres, a spasmodic increase in the mass of the dense fibrillar component, which is associated with the formation of the continuous strand of the nucleolonema. All this is responsible for the complete changes in structural organization of nucleoli commonly seen in mouse hepatocytes. It is suggested that the cause of such a structural reconstruction may be due to the changes in topography of transcriptionally active regions of rDNA.