Changes in the number and volume of fibrillar centres with the inactivation of nucleoli at erythropoiesis.

The number and volume of fibrillar centres, the structural components of interphase cell nucleoli on the surface of which rRNA is synthesized, have been studied in differentiating erythroblasts of mouse embryo liver. Complete series of ultrathin sections of erythroblast nuclei have been used at the main stages of differentiation: proerythroblast, basophilic erythroblast, polychromatophilic erythroblast and normoblast. It has been shown that in the active nucleoli of proerythroblasts the number of fibrillar centres is correlated with cell ploidy and exceeds by several-fold the number of nucleolus-organizing regions of chromosomes. The total volumes of fibrillar centres in 2C (0.369 micron 3) and 4C (0.749 micron 3) proerythroblasts are proportional to number of nucleolus-organizing regions. With the maturation of erythroblasts the total number of fibrillar centres declines and in normoblasts it is 3- to 10-fold less than that of the nucleolus-organizing regions. The total volume of fibrillar centres in normoblasts (0.102 micron 3) is threefold smaller than that in proerythroblasts (0.369 micron 3), even though the mean volumes of individual fibrillar centres are significantly higher (0.0042 micron 3 in proerythroblasts and 0.039 in normoblasts). The optical density of fibrillar centres in normoblasts can be higher compared with that of proerythroblasts. It has been suggested that the inactivation of nucleoli at erythropoiesis is accompanied by the fusion of individual fibrillar centres and, possibly, by the compaction of their material.