Changes in nucleoli and nucleolar fibrillar centres of chromaffin cells in rat adrenal medulla over a 24-hour period: an ultrastructural and stereological analysis.

A stereological and ultrastructural study was performed on the nucleoli of the adrenal medulla chromaffin cells of rats exposed to a standardized 12 h light/12 h dark cycle with free access to food and water. The animals were killed three at a time, every 4 h during the 24-h span and fixed by intracardiac perfusion. In these reticulated nucleoli, the stereological analysis over a 24-h period showed a variation dependent on the time of killing for the two parameters investigated, the mean nucleolar volume, Vnu, and the mean volume of the fibrillar centres, Vfc(nu). The minimal value occurred at 0300 h (dark span) and the maximal one at 0700 h (at the onset of the light span). Between these two extreme values, Vnu increased 1.8-fold and Vfc(nu) 5.3-fold. These data are compared with a previous description from our laboratory of circadian rhythm in nucleoli of sympathetic neurons of superior cervical ganglion in the same animals. Analogies and differences are pointed out, but apart from these considerations the present study provides a new example of temporal organization at the cellular level in the organelle involved in ribosomal RNA synthesis and ribosome assembly.