Changes in nucleolar morphology and volume of the supraoptic nucleus neurons during postnatal development of the rat

The changes in the volume and nucleolar ultrastructure of neurons of the supraoptic nucleus were examined during the postnatal period of the rat. This morphological maturation involved the gradual transformation from relatively compact nucleoli to reticulate ones which exhibited a typical nucleolonemal configuration with numerous nucleolar interstices and fibrillar centers. A progressive augmentation of the nucleolar volume was also observed during postnatal life; the average value on the first postnatal day was 2.94 +/- 0.42 micron3 while by the 40th day the mean had increased to 15.01 +/- 0.45 micron3. We suggest that this cytodifferentiation process is the morphological expression of the activation of nucleolar ribosomal RNA synthesis, which is required for the growth and maturation of the neurosecretory cells.