Changes in nuclear ribonucleoprotein constituents and chromatin disposition during neuronal differentiation and maturation.

Morphological and quantitative changes of ribonucleoproteic (RNP) structures and chromatin are studied in the nuclei of undifferentiated cells, neuroblasts and neurons in several degrees of maturation, in order to relate them to the drastic modifications in transcription and/or RNA processing taking place during cell differentiation. Undifferentiated (matrix) cells of 2-day embryos differ from that of 4-day embryos in their nucleolar volume and in the amount of compact chromatin. These differences are interpreted as the earliest signs of neuroblast differentiation. All over the process of matrix cell-bipolar-multipolar neuroblast differentiation there is a large spreading of compact chromatin well before any important change in RNP structures or in nuclear volume. The most remarkable increase in nuclear volume and in the amount of RNP particles occurs during the differentiation of multipolar neuroblasts to immature neurons, which is characterized by large synaptogenic activity. The interpretation of these changes is discussed in connection, on one hand with the metabolic effects of synapses, and on the other hand with the variations of gene expression taking place in cell differentiation and under other natural and experimental situations.