INFLUENCE OF BASIC FIBROBLAST GROWTH-FACTOR AND ASTROGLIAL CELLS ON THE ULTRASTRUCTURE OF DEVELOPING RAT-BRAIN NEURONAL PRECURSORS IN-VITRO

We have examined the ultrastructural aspect of neuronal precursors der ived from 14-day-old rat embryos during their development under variou s culture conditions. Cells maintained in serum-free medium which have developed for 1 week in vitro present ultrastructural features of you ng neurons. They contain many free ribosomes and microtubules, but few other organelles and incompletely developed Golgi apparatus. In the p resence of basic fibroblast growth factor (bFGF), besides cells remain ing in aggregates and displaying morphological features of undifferent iated cells, dispersed neuroblasts underwent accelerated ultrastructur al maturation. They present well-developed Golgi apparatus, axodendrit ic synapses and dense-core vesicles already after 3 days in culture. B y contrast, in the presence of astroglial-conditioned medium a more ho mogeneous population developed showing ultrastructural features of rel atively mature neurons. However, the neuronal precursors acquired the most mature ultrastructural. aspect when they were cocultured with ast roglial cells. The neuronal cell bodies contain highly developed Golgi complexes, well-differentiated ergastoplasm and Nissl body formations , while in the complex neurite network much more numerous mature synap ses with clear and dense-core vesicles are visible. These observations indicate that a combination of soluble factors and membrane-bound fac tors is essential for extensive ultrastructural development of neurona l precursors in vitro. Another finding was that in these cultured neur ons neurofilaments (NF) were never seen, while NF protein subunits wer e found. These data suggest that the polymerization of the three NF su bunits into intermediate filaments might need particular cellular fact ors which probably do not exist under our in vitro conditions.