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 derived from 14-day-old rat embryos during their development under various culture conditions. Cells maintained in serum-free medium which have developed for 1 week in vitro present ultrastructural features of young neurons. They contain many free ribosomes and microtubules, but few other organelles and incompletely developed Golgi apparatus. In the presence of basic fibroblast growth factor (bFGF), besides cells remaining in aggregates and displaying morphological features of undifferentiated cells, dispersed neuroblasts underwent accelerated ultrastructural maturation. They present well-developed Golgi apparatus, axodendritic synapses and dense-core vesicles already after 3 days in culture. By contrast, in the presence of astroglial-conditioned medium a more homogeneous population developed showing ultrastructural features of relatively mature neurons. However, the neuronal precursors acquired the most mature ultrastructural aspect when they were cocultured with astroglial cells. The neuronal cell bodies contain highly developed Golgi complexes, well-differentiated ergastoplasm and Niss1 body formations, while in the complex neurite network much more numerous mature synapses with clear and dense-core vesicles are visible. These observations indicate that a combination of soluble factors and membrane-bound factors is essential for extensive ultrastructural development of neuronal precursors in vitro. Another finding was that in these cultured neurons neurofilaments (NF) were never seen, while NF protein subunits were found. These data suggest that the polymerization of the three NF subunits into intermediate filaments might need particular cellular factors which probably do not exist under our in vitro conditions.