Distribution of fibroblast growth factor in cultured dorsal root ganglion neurons and Schwann cells. I. Localization during maturation in vitro.

The distribution of acidic and basic fibroblast growth factor in co-cultures of dorsal root ganglion neurons and Schwann cells was examined as a function of time in culture. After two days in vitro, the cytoplasm of the neuronal cell bodies demonstrated both acidic and basic FGF immunoreactivity, whereas the cytoplasm of the neurites was not immunoreactive. Schwann cells, in contrast, exhibited both acidic and basic fibroblast growth factor cytoplasmic immunoreactivity. After two days in culture, immunoreactivity was not detected on the plasma membrane surface of either the neurons or the Schwann cells. By 10 days in vitro, fibroblast growth factor immunoreactivity was observed in the cytoplasm of the most proximal portion of some, but not all, neurites but was unchanged in Schwann cells. At 20 days in vitro, immunoreactivity was still restricted to the intracellular compartment of both Schwann cells and neurons. Acidic fibroblast growth factor was primarily localized to the cytoplasm of Schwann cells, neuron cell bodies and along the entire length of the neurites. In contrast, basic fibroblast growth factor was predominantly localized to the nuclei of Schwann cells and small to medium size neurons. In many cases, the nucleolar region demonstrated the most intense basic fibroblast growth factor. The cytoplasm of the neurites was also immunoreactive for basic fibroblast growth factor. At 30 days in vitro the intracellular distribution of fibroblast growth factor immunoreactivity was similar to that observed at 20 days. However, both acidic and basic fibroblast growth factor were detected on the surface of the neurites. In contrast, no fibroblast growth factor immunoreactivity was detected at the Schwann cell surface at any time point examined. The distribution of fibroblast growth factor in Schwann cells cultured by themselves was similar to that of Schwann cells co-cultured with neurons after 20 days in vitro. Both Schwann cells and dorsal root ganglia exhibited increased fibroblast growth factor immunoreactivity with increased time in culture and an increased expression of basic fibroblast growth factor in the nucleus. Of particular interest was the appearance of fibroblast growth factor on the surface of neurites after 30 days in vitro where it could function to modulate neuron-glial cell interactions.