A single peripheral injection of basic fibroblast growth factor (bFGF) stimulates granule cell production and increases cerebellar growth in newborn rats

The control of neuronal number is critical for coordinating innervation and target organ requirements. Although basic fibroblast growth factor (bFGF) is known to regulate neuron number in the developing embryonic cortex, its potential role during postnatal brain development remains undefined. To address this issue, the cerebellum, a site of postnatal neurogenesis, was used. Previously, we found that a single peripheral injection of bFGF in newborn rats elicited mitosis of neuronal precursors in the external germinal layer (EGL) 8 h after administration. We now define the sustained effects of bFGF treatment on postnatal granule cell production and cerebellar growth. Seventy-two h after a single injection of bFGF (20 ng/g) in newborn rats, the fraction of BrdU-labeled cells in the EGL increased by 46% without altering apoptotic cell number, consistent with enhanced precursor proliferation. Moreover, bFGF increased mitotically labeled cells by 100% and total cell density by 33% in the internal granular layer (IGL), the final destination of the EGL precursors. Because cerebellar volume also increased by 22%, bFGF-induced proliferation enhanced generation of total IGL neurons and increased cerebellar growth. These morphometric measures were corroborated independently by using DNA quantitation: cerebellar DNA content increased 16% after bFGF injection, consistent with increased neuron number. Furthermore, using DNA quantitation as an index, increased total cerebellar cell number elicited by bFGF injection persisted beyond the neurogenetic period, until P35. We conclude that a single postnatal injection of bFGF increases granule neuron number and enhances cerebellar growth following mitotic stimulation.