Conditioned medium enhances neuritic outgrowth from rat spinal cord explants

Medium conditioned by primary cultures of fetal or neonatal rat skeletal muscle, fibroblasts, or lung cells dramatically increases the neuritic outgrowth from spinal cord explants. After 7 days in vitro, the outgrowth of neurites from 15- to 16-day fetal rat spinal cord slices grown in conditioned medium (CM) covers a 3- to 4-fold greater area than that from slices grown in fresh, nonconditioned (control) medium. Moreover, the pattern of neuritic outgrowth is markedly different in CM-treated slices. In control slices, the neurites form a tangled, dense network of neurites which usually extend only a small distance from the slice edge, while in CM-treated slices, the neurites form a more open network, with the majority of neurites extending radially for long distances (up to several millimeters) from the slice edge. The effect of CM on neuritic outgrowth is not due to a detoxification or modification of the serum in the medium, because increased neuritic outgrowth was observed in slices grown in medium conditioned in the presence or absence of 10% fetal calf serum. The outgrowth-enhancing factor(s) in CM has a high molecular weight, since all outgrowth-enhancing activity is retained by membrane filters with a nominal molecular weight cutoff of 10⁵ daltons. This factor(s) is stable at 58°C for 30 min, and does not appear to be βNGF or fibronectin.