Two components of conditioned medium increase neuritic outgrowth from rat spinal cord explants

Conditioned medium (CM) from muscle or fibroblast cultures dramatically increases the outgrowth of neurites from fetal rat spinal cord slices in vitro. We report here that there are two separable fractions in conditioned medium that cause this increase in neuritic outgrowth. One fraction, with a molecular weight of approximately 50,000 daltons, enhances neuritic outgrowth only when it is present in the culture medium so that the slices are directly exposed to it. The second component has a much larger molecular weight (above 300,000 daltons), and can enhance neuritic outgrowth by directly binding to the culture substrate on pretreatment. Only when these two fractions are combined by pretreating the substrate with the higher molecular weight fraction and then growing the slices in the lower molecular weight fraction is the full outgrowth promoting activity of whole conditioned medium reconstituted. These two components act synergistically to reproduce nearly the full outgrowth promoting activity of non-fractionated, whole conditioned medium.