Schwann cells and their precursors emerge as major regulators of nerve development

It is becoming ever clearer that Schwann cells and Schwann-cell precursors are an important source of developmental signals in embryonic and neonatal nerves. This article reviews experiments showing that these signals regulat e the survival and differentiation of other cells in early nerves. The evid ence indicates that glial-derived signals are necessary for neuronal surviv al at crucial periods of development, that they regulate the molecular and functional specialization of axons and that they control the maturation of the perineurial sheath that protects nerves from inflammation and unwanted macro-molecules produced in the surrounding tissues. Furthermore, an autocr ine survival circuit enables Schwann cells in postnatal nerves to survive i n the absence of axons, a vital requirement for successful nerve regenerati on following injury. The molecular identity of these signals and their rece ptors is currently being determined.