Schwann cell surfaces but not extracellular matrix organized by Schwann cells support neurite outgrowth from embryonic rat retina.

Despite evidence that glial cell surfaces and components of the extracellular matrix (ECM) support neurite outgrowth in many culture systems, the relative contributions of these factors have rarely been compared directly. Specifically, it remains to be determined which components of peripheral nerve support growth of central nerve fibers. We have directly compared neurite outgrowth from embryonic day 15 rat retinal explants placed onto beds of (1) Schwann cells without ECM, (2) Schwann cells expressing ECM (including a basal lamina), (3) cell-free ECM prepared from neuron-Schwann cell cultures, (4) nonglial cells (fibroblasts), and (5) 2 isolated ECM components, laminin and type I collagen. From the first day in culture, retinal explants extended neurites when placed on Schwann cells without ECM. Outgrowth on Schwann cells expressing ECM was also extensive, but not obviously different form that on Schwann cells alone. Ultrastructural study revealed that 95% of retinal neurites in ECM-containing cultures contacted other neurites and Schwann cell surfaces exclusively. On cell-free ECM prepared from neuron-Schwann cell cultures, neurite extension was poor to nonexistent. No neurite outgrowth occurred on fibroblasts. Retinal explants also failed to extend neurites onto purified laminin and ammoniated type I collagen substrata; however, growth was rapid and extensive on air-dried type I collagen. In cultures containing islands of air-dried type I collagen on a laminin-coated coverslip, retinal explants attached and extended neurites on collagen, but these neurites did not extend off the island onto the laminin substratum. We conclude from these experiments that neurite extension from embryonic rat retina is supported by a factor found on the surface of Schwann cells and that neither organized nor isolated ECM components provide this neurite promotion. These findings are discussed in relation to possible species differences in growth requirements for retinal ganglion cell neurites and to the specificity of response of different CNS neurites to ECM substrata.