Abstract
<p>I have utilized in vitro techniques, including a documented neuron-Schwann cell coculture system, to study Schwann cells derived from adult rats and humans. Comparisons are made between these cells and the well characterized embryo-derived rat Schwann cell, concerning procurement techniques, proliferative capacities and differentiation in response to axonal contact. Cell populations highly enriched in Schwann cell content ($>$90%) can be derived by enzymatic dissociation of pieces of peripheral nerve (rat or human) that have been maintained in culture for several weeks. As compared to dissociation of nerve immediately after harvest, this method leads to greatly increased cell yield and a much smaller population of contaminating fibroblasts. Adult-derived rat Schwann cells (RASCs) proliferate in response to growth factors or axonal contact, serve as a promotional substrate for the growth of retinal axons, and fully differentiate in coculture with sensory neurons (including ensheathment and myelination of axons). By these standards, RASCs do not appear to differ from their embryo-derived counterpart. Human adult-derived Schwann cells (HASCs), in contrast, show limited proliferative potential in response to axonal contact (24h HASC $\sp3$H-thymidine incorporation labelling indices typically 6-10% vs. RASC labelling indices of 40-80%). The addition of antibodies against heregulin, or a putative heregulin receptor (p185$\sp{erbB2}$), attenuates the incorporation of $\sp3$H-thymidine, indicating a role for this receptor ligand complex in axon-induced stimulation of Schwann cell proliferation. Under the coculture conditions that support RASC differentiation, primary HASCs do not proceed to fully differentiate. Only limited ensheathment and no myelin formation is observed. Further, a detrimental effect on the state of the neurons, characterized by cell body shrinkage and decrease in neuritic field density is seen in cultures containing these HASC populations. This shrinkage can be elicited by coculture with HASCs or human fibroblasts, or conditioned medium from either of these cell types, and is not caused by RASCs. By extensive purification and heregulin-induced expansion of cell numbers, I have generated a population of HASCs which (a) does not cause neuronal shrinkage, and (b) differentiates (ensheathes and myelinates axons) in coculture with sensory neurons. These results indicate that Schwann cells derived from adult humans are capable of redifferentiating after removal from axonal contact in vivo. Studies of these cells, and their differences from rat-derived cells, are likely to yield insights into both axon-Schwann cell interactions and human-specific cellular biology.</p>