Abstract
Studies over the last century have shown that peripheral nerve grafts containing viable Schwann cells (SCs) are capable of promoting regeneration from both injured peripheral and central nervous system axons. Only recently has it been possible to isolate and expand SCs from adult rat peripheral nerve in cell culture. This approach has allowed the construction of cellular prostheses consisting of cultured SCs, which when transplanted into both the PNS and CNS of the rat, engender axonal regrowth. Using this technology to repair nervous system injuries in man is contingent upon a thorough understanding of the biology of cultured human SCs. Segments of human peripheral nerve were stored in University of Wisconsin storage solution at 4$\sp\circ{\rm C}$. Using an in vitro assay system, numerous viable cells were obtained from nerves stored for short intervals ($<$ 1 week), while lengthier storage times resulted in few viable cells. The heregulins (HRGs) are human gene products which are closely related to glial growth factor. Using DNA synthesis assays, HRG was found to be an effective mitogen for human SCs. With the addition of agents which elevate cAMP, it is possible to expand these cells over multiple passages without overwhelming fibroblast contamination. A monoclonal antibody (2C4) to the p185$\sp{\rm erbB2}$ protein inhibited HRG stimulated $\sp3$H-thymidine incorporation in human SCs suggesting an essential role for this receptor in transducing the HRG signal. The functional capacity of human SCs prepared in culture was tested by placing the cells within guidance channels into a peripheral nerve gap in two species of immune deficient rodents. In nude rats the human SCs survived and substantially enhanced axonal regeneration. There was, however, invasion of host SCs into the guidance channels and only a minority of the transplanted human SCs formed myelin. Primary and expanded human SCs as well as xenografts of human peripheral nerve transplanted into severe combined immunodeficient (scid) mice resulted in the formation of substantial amounts of human myelin. In the guidance channels as well as the xenografts there again was invasion of the transplant by host SCs which also formed myelin around regenerating mouse axons.