This chapter reviews the developments in laboratory over the past two years using adrenal chromaffin cell implants and the evolution of the way in which the chromaffin cell transplants may interact with the host brain. Some chromaffin cells do survive when transplanted intraparenchymally into the rat models of Parkinson's disease but exhibit very poor survival in other mammalian species. Chromaffin cells survive capriciously and continue to exhibit their endocrine phenotype when cultured in the absence of nerve growth factor (NGF). However, the addition of NGF to the culture medium induces a phenotypic change in chromaffin cells that results in the extension of neuritic processes and a decrease in the size of their cytoplasmic dense-core vesicles. If chromaffin cell survival augments the trophic action of implant surgery and continues to provide a source of trophic molecules for a host sprouting response, then it might still be important to achieve the maximum amount of chromaffin cell survival possible. Several laboratories have demonstrated in rodent models that the addition of NGF or NGF-producing cells with implanted chromaffin cells will significantly enhance behavioral improvement. The advantage of NGF-producing cells is that they potentially may provide a continuous source of NGF, thus eliminating the need to administer NGF exogenously. One such source of species-specific NGF is the host's Schwann cells that ensheath peripheral nerve fibers. Schwann cells synthesize NGF and express an increase in NGF receptors following transection of a peripheral nerve, a response that facilitates survival and regeneration in the peripheral axons.
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