Growth factors in chromaffin cells.

Chromaffin cells, the neuroendocrine cells of the adrenal medulla and paraganglia, occupy paradigmatic roles in molecular, cellular and developmental neurobiology. The fact that they are very effective 'minipumps', secreting numerous bioactive substances including amines, neuropeptides and proteins, has made them interesting and useful for the treatment of chronic pain and Parkinsonism. An essential advantage of chromaffin cells is that they can be isolated and purified in extremely high numbers, which can never be achieved for peripheral or central nervous system neurons. Growth factors (cytokines) and peptides with growth factor-like efficacies constitute an important component of bioactive materials released from chromaffin cells. Not only their presence, but also neural and humoral mechanisms regulating their expression and release, are now being revealed. Prominent examples include fibroblast growth factors (FGFs), transforming growth factor-beta s (TGF-beta s) and interleukins. Functions that can be assigned to these factors in the adrenal medulla are gradually emerging. For example, FGFs and TGF-beta s can regulate chromaffin cell proliferation and differentiation and participate in the neurotrophic maintenance of neurons innervating chromaffin cells. In contrast, the functions of the predominant secretory proteins of chromaffin cells, the chromogranins, are still largely unknown, but might include cytokine-like roles. Thus, chromaffin cells continue to teach neurobiologists about the fundamental capacity of neurons to secrete bioactive molecules with a wide range of functions as well as modes of their secretion underscoring the close relationship of endocrine and neuronal systems.