The ability to grow primary neurons under serum-free conditions is facilitating better control in studies of neuronal development, mechanisms of neuronal signaling, electrophysiology, pharmacology, plasticity, in vitro growth requirements, gene expression, and neurotoxicity. Some of the new commercially available media combinations allow for the growth of sparse populations of neurons, which in turn allow for the study of individual neurons and synapses. This has not been possible using serum-supplemented media without a feeder layer of glial cells. In serum-supplemented media, glial cells continue to multiply, necessitating the use of cytotoxic mitotic inhibitors (Wallace and Johnson, 1989 ). Serum also contains unknown and variable levels of growth factors, hormones, vitamins, and proteins.
