This chapter discusses selected aspects of the influences of neuronal age, substratum, and nonneuronal cells on nerve fiber growth. Neurite growth from sympathetic ganglia removed from perinatal rats starts earlier and initially progresses faster than that from either embryonic or postnatal ganglia under identical conditions. These age dependent results reflect the morphology and behavior of individual growth cones. With increasing embryonic age, the chick sensory ganglion neurons increase in total axon length and complexity when cultured for equal periods. Age dependent growth is also expressed by some central nervous tissues, the olfactory bulb, and the retina. Collagen, laminin, and Schwann cell generated extracellular matrix all promote neurite outgrowth, but to varying degrees depending upon the type of neuron. Neurites from some types of central neurons grow faster on Schwann cells and/or their extracellular matrix than on two-dimensional collagen and faster on three-dimensional collagen than on laminin. Thus, the neurons of different age express differences in their intrinsic developmental program when placed in culture, but this program may be modified by environmental influences, such as the surface they contact.
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