Differences and similarities in the neurotrophic growth factor requirements of sensory neurons derived from neural crest and neural placode.

This article reviews recent studies that have examined differences and similarities in the neurotrophic growth factor requirements of neural crest- and neural placode-derived sensory neurons of the developing chick embryo. From in vitro experiments using both explant and dissociated, neuron-enriched cultures of spinal and cranial nerve sensory neurons, it has been established that only sensory neurons of neural crest origin are responsive, at least in terms of survival and neurite outgrowth, to mouse submandibular gland nerve growth factor (NGF). Sensory neurons derived from neural placodes (neurons of the ventrolateral portion of the trigeminal ganglion and the entire neuronal population of the vestibular, geniculate, petrosal and nodose ganglia) are largely unresponsive to NGF throughout embryonic development, but do respond to neurotrophic activity present in extracts of brain and various peripheral 'end-organs', such as heart or liver. By incubation of neuron-enriched cultures with radiolabelled [125I]NGF, followed by autoradiographic exposure, it has been demonstrated that placode-derived neurons, in marked contrast to those of neural crest origin, are completely devoid of specific cell surface receptors for NGF. In contrast to differences in their requirement and responsiveness to NGF, both placode- and crest-derived sensory neurons are responsive to the survival and neurite-promoting activity of a recently purified brain-derived neurotrophic factor (BDNF). It is postulated that all primary sensory neurons have a dual growth factor requirement during development; their survival being dependent on a supply of both a peripheral and a central 'target'-derived neurotrophic factor. It appears that BDNF may act as common 'central target-derived' neurotrophic factor for both placode- and crest-derived sensory neurons, but that within peripheral tissues there are specific neurotrophic factors for each of these two classes of primary sensory neurons.