General considerations

The sensory ganglia of the peripheral nervous system (PNS) transmit information from peripheral targets to higher somatosensory areas in the spinal cord and brain. They include the dorsal root ganglia (DRG) organized as bilateral metameric units along the spinal cord, and the ganglia located along the path of cranial nerves. In development, sensory neurons originate from progenitors that migrate from the neural crest and certain ectodermal placodes to the homing sites where they differentiate. Nascent sensory ganglia are colonized by subsets of neural and glial progenitors with heterogeneous developmental potentialities. Knowledge of the state of commitment of neural crest precursors invokes a critical role for the local environment encountered along the migratory routes and at the target sites in regulating neural crest development into ganglionic derivatives. The pathways and mechanisms of neural crest cell migration that lead to the formation of segmentally organized ganglia, as well as the factors that regulate the differentiation of progenitor cells into neurons and satellite cells, have been the subject of intensive research during the past 10 years and will be discussed in this chapter.

Upon differentiation, sensory neurons initially extend two axonal processes that grow in opposite directions from the cell bodies to reach peripheral and central target fields. The innervation of the targets is executed with exquisite precision, raising the possibility that sensory neurons become specified at early stages prior to innervation.