Developmental regulation of sensory neuron number and limb innervation in the mouse.

Although used widely in studies of naturally occurring cell death, systematic descriptions of the time course of changes in sensory neuron number and of limb innervation in the mouse are not available. The development of sensory innervation to the mouse forelimb was traced using the lipophilic carbocyanine dye, DiI, and correlated with neuron number in dorsal root ganglia contributing to the cervical enlargement. Axon invasion of the forelimb began at E10.5. Sensory axons reached the distal margin of the forelimb by E13.5. The difficulty of identifying immature neurons precluded estimating neuron numbers during the period of limb innervation. Neuron numbers in dorsal root ganglia (DRGs) C5-C8 increased from E14 to E16 and from E18 to P4. No evidence of a decline in neuron numbers was found during the developmental periods studied. Neuron number was compared in neonates and adults to determine if sensory neurons are added as body size increases as found in the frog [J. Comp. Neurol. 314 (1991) 106] and the rat [J. Comp. Neurol. 386 (1997) 8]. In contrast to previous findings, no difference was found in sensory neuron number between neonate and adult mice in either cervical or lumbar DRGs.