Demonstration of segmental arrangement of thoracic spinal motor neurons using lipophilic dyes.

Unlike the afferent input into the spinal cord, it is generally believed that the motor efferent system in the mammalian spinal cord is more segmental in arrangement. Isolated comments in recent reports suggest that motor neurons in the mammalian spinal cord may be strictly segmental, contrary to many earlier reports using less sophisticated methods. We have attempted to address the question of the segmental arrangement of motor neurons in a mammalian spinal cord using single and double labelling with the fluorescent lipophilic dyes, Di I and Di A. These dyes have the ability to diffuse along and stain the plasma membrane of the neural elements in aldehyde-fixed specimens. Sprague-Dawley rat fetuses and early postnatal pups were used. The study was conducted on the thoracic spinal cord. The results confirm that thoracic spinal motor neurons are restricted to the segments from which their axons exit via the ventral roots in the intrauterine stages of development. The segmental arrangement of motor neurons in the thoracic spinal cord corresponded well with that of the sympathetic preganglionic neurons which were stained simultaneously.