Dorsal root ganglia, neural crest migration, and spinal cord form in snakes

The classic view of the vertebrate dorsal root ganglion is that it arises from trunk neural crest cells that migrate to positions lateral to the spinal cord, sending axons dorsally into the spinal cord and dendrites ventrally to meet with motor axons in the ventral root to form spinal nerves. As a result, the ganglion ends up lying in the dorsal root of the spinal nerve. Serial histological sections of parts of the trunk of juveniles of different snake species revealed that the ganglia lie distal to the junction of dorsal and ventral roots of spinal nerves and outside the neural canal. The anatomical position of spinal ganglia in snakes suggests that regulation of trunk neural crest migration in snakes differs from that in the model endotherms in which it has been most thoroughly explored. Dorsal roots have no distinct rootlets and the span of root entry to the spinal cord is short compared to that of ventral rootlets in the same segment. Comparing early developmental stages to juvenile spinal cords shows an increased separation of spinal nerve root sites and ventral migration of the ganglion in later development. Dorsal rami of the spinal nerves leave directly from the dorsal edge of the ganglion, and the ventral ramus leaves from the ventral tip of the ganglion. How these features relate to the developmental regulation of ganglion form and position and the extraordinary locomotor capabilities of the snake trunk are unclear.