Early stages of spinal ganglion formation during tail regeneration in the newt, Notophthalmus viridescens.
Stages in the development of sensory ganglia in the regenerating newt tail after amputation are described by taking advantage of the rostrocaudal developmental gradient of the regenerating tail. A series of ganglia, beginning at the tip of the regenerate and progressing rostrally, were examined. Eight-week regenerates were used because they showed the most complete array of stages. The first recognizable ganglia appear as small clusters of cells sitting dorsally on the already established ventral roots. The cluster of ganglionic cells steadily expands with the addition of many new cells. Signs of cell differentiation within the ganglion precede the formation of the dorsal root rudiment, which assumes several different configurations but most commonly enters the cord close to the ventral root. Our material suggests that ganglion precursor cells originate in the ventral region of the developing spinal cord and migrate out of the cord by travelling along the ventral root until, at a suitable distance from the cord, they halt, proliferate, and eventually differentiate. In the regenerate, we saw no evidence of neural crest cells--such as those that give rise to ganglia in the trunk region during development--forming at the dorsal region of the regenerated neural tube. Nor was there any morphological evidence of mesenchymal contribution to the ganglion cell clusters.