Are the neurons in the dorsal root ganglion pseudounipolar? A comparison of the number of neurons and number of myelinated and unmyelinated fibres in the dorsal root.

The neurons in the dorsal root ganglion have classically been described as pseudounipolar. Previous studies have questioned this simple organisation because an equality between the number of fibres in the dorsal root and neurons could not be established. In this study the number of neurons in the fifth lumbar dorsal root ganglion of the adult rat is compared to the number of fibres in the dorsal root. The methods used are founded on unbiased stereological principles and includes the optical disector, the Cavalieri principle, unbiased counting rules in two and three dimensions, and systematic random sampling. The number of A- and B-cells is estimated with light microscopy, and the number of myelinated and unmyelinated fibres is estimated with electron microscopy. The present study demonstrates that there is a 1:1 ratio (mean: 0.98, CV: 0.12, 95% confidence interval: 0.90-1.07) of fibres in the dorsal root to neurons in the dorsal root ganglion, as the classical theory predicts. Furthermore, the study of the two neuron subtypes supports the hypothesis that myelinated fibres originate from the A-cells and the unmyelinated fibres from the B-cells.