ARE THE NEURONS IN THE DORSAL-ROOT GANGLION PSEUDOUNIPOLAR - A COMPARISON OF THE NUMBER OF NEURONS AND NUMBER OF MYELINATED AND UNMYELINATED FIBERS IN THE DORSAL-ROOT

The neurons in the dorsal root ganglion have classically been describe d as pseudounipolar. Previous studies have questioned this simple orga nisation because an equality between the number of fibres in the dorsa l 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 i s compared to the number of fibres in the dorsal root. The methods use d are founded on unbiased stereological principles and includes the op tical disector, the Cavalieri principle, unbiased counting rules in tw o and three dimensions, and systematic random sampling. The number of A- and B-cells is estimated with light microscopy, and the number of m yelinated and unmyelinated fibres is estimated with electron microscop y. The present study demonstrates that there is a 1:1 ratio (mean: 0.9 8, CV: 0.12, 95% confidence interval: 0.90-1.07) of fibres in the dors al root to neurons in the dorsal root ganglion, as the classical theor y 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. (C) 1995 Wiley-Liss, Inc.