Visualization of six unique morphological subpopulations of adult frog dorsal root ganglion neurons at the light microscopic level.

Subpopulations of adult frog dorsal root ganglion (DRG) neurons respond to different physiological stimuli, and have unique biophysical and pharmacological properties. Two broad-based subpopulations of DRG neurons appear under phase optics, "large clear" and "small dark" neurons, while immunochemical and electrophysiological techniques allow identification of additional subpopulations. Nevertheless, most studies of DRG neurons involve randomly selected neurons. Under bright field illumination, we found dark and clear DRG neurons are distinctly different, with dark neurons composed of four subpopulations, each with unique numbers and distribution of bright rusty-colored cytoplasmic granules, and statistically significant difference in the soma diameter distribution. The clear neurons are granule-free, but the two subpopulations have statistically significant differences in soma size distributions. Thus, morphological criteria alone allow identification of six distinct subpopulations of DRG neurons in the light microscope, although further studies are required to determine whether they correspond to physiologically different subpopulations of sensory neurons.