Morphological characterization of six subpopulations of adult human DRG neurons at the light microscopic level.

Dorsal root ganglion (DRG) neurons are composed of physiologically distinct subpopulations, each responding to a different sensory stimulus. One can morphologically discriminate between two broad populations of adult rat and frog DRG neurons by their appearance under the light microscope. These groups are called large clear and small dark. However, additional subpopulations have not been identified by visual observation. Such identification requires application of immunochemistry or biophysical techniques. Although these are useful techniques, they do not allow the rapid discrimination of different neuron subpopulations, which would be useful for pharmacological studies on unique neuron subpopulations. Such experiments would be greatly facilitated if viable DRG neuron subpopulations could be identified based on their morphology at the light microscopic level. Just as for adult frog and rat DRG neurons, when adult human DRG neurons are observed under phase optics, two subpopulations can be seen, small dark and large light. However, under bright-field illumination, six distinct subpopulations can be distinguished based solely on morphological features. Five subpopulations contain rusty-colored cytoplasmic inclusions with different sized granules and differences in the size and density of the granule clusters, while one is granule-free. Analysis of the soma diameter distribution shows each of the six granule-containing and the non-granule-containing (clear) neuron subpopulations has a statistically significant difference in size distribution. We propose that neurons with different morphologies correspond to unique physiological subpopulations of DRG neurons. Experiments are underway using immunochemical techniques to determine whether neurons with the unique morphologies correspond with unique physiological functions.