Onion bulb formation in the initial complex of neurons in human dorsal root ganglion: their significance and alterations in amyotrophic lateral sclerosis.

We have demonstrated onion bulb-like structures in human dorsal root ganglia (DRG). These onion bulbs morphologically consist of non-continuous layers of supporting-cell cytoplasms that encase thinly myelinated axons and are immunocytochemically recognized by anti-S-100 protein and anti-glial fibrillary acidic protein antibodies. These structures are present in normal controls and preferentially involve the initial complex of the large, light, neurofilament-rich neurons. The number of onion bulbs and their average number of lamellae reach a peak in the third decade and then decline. In three cases of amyotrophic lateral sclerosis (ALS), the onion bulbs involve non-myelinated axons as well as the thinly myelinated portion of the initial complex and are increased both in frequency and in average number of lamellae. Our studies suggest that these onion bulbs represent a normal biological process of DRG neurons that may be accentuated in ALS.