Principles of glomerular organization in the human olfactory bulb--implications for odor processing

Abstract

Olfactory sensory neurons (OSN) in mice express only 1 of a possible 1,100 odor receptors (OR) and axons from OSNs expressing the same odor receptor converge into approximately 2 of the 1,800 glomeruli in each olfactory bulb (OB) in mice; this yields a convergence ratio that approximates 2:1, 2 glomeruli/OR. Because humans express only 350 intact ORs, we examined human OBs to determine if the glomerular convergence ratio of 2:1 established in mice was applicable to humans. Unexpectedly, the average number of human OB glomeruli is >5,500 yielding a convergence ratio of approximately 16:1. The data suggest that the initial coding of odor information in the human OB may differ from the models developed for rodents and that recruitment of additional glomeruli for subpopulations of ORs may contribute to more robust odor representation.

Figure 1. Laminar organization of HOB glomeruli.

Laminar organization DAPI stained mouse (a) and human (b and c) OBs. a) Coronal section of the adult mouse OB shows the olfactory nerve layer (ONL), glomerular layer (GL), external plexiform layer (EPL), internal plexiform layer (IPL), granule cell layer (GCL) and rostral migratory stream (RMS). b,c) Coronal HOB sections show equivalent laminae. Scale bars = 500 µm.

Figure 2. Distribution of glomeruli in the HOB.

Triple-labeling for NCAM (green), VGlut2 (red) and DRAQ5 (blue) reveals complex distributions of HOB glomeruli. Scale bar = 250 µm.

Figure 3. Morphology and variability among HOB glomeruli.

Glomeruli shown with NCAM (green), VGlut2 (red) and DRAQ5 (blue). a) Mouse glomeruli are spherical and evenly spaced. b,c,d,e) HOB glomeruli show a broader range of morphology and variability. f) The number of HOB glomeruli. Scale bars = 100 µm.