Glomeruli of the Mouse Olfactory Bulb: Numbers, Sizes, Shapes

The discovery of olfactory bulb glomeruli was reported by the neuroanatomist Camillo Golgi in 1875, but the number of glomeruli in a mammalian olfactory bulb remains unknown. The 1141 intact odorant receptor genes in the mouse genome are key determinants of the coalescence of axons of olfactory sensory neurons into glomeruli at recognizable positions in the olfactory bulb. Here, we devised a methodology for quantifying the numbers, sizes, and shapes of the glomeruli of mouse olfactory bulbs. Using serial two-photon tomography, we generated complete image stacks of 12 mouse olfactory bulbs labeled immunohistochemically as whole mounts with antibodies against the glomerular marker VGLUT2. The mouse ages were postnatal days 14, 21, and 56. We manually segmented 33,137 glomeruli based on the VGLUT2-immunoreactive signal. This manual segmentation resulted in complete empirical counts of the heterogeneously-sized glomerular 3D objects instead of providing only statistical estimates. We counted a median of 2851 glomeruli per olfactory bulb at postnatal day 56, corresponding to 2.50 glomeruli per intact odorant receptor gene per olfactory bulb. For a 3D object of any shape, the effective diameter is defined as the diameter of a sphere that possesses the equivalent volume. We found that the effective diameters of the glomeruli are well modeled by a Gaussian distribution, with a median of 77.54 μm at postnatal day 56. By quantifying glomerular shape with aspect-length ratios and sphericity, we demonstrate that glomeruli do not conform to spherical shapes. We propose the descriptor "tuberiform" to encompass the diversity of glomerular shapes.