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

Area of Science:

• Neuroscience
• Olfactory System Research
• Mammalian Anatomy

Background:

• Olfactory bulb glomeruli, discovered by Camillo Golgi in 1875, are vital for olfactory processing.
• The precise number of glomeruli in mammalian olfactory bulbs has remained unknown.
• Odorant receptor genes influence glomerular organization within the olfactory bulb.

Purpose of the Study:

• To develop a methodology for quantifying the number, size, and shape of glomeruli in mouse olfactory bulbs.
• To provide empirical counts of glomeruli, moving beyond statistical estimates.
• To characterize the three-dimensional morphology of olfactory bulb glomeruli.

Main Methods:

• Serial two-photon tomography was employed to generate complete image stacks of 12 mouse olfactory bulbs.
• Whole-mount olfactory bulbs were labeled immunohistochemically using antibodies against the glomerular marker VGLUT2.
• Manual segmentation of 33,137 glomeruli was performed based on VGLUT2-immunoreactive signals.

Main Results:

• A median of 2851 glomeruli per olfactory bulb was counted in mice at postnatal day 56.
• This equates to approximately 2.50 glomeruli per intact odorant receptor gene.
• Glomeruli effective diameters followed a Gaussian distribution (median 77.54 μm), but their shapes were not spherical, leading to the proposed descriptor 'tuberiform'.

Conclusions:

• This study provides the first complete empirical counts and detailed morphological characterization of olfactory bulb glomeruli in mice.
• The findings offer a quantitative foundation for understanding olfactory sensory map formation and function.
• The proposed 'tuberiform' descriptor captures the complex, non-spherical shapes of olfactory bulb glomeruli.