Structure of the Antennal Lobe

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Only a single type of olfactory receptor neurons projects to a glomerulus, which forms a dense neuropil isolated by surrounding glia cells from other glomeruli. The macroglomerular complex receives input from pheromone-sensitive olfactory receptor neurons and has been an important model system for investigating olfactory information processing.

Cell bodies of antennal lobe neurons are arranged more or less clearly in clusters. In the silkmoth, three main clusters are found, the anterior (AC), medial (MC), and lateral (LC) clusters, named according to their position with respect to the antennal lobe neuropil.

In the macroglomerular complex, there are three regions: cumulus, toroid, and horseshoe. Receptor neurons responsive to bombykol project to the toroid while receptor neurons sensitive to bombykal project to the cumulus [Kanzaki et al., 2003]．Anatomically, toroid and cumulus appear somewhat intermingled dorsally.

The glomeruli in the silkmoth antennal lobe have been mapped and can be identified based on their shape and position [Kazawa et al., 2009]．There is a small amount of variability in the number of glomeruli as some glomeruli appear to have a tendency to split up into smaller glomeruli in some individuals. Glomeruli gradually depart from an ideal spherical shape which is seen near the anterior surface of the brain in deeper regions of the antennal lobe, possibly due to the fact that there are also other elements such as fiber tracts to accommodate for in a spatially limited volume.


　　　　　　　　　　　

Figure. Glomerular structure of the silkmoth antennal lobe. Optical sections obtained by confocal microscopy (left) have been segmented and combined into a 3D reconstruction map (right)．D，Dorsal；MC, Medial cell cluster；MGC, Macroglomerular complex；L，Lateral；LC，Lateral cell cluster；OG，Ordinary glomeruli．Modified from Kazawa et al., 2009．

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Figure. Identification of ordinary glomeruli. A map was produced from confocal sections and the glomeruli were assigned names. D，Dorsal；L，Lateral．Modified from Namiki & Kanzaki, 2008．

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References

Kanzaki R, Soo K, Seki Y and Wada S (2003) Projections to higher olfactory centers from subdivisions of the antennal lobe macroglomerular complex of the male silkmoth. Chem Senses 28: 113-130.

Kazawa T, Namiki S, Fukushima R, Terada M, Soo K and Kanzaki R (2009) Constancy and variability of glomerular organization in the antennal lobe of the silkmoth. Cell Tissue Res 336:119-136.

Seki Y, Aonuma H and Kanzaki R (2005) Pheromone processing center in the protocerebrum of Bombyx mori revealed by NO-induced anti-cGMP immunocytochemistry. J Comp Neurol 481: 340-351.