BIO254:Glomerulus

Olfactory Glomerulus

Anatomy

As observed by Cajal, the olfactory glomerulus is the location of the first synapse in the olfactory circuit. The synapse, in mammals, is between the olfactory receptor cells and the dendritic tufts of the mitral, tufted, and periglomerular cells. This synapse forms a characteristic spheroid shape, which is created by the surrounding glial cells and the cell bodies of the periglomerular cells. In mammals the glomeruli range in size from 50-200 μm, depending on the species. Vassar and colleagues, and Ressler and colleagues, used in situ hybiridization to odorant receptor genes to trace the path of the axons of the olfactory receptor cells and determine the composition of the glomeruli. Their studies indicated that the axons in a glomerulus are traced from cells expressing a single odorant receptor. The location of a specific glomerous is also relatively fixed in location. A glomerulus of a certain receptor is located in the same place between different animals.

Development

Experiments by Mombaerts and colleagues showed the role of the olfactory receptor itself in targeting the olfactory receptor cells to their proper glomeruli. By genetically switching receptors they were able to observe mistargeting of the axons; however, the process is not fully dependant upon the receptor, because cells with swapped receptors were neither targeted to their correct location nor to the location of the swapped receptor. In addition to the receptors, an assembly line of molecules is proposed to play a role in correct targeting. Among them are cell-adhesion molecules, cell-surface carbohydrates, and even endogenous concentrations of second messenger molecules like cAMP.

Function

As with all of the other cortical organizations, a direct role for these glomeruli remains elusive. However a few suggestions seem reasonable. Since olfaction has been shown to be a combinatorial process, the gathering of single receptors to glomeruli, allows for the decoding of the glomeruli as a pattern associated with a certain smell. It has also been suggested that the glomerulus role is to enhance the signal to noise ratio, an inevitable necessity in the detection of a diffusion based sensory modality.

Useful Sources: Squire Fundamental Neuroscience Chapter 24; particularly pages 653-659

Chen WR, Shepherd GM. “The olfactory glomerulus: a cortical module with specific functions.” J Neurocytol. 2005 Sep;34(3-5):353-60.

Mombaerts P, Wang F, Dulac C, Chao SK, Nemes A, Mendelsohn M, Edmondson J, Axel R. “Visualizing an olfactory sensory map.” Cell. 1996 Nov 15;87(4):675-86.

Ressler KJ, Sullivan SL, Buck LB. “Information coding in the olfactory system: evidence for a stereotyped and highly organized epitope map in the olfactory bulb.” Cell. 1994 Dec 30;79(7):1245-55.

St John JA, Clarris HJ, Key B. “Multiple axon guidance cues establish the olfactory topographic map: how do these cues interact?” Int J Dev Biol. 2002;46(4):639-47.

Vassar R, Chao SK, Sitcheran R, Nunez JM, Vosshall LB, Axel R. “Topographic organization of sensory projections to the olfactory bulb.” Cell. 1994 Dec 16;79(6):981-91.

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15 April 2024

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14 April 2024