Abstract
This study describes the microscopic organization of a wedge-shaped area at the intersection of the main (MOB) and accessory olfactory bulbs (AOBs), or olfactory limbus (OL), and an additional component of the anterior olfactory nucleus or alpha AON that lies underneath of the AOB. The OL consists of a modified bulbar cortex bounded anteriorly by the MOB and posteriorly by the AOB. In Nissl-stained specimens the OL differs from the MOB by a progressive, antero-posterior decrease in thickness or absence of the external plexiform, mitral/tufted cell, and granule cell layers. On cytoarchitectual grounds the OL is divided from rostral to caudal into three distinct components: a stripe of glomerular-free cortex or preolfactory area (PA), a second or necklace glomerular area, and a wedge-shaped or interstitial area (INA) crowned by the so-called modified glomeruli that appear to belong to the anterior AOB. The strategic location and interactions with the main and AOBs, together with the previously noted functional and connectional evidence, suggest that the OL may be related to both sensory modalities. The alpha component of the anterior olfactory nucleus, a slender cellular cluster (i.e., 650 × 150 μm) paralleling the base of the AOB, contains two neuron types: a pyramidal-like neuron and an interneuron. Dendrites of pyramidal-like cells (P-L) organize into a single bundle that ascends avoiding the AOB to resolve in a trigone bounded by the edge of the OL, the AOB and the dorsal part of the anterior olfactory nucleus. Utrastructurally, the neuropil of the alpha component contains three types of synaptic terminals; one of them immunoreactive to the enzyme glutamate decarboxylase, isoform 67.
Highlights
While the cytological organization of the rodent main and accessory olfactory bulbs (AOB) has been depicted and most of its constituent neurons are known, growing evidence suggests that the structural and functional differentiation of the bulbar cortex is not homogeneous
Given the fact that the transition between the AOB with the Main olfactory bulb (MOB) is not sharp (Larriva-Sahd, 2008), and since this territory has not previously been described, it is necessary to provide a general description of this frontier before dealing with the native cellular elements
The term olfactory limbus (OL) will be used to refer to it throughout this study. The use of this term is further justified by the fact that in adult rodents the cytological characteristics of the OL differ from those of the MOB and AOB as presented
Summary
While the cytological organization of the rodent main and accessory olfactory bulbs (AOB) has been depicted and most of its constituent neurons are known, growing evidence suggests that the structural and functional differentiation of the bulbar cortex is not homogeneous. Both the main (MOB) and AOBs possess distinct neurons embedded in a neuropil that yields the characteristic laminated appearance when studied with both silver and aniline stains (Macrides and Davis, 1983). The mitral cell layer, there is a cell-poor stratum or internal plexiform area bounding the deeper granule cell layer that arbors the homonymous cells and Abbreviations: aAON, anterior olfactory nucleus, alpha component; AOB, Accessory olfactory bulb; EPL, external plexiform layer; GAD67, enzyme glutamic acid decarboxylase, isoform 67; GAD-i, immunoreactivity to the enzyme glutamic acid decarboxylase, isoform 67; INA, interstitial area of the olfactory limbus; INBi, Interstitial neuron bulbi, interneuron; INBp, Interstitial neuron bulbi, projecting cell; LOT, lateral olfactory tract; MOB, Main olfactory bulb; NG, necklace glomerulus; NGA, Olfactory limbus, area of the necklace glomeruli; OL, olfactory limbus; PA, Preolfactory area of the olfactory limbus; P-L, pyramidal-like cell
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