Abstract
The vomeronasal organ (VNO) is sensory organ located in the ventral region of the nasal cavity in rodents. The VNO develops from the olfactory placode during the secondary invagination of olfactory pit. The embryonic vomeronasal structure appears as a neurogenic area where migratory neuronal populations like endocrine gonadotropin-releasing hormone-1 (GnRH-1) neurons form. Even though embryonic vomeronasal structures are conserved across most vertebrate species, many species including humans do not have a functional VNO after birth. The vomeronasal epithelium (VNE) of rodents is composed of two major types of vomeronasal sensory neurons (VSNs): (1) VSNs distributed in the apical VNE regions that express vomeronasal type-1 receptors (V1Rs) and the G protein subunit Gαi2, and (2) VSNs in the basal territories of the VNE that express vomeronasal type-2 receptors (V2Rs) and the G subunit Gαo. Recent studies identified a third subclass of Gαi2 and Gαo VSNs that express the formyl peptide receptor family. VSNs expressing V1Rs or V2Rs send their axons to distinct regions of the accessory olfactory bulb (AOB). Together, VNO and AOB form the accessory olfactory system (AOS), an olfactory subsystem that coordinates the social and sexual behaviors of many vertebrate species. In this review, we summarize our current understanding of cellular and molecular mechanisms that underlie VNO development. We also discuss open questions for study, which we suggest will further enhance our understanding of VNO morphogenesis at embryonic and postnatal stages.
Highlights
The vomeronasal organ (VNO) is a specialized olfactory organ responsible for detecting pheromones and kairomones; stimuli that can trigger a wide range of behaviors [1–5] and hormonal responses [6–8]
As the VNO is formed after a secondary invagination from olfactory pit (Fig. 1a, b), all the transcription factors and signaling molecules that affect the formation of the olfactory placode/pit may control VNO development (Table 1)
By using AP-2εCre mice to conditionally knock out Smad4 in maturing basal vomeronasal neurons, we found that Smad4 mediated TGF/Bmp signaling is important for proper dendritic knob formation, pheromone-induced vomeronasal sensory neurons (VSNs) activation, survival, and correct glomerular formation of Gαo + basal VSNs in the posterior accessory olfactory bulb (AOB)
Summary
The vomeronasal organ (VNO) is a specialized olfactory organ responsible for detecting pheromones and kairomones; stimuli that can trigger a wide range of behaviors [1–5] and hormonal responses [6–8]. V1R positive neurons lie mostly in the apical zone of the vomeronasal epithelium and express the G-protein subunit Gαi and neuropilin-2 (Nrp-2) and project to the anterior portion of the AOB [33, 34]. A Schematic showing olfactory placode specification at E9.5, invagination of olfactory placode to form olfactory pit at E10.5 followed by vomeronasal thickening and invagination at E11.5 At this stage, most of the proliferative progenitors are localized to apical side of the epithelium (magenta dots). C Schematic showing Gαi2 + apical and Gαo + basal VSNs respectively sending their axons to the anterior (green) and posterior (red) portions of the AOB. In the vomeronasal sensory epithelium, basal VSNs are indicated in red, while apical VSNs are shown in green and sustentacular cell layer is labelled as S.Cs. Adult neurogenesis mostly occurs in the marginal zones (circled) of the VNO. Our goal is to highlight underexplored areas of developmental biology from our perspective
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