Abstract
Within the central nervous system, the olfactory system fascinates by its developmental and physiological particularities, and is one of the most studied models to understand the mechanisms underlying the guidance of growing axons to their appropriate targets. A constellation of contact-mediated (laminins, CAMs, ephrins, etc.) and secreted mechanisms (semaphorins, slits, growth factors, etc.) are known to play different roles in the establishment of synaptic interactions between the olfactory epithelium, olfactory bulb (OB) and olfactory cortex. Specific mechanisms of this system (including the amazing family of about 1000 different olfactory receptors) have been also proposed. In the last years, different reviews have focused in partial sights, specially in the mechanisms involved in the formation of the olfactory nerve, but a detailed review of the mechanisms implicated in the development of the connections among the different olfactory structures (olfactory epithelium, OB, olfactory cortex) remains to be written. In the present work, we afford this systematic review: the different cellular and molecular mechanisms which rule the formation of the olfactory nerve, the lateral olfactory tract and the intracortical connections, as well as the few data available regarding the accessory olfactory system. These mechanisms are compared, and the implications of the differences and similarities discussed in this fundamental scenario of ontogeny.
Highlights
A BRIEF SKETCH OF THE OLFACTORY SYSTEM AND ITS DEVELOPMENT The basic circuit in the mammalian olfactory system (Figure 1) commences at the olfactory sensory neurons (OSNs) in the olfactory epithelium, a structure that develops from the olfactory placode during embryogenesis
In this essay, we have presented the full scenario of molecular mechanisms involved in the formation of different connections within the olfactory system during development: the olfactory nerve, the lateral olfactory tract (LOT) and the intra-cortical connections, as well as a brief summary of the mechanisms studied in the vomero-nasal nerve
We can conclude that the expectations raised by the mechanisms that we have considered as “special mechanisms” have become somewhat tarnished with the progress made in the field
Summary
A BRIEF SKETCH OF THE OLFACTORY SYSTEM AND ITS DEVELOPMENT The basic circuit in the mammalian olfactory system (Figure 1) commences at the olfactory sensory neurons (OSNs) in the olfactory epithelium, a structure that develops from the olfactory placode during embryogenesis. We extensively reviewed the bulk of the experimental data to suggest that the protomap hypothesis explains how the olfactory system develops, with the influence exerted by the olfactory epithelium on the OB only becoming relevant once the different structures (olfactory epithelium, OB, olfactory cortex) have been independently generated (López-Mascaraque and de Castro, 2002, 2004) In this sense, the latest genetic approaches, such as mice with a “monoclonal nose” (in which more than 95% of the OSNs express a single odorant receptor), seem to confirm that mechanisms intrinsic to the OB are responsible for building glomeruli (Fleischmann et al, 2008), while a member of the TGF family that is secreted by OSN axons selectively induces the outgrowth of the dendrites from mitral and tufted cells in the embryo (Tran et al, 2008).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
