The peripheral olfactory system is able to recover after injury, i.e., the olfactory epithelium reconstitutes, the olfactory nerve regenerates, and the olfactory bulb is reinnervated, with a facility that is unique within the mammalian nervous system. Cell renewal in the epithelium is directed to replace neurons when they die in normal animals and does so at an accelerated pace after damage to the olfactory nerve. Neurogenesis persists because neuron-competent progenitor cells, including transit amplifying and immediate neuronal precursors, are maintained within the population of globose basal cells. Notwithstanding events in the neuron-depleted epithelium, the death of both non-neuronal cells and neurons directs multipotent globose basal cell progenitors, to give rise individually to sustentacular cells and horizontal basal cells as well as neurons. Multiple growth factors, including TGF-α, FGF2, BMPs, and TGF-βs, are likely to be central in regulating choice points in epitheliopoiesis. Reinnervation of the bulb is rapid and robust. When the nerve is left undisturbed, i.e., by lesioning the epithelium directly, the projection of the reconstituted epithelium onto the bulb is restored to near-normal with respect to rhinotopy and in the targeting of odorant receptor-defined neuronal classes to small clusters of glomeruli in the bulb. However, at its ultimate level, i.e., the convergence of axons expressing the same odorant receptor onto one or a few glomeruli, specificity is not restored unless a substantial number of fibers of the same type are spared. Rather, odorant receptor-defined subclasses of neurons innervate an excessive number of glomeruli in the rough vicinity of their original glomerular targets. Anat Rec (New Anat) 269:33–49, 2002. © 2002 Wiley-Liss, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ar.10047

Neural regeneration and the peripheral olfactory system

The olfactory system has been long reported as one of the most phylogenetically primitive (Ariens Kappers et al., 1967), because of the peripheral location of its neurons and the relatively conspicuous development in lower vertebrates

Continuous Nerve Cell Renewal in the Olfactory System

https://www.cell.com/trends/neurosciences/pdf/0166-2236(90)90017-5.pdf

Olfactory neurogenesis: genetic or environmental controls?

The olfactory organ-unlike most receptor sheet-must be directly exposed to airborne chemicals. But this requirement also mates vulnerability to bacterial and viral invasion, to the mechanical impact of the respiratory airstream, and, perhaps, to the toxic effects of environmental pollutants. In fact, rhinitic infections, with accompanying pathological changes in the olfactory epithelium, are common in at least some species of laboratory animals.12*26* 2% 80 Despite this exposure, the olfactory system of most individuals appears to retain its capacity of detecting and recognizing odors throughout life. Does it, therefore, possess special mechanisms for maintaining function in the face of cell loss? If so, what are these mechanisms? And what effect, if any, would they have on olfactory function. I t is true that the number of olfactory receptors is vast (106 in man, 10s in rabbits), so perhaps even extensive attrition need not seriously compromise function. On the other hand, most tissues regenerate or renew at either the biochemical or cellular level or both. The olfactory epithelium may be no exception. If renewal is confined to subcellular biochemical events, the functional consequences could be significant but are likely to be minimal, If, however, receptor renewal or regeneration occurs, the epithelium and primary neurones must undergo extensive reorganization if normal function is to be maintained or restored. In particular, olfactory receptors are not synaptically isolated from the neurones that connect them to the brain (as is the case with auditory, visual, and taste receptors). Thus, if a receptor is to be replaced, the entire axonal connection to the olfactory bulb must also be replaced. There are good reasons for examining this possibility with caution: neurones do not generally regenerate after loss of the cell body. Nevertheless, the olfactory receptors are specialized neurones and may, in this regard, conform more to the behavior of, say, respiratory epithelial cells. Inquiries into the properties of cell populations in a variety of epithelial and other tissues have long depended on concepts such as that of the cell cycle and on techniques such as that of autoradiography. Until recently, however, these aspects of cell biology had not informed work on the olfactory epithelium: there was essentially no discussion of the properties of its constituent cell populations in quantitative and dynamic terms, and there was little apparent awareness of either the criteria necessary for distinguishing between certain types of proliferating cell populations or the importance of these distinctions for an understanding of olfactory function. This earlier literature has been revised in depth by Takagi.52 During the last few years the picture has begun to change. We now have quantitative information concerning proliferating cell populations in the olfactory epithelium, and some understanding of the dynamic properties of this system is beginning to emerge. Perhaps, then, the time is appropriate to take stock of the

Dynamics of cell populations in the olfactory epithelium

The capacity of the olfactory epithelium (OE) for lifelong neurogenesis and regeneration depends on the persistence of neurocompetent stem cells, which self-renew as well as generating all of the cell types found within the nasal epithelium. This Review focuses on the types of stem and progenitor cells in the epithelium and their regulation. Both horizontal basal cells (HBCs) and some among the population of globose basal cells (GBCs) are stem cells, but the two types plays vastly different roles. The GBC population includes the basal cells that proliferate in the uninjured OE and is heterogeneous with respect to transcription factor expression. From upstream in the hierarchy to downstream, GBCs encompass 1) Sox2+ /Pax6+ stem-like cells that are totipotent and self-renew over the long term, 2) Ascl1+ transit-amplifying progenitors with a limited capacity for expansive proliferation, and 3) Neurog1+ /NeuroD1+ immediate precursor cells that make neurons directly. In contrast, the normally quiescent HBCs are activated to multipotency and proliferate when sustentacular cells are killed, but not when only OSNs die, indicating that HBCs are reserve stem cells that respond to severe epithelial injury. The master regulator of HBC activation is the ΔN isoform of the transcription factor p63; eliminating ΔNp63 unleashes HBC multipotency. Notch signaling, via Jagged1 ligand on Sus cells and Notch1 and Notch2 receptors on HBCs, is likely to play a major role in setting the level of p63 expression. Thus, ΔNp63 becomes a potential therapeutic target for reversing the neurogenic exhaustion characteristic of the aged OE. J. Comp. Neurol. 525:1034-1054, 2017. © 2016 Wiley Periodicals, Inc.

/pdf/stem-and-progenitor-cells-of-the-mammalian-olfactory-4ghlhenrno.pdf

Stem and progenitor cells of the mammalian olfactory epithelium: Taking poietic license.

Repair of the Olfactory Mucosa with Special Reference to Regeneration of Olfactory Cells (Sensory Neurons)

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Repair of the Olfactory Mucosa with Special Reference to Regeneration of Olfactory Cells (Sensory Neurons)