Abstract:
The zebrafish olfactory epithelium (OE) can recover from the near total loss of olfactory sensory neurons (OSNs) within only a week when the OE is damaged by external causes. This remarkable neurogenic activity depends on the differential activation of horizontal basal cell (HBC) progenitors, which are quiescent in the intact tissue. Under physiological conditions, an independent pool of mitotically active progenitors, the globose basal cells (GBCs), constitutively generate new neurons to replace OSNs that reach the end of their natural life span. Unique to the zebrafish OE, the two progenitor types are spatially segregated, and HBC and GBC activity can easily be discriminated by analyzing the epithelial position of mitotic activity under different tissue conditions. However, detailed knowledge about the diversity of non-neuronal cells and the molecular cues that regulate the rate of maintenance neurogenesis or trigger OE regeneration, is largely missing. This study shows that HBCs, GBCs and sustentacular glial cells (SCs) express Sox2, that they segregate within distinct layers of the basal OE, and that they are selectively sensitive to purine compounds. HBCs occupy the basal OE and respond strongly to ATP and 2- MeSATP, while GBCs and SCs occupy suprabasal strata and show high and low ATP sensitivity, respectively. ATP stimulates GBC activity but does not affect HBC proliferation, indicating that purinergic signaling has a selective function in regulating maintenance neurogenesis. In contrast, exposure to the cytokine IL-6 results in HBC expansion and subsequent OSN generation. Detailed analysis of the tissue dynamics in the intact OE revealed that HBCs are more active than anticipated but only rarely, if at all, contribute to OSN or SC neurogenesis. A preliminary model of maintenance neurogenesis could be developed, which suggests the existence of a novel SC progenitor within the neurogenic niche of the intact OE.
