Abstract:
The olfactory epithelium (OE) provides a unique exception to the limited ability of the nervous system to regenerate itself. Olfactory sensory neurons (OSN) are directly exposed to the outside world, prone to environmental insult, and need to be replaced continuously to maintain a sense of smell. An open question is, how proliferation of neuronal progenitors is regulated at the tissue level. Generally, the vertebrate olfactory sensory tissue is composed of OSN, sustentacular cells (Sus), which is a unique type of olfactory glia, as well as neuronal stem cell and precursor populations. However, olfactory stem cells and Sus have not been described in zebrafish. To gain more insight into the cellular architecture and physiological role of different cell types in the zebrafish OE, immunohistochemical stainings with cell type-specific markers and physiological studies of intercellular signaling were performed. Immunohistochemistry against the intermediate filament cytokeratin II, for the first time, visualizes the entire population of zebrafish Sus. Sus are abundant, span the entire apicobasal dimension of the tissue and are regularly spaced throughout the sensory region of the epithelium. Interestingly, Sus also appear to be positive for the progenitor marker Sox2. Thus, Sus may contribute to olfactory sensory neuron regeneration in two, not necessarily exclusive ways. Sus could dedifferentiate and reacquire neuronal progenitor cell identity or they could communicate signals from OE tissue to basal stem cell / progenitor populations. To investigate these possibilities, cytokeratin II immunohistochemistry was combined with BrdU cell proliferation assays and immunohistochemistry for neuronal progenitor markers, such as nestin and sox2. To examine if Sus can communicate between the tissue and stem cells, functional Ca2+- imaging and pharmacological manipulation was performed on olfactory tissue slices upon purine stimulation. The outcome of these experiments suggests that both Sus and a basal cell population respond to purine stimulation but may utilize different receptors to mediate responses, supporting the possibility of direct signaling Sus onto basal neuronal precursors.
