THE NEURONAL STEM-CELL OF THE OLFACTORY EPITHELIUM

The vertebrate olfactory epithelium (OE) is a system in which behavior of neuronal progenitor cells can be observed and manipulated easily. It is morphologically and functionally similar to embryonic germinal n euroepithelia, but is simpler in that it produces large numbers of a s ingle type of neuron, the olfactory receptor neuron (ORN). The OE is a menable to tissue culture, gene transfer, and in vivo surgical approac hes, and these have been exploited in experiments aimed at understandi ng the characteristics of OE neuronal progenitor cells. This has led t o the realization that the ORN lineage contains at least three distinc t stages of proliferating neuronal progenitor cells (including a stem cell), each of which represents a point at which growth control can be exerted. Neurogenesis proceeds continually in the OE, and studies in vivo have shown that this is a regulated process that serves to mainta in the number of ORNs at a particular level. These studies suggest tha t OE neuronal progenitors-which are in close physical proximity to ORN s-can ''read'' the number of differentiated neurons in their environme nt and regulate production of new neurons accordingly. Putative neuron al stem cells of the OE have been identified in vitro, and studies of these cells indicate that ORNs produce a signal that feeds back to inh ibit neurogenesis. This inhibitory signal may be exerted at the level of the stem cell itself. Recent studies to identify this signal, as we ll as endogenous stimulatory signals that may be important in regulati ng OE neurogenesis, are also discussed, (C) 1998 John Wiley & Sons, In c.