RETROVIRAL LINEAGE STUDIES OF THE RAT OLFACTORY EPITHELIUM

Replication-incompetent retroviral vectors that encode the heritable m arker enzyme, beta-galactosidase, were used to study the lineage relat ionships of cells in the olfactory epithelium of unmanipulated animals and in the olfactory epithelium as it reconstitutes after lesion. Vir ally-marked cells are categorized as to type based on their position i n the epithelium and on expression of NCAM (limited to neurons) and th e carbohydrate moiety recognized by Griffonia lectin (limited to the d ark/horizontal basal cells and the microvillar class of supporting cel ls). Direct injections of the vectors into the olfactory epithelium of otherwise intact animals produce clusters of beta-galactosidase-label ed cells when assessed 6-10 days after infection; these clusters are c omposed of neurons and NCAM-negative/lectin-negative light/globose bas al cells exclusively. In contrast, clusters of virally-marked cells af ter MeBr-induced lesion of the epithelium frequently contain both neur ons and supporting cells, as well as both types of basal cells. Other clusters contain supporting cells and/or Bowman's gland/duct cells. It is likely that the clusters of marked cells are derived from a single founder cell, i.e. the cells are clonal and lineally related, since t he clusters are widely dispersed. Furthermore, infusion of mixtures of viruses that can be distinguished on the basis of the type and subcel lular localization of the marker enzyme that is expressed produce clus ters that are homogenous with respect to enzyme type, providing strong evidence in favor of the notion that the clusters are clonal in natur e. Thus, the founders of the clones that contain neurons, supporting c ells and basal cells are pluripotent in their capacity for differentia tion. It is unlikely that the pluripotent cells are found in Bowman's gland/duct, since we have yet to observe a clone that contains neurons and cells in Bowman's gland/duct. Hence, the pluripotent stem cells a re to be found in the basal cell compartment of the epithelium. Howeve r, the exact nature of these stem cells remains unknown and a subject for future investigation.