ABSTRACT
Basal cells of the tracheal epithelium are poorly differentiated and similar to basal cells of skin, and, as such, are traditionally considered as multipotential cells. The confirmation that basal cells function as stem cells, at least Ln the rabbit tracheal epithelium, seemed to be provided by Inayama et al. (86). They used a population of tracheal basal cells purified by centrifugal elutriation to inoculate heterotopic tracheal grafts denuded of their own epithelium and transplanted to the backs of nude mice. a few weeks the hasal ceBs were able to reestablish a complete differentiated mucociliary epithelium, demonstrating that they could give rise to all major tracheal cell types. However, when flow cytometry was
to sor! out pure subpopulalions of rat tracheobronchial cells (87), the basal cell inoculum into tracheal grafts gave rise to an epithelium composed of only basal and ciliated cells. The basal cells showed only a limited pmgenitorial capacity, while secretory cells gave rise an epithelium comprised of secretory, basal, and ciliated cells. These results supported previous conclusions based on detailed studies of the proliferation kinetics of epithelial cells of rat bronchi after nitrogen oxide (88) or ozone injury and of hamster trachea after mechanical injury (90) or after vitamin A deficiency (81). In early vitamin A deficiency, secretory cells are the first to slow down in their division rate, eventually followed by reduction in the proliferation of basai cells (85), and they are also apparently the first to resume normal rates of cell division upon readministration of retinoic acid (81).
