Studies of cellular function in the respiratory tract lining have traditionally been limited by the small tissue mass and functional diversity of the epithelium. Recent improvements in culture conditions have permitted long-term maintenance of epithelial cells derived from the upper respiratory tract of rats. The present study determined the extent to which proliferation and differentiation took place in such epithelial cell cultures. The labelling index obtained after 3H-dThd administration was approximately 100-fold higher than that of the quiescent epithelium in vivo; therefore, a large proportion of the cells were probably in the cycling population. Ultrastructural studies showed that features unique to the specialized mucous secretory cells and ciliated cells were lost rapidly with entry of these cells into the in vitro environment. With long-term maintenance, the cultures were reorganized into a stratified epithelium containing large, squamous, apical cells and small basal cells. The ultrastructural appearance of basal cells in vitro was nearly identical to that of basal cells in vivo. Squamous cells were frequently joined by tight junctions. Because hemicysts originated by detachment of squamous cells from the basal layers but not from adjacent squamous cells, they were considered to indicate stratification in the cultures. The proliferative and differentiative status of the mucociliary epithelium was altered by in vitro conditions, and came to resemble that of early squamous metaplasia in the respiratory tract epithelium.

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