Abstract. The fate of [3H]thymidine ([3H]Tdr) pulse‐labelled cells was followed in tracheal epithelium of young male rats. The time course for cell differentiation, and the relation of events to tissue composition were studied. In vivo labelling and light microscope autoradiography of epoxy embedded sections were used. Labelled and total nuclei for each cell type, and combinations of labelled cells which were adjacent to one another, were tallied. Hierarchical analyses of variance were performed on the several data sets. All cell types, except ciliated, were labelled at 1 hr. A few labelled ciliated cells were seen 24 hr post‐label. The frequency of labelled intermediate cells peaked at day 2; goblet and ciliated cells at day 3. No significant changes occurred in the labelling index, but at 24 hr the frequency of adjacent labelled cells (ALC) had increased > 5‐fold, and changes had occurred in patterns of ALC combinations. The labelled ciliated cells which were seen at 24 hr were adjacent to labelled intermediate cells. No labelled basal‐ciliated cell combinations were seen at any time. Data indicated that ciliated cells can develop from S‐phase intermediate cells within 24 hr, and neither basal nor superficial goblet cells are progenitors of ciliated cells. It is proposed that both superficial goblet cell and ciliated cell development is preceded by two divisions: a basal cell division followed by an intermediate cell division.

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