Effects of retinoic acid on the growth and morphology of hamster tracheal epithelial cells in primary culture

Hamster tracheal epithelial cells were grown in primary culture on a collagen gel substrate in hormone-supplemented serum-free Ham's F12 medium with 10^-8 M retinoic acid (RA +), or without retinoic acid (RA -). On days 1 and 2, the colonies were composed of large (secretory) cells and lesser numbers of small (basal) cells; ciliated cells were rare. At these times, cell number, thymidine incorporation, and total labelling indices (small and large cells, combined) were similar in RA+ and RA-cultures, but the large cells became flat in RA-medium on day 2. On days 3-5, thymidine incorporation and total labelling indices were less in RA-than RA+ cultures, and on days 4-6, cell numbers were decreased in RA-cultures. On day 3, the large cells of the RA-colonies had flattened further and clusters of small basal cells had formed. On day 4, the RA+ colonies were composed of densely-packed cuboidal secretory cells, small basal cells were inconspicuous; the total labelling index was about 27%. The RA-colonies were composed of large flat secretory cells and numerous small basal cells which were clustered in groups; the total labelling index was about 7%. Since large and small cells could be discriminated by size in RA-colonies, a labelling index was generated based on cell size. On days 2, 3 and 4, the labelling index of the small basal cells in the RA-colonies was 44%, 43% and 24% respectively, whereas the labelling index of the large secretory cells fell rapidly over the same period (56%, 14% and 2%). On days 5 and 6, the cuboidal secretory cells in the RA + cultures had differentiated further and the cells were stratified focally. Some new ciliated cells had formed on day 6. In RA cultures, mucous granules were not observed in the large flat cells and ciliated cells were not seen. The total labelling indices were 11% and 0.35% in RA+ cultures, and 0.5% and 0.25% in RA-cultures on days 5 and 6, respectively. The study shows that the target cell for vitamin A in the hamster tracheal epithelium is the secretory (mucous) cell. When retinoic acid was deficient, the secretory cells flattened and their capacity to divide was greatly diminished. Since the basal cells continued to replicate when the secretory cells did not, the population density of the basal cells increased disproportionally, which could be interpreted erroneously as a "basal cell hyperplasia." Real hyperplasia and epidermoid metaplasia were late secondary events which occurred in this study following focal disintegration of the epithelial cell sheet. Then the large secretory cells became keratinized and expressed an epidermoid phenotype.