Vitamin A deficiency and inflammation: the pivotal role of secretory cells in the development of atrophic, hyperplastic and metaplastic change in the tracheal epithelium in vivo

We showed previously that the proliferation of hamster airway secretory cells decreases during vitamin A deficiency (VAD) but later increases when submucosal inflammation develops (Virchows Arch [B] 59:231-242, 1990). This observation has important biological implications since two morphological extremes (atrophy and quiescence versus hyperplasia and hyperproliferation) are reported in the literature for VAD tracheal epithelium in vivo. In the present study, histological slides of tracheal rings from 35-day-old control and VAD hamsters (Virchows Arch [B] 45:197-219, 1984) were reviewed again. Rings from VAD hamsters were selected based on the absence or presence of a florid submucosal inflammation. Quantitative analyses were made on the cartilaginous part of rings from the anterior third of the trachea. When inflammation was absent, a mucociliary pseudostratified epithelium was, for the most part, maintained. The mitotic rate (MR, 6 h colchicine blockade) of secretory cells was markedly reduced (29-fold) but that of basal cells was not changed significantly. Moreover, cell density was not changed by VAD but ciliated cells and secretory cells were decreased and basal cells were increased, proportionally. We call this "minimal morphological change." Thinning (atrophy) of the minimally changed epithelium was associated with focal cell sloughing. Small scattered foci of epidermoid metaplasia (multiple layers of highly keratinized cells which were extremely flat, so that the epithelium was thin and attenuated) were also seen. We call this "atrophic epidermoid metaplasia." When inflammation was present, hyperplastic changes (stratification and epidermoid metaplasia) predominated and cells were in mitosis at all epithelial levels (low, middle, superficial) except in the most superficial (terminally differentiated) squames. The tracheal epithelium was thickened and hypercellular. The cells were piled up at the stratified lesions, and epithelial height, cell density and epithelial MR were significantly increased compared with the non-inflamed VAD epithelium. The effects of VAD and inflammation on cell proliferation were analyzed further by studying 7 h bromodeoxyuridine (BrdU) labelling patterns of cells in VAD tracheal epithelium, with and without submucosal inflammation. In addition, inflammation was induced in "minimally changed epithelium" by mild mechanical injury. The BrdU labelling patterns confirmed that DNA synthesis by secretory cells is reduced markedly by VAD. However, this suppression is overidden by the influx of inflammatory cells (the nature of the stimulus is unknown). The results indicate that the morphological contrasts (atrophy and hyperplasia) seen in the trachea during VAD in vivo are related to extremes in proliferation rates of tracheal secretory cells, regulated by VAD alone (minimal replication) and by inflammation (maximal replication).