Regeneration of hamster tracheal epithelium after mechanical injury

Summary

Regeneration of hamster tracheal epithelium following multifocal mechanical injury was studied with the combined use colchicine metaphase blockade and single pulse tritiated thymidine (³HTdR) labeling. Control hamster tracheal epithelium had a low labeling index (LI = 0.63%) and a low mitotic rate (MR = 0.19%), but of the cells labeled and in mitosis, more were secretory cells (LI = 0.36%, MR = 0.14%) than basal cells (LI = 0.26%, MR = 0.05%).

Following injury, 38% of the epithelial cells was lost by 6 h, and this loss increased to 43% by 12 h. During these times LI and MR remained near control values. However, at 24 h there was a dramatic rise in LI (14.1%) and MR (10.6%) as a broad peak of proliferation with rapid movement through the cell cycle. Secretory cells, which comprised 71 % of the epithelial cells, accounted for about 77% of the mitoses observed and 65% of the³HTdR labeling. In contrast, basal cells comprised 13% of the total cells present, 12% of the mitoses, and 21% of the³HTdR labeling. Non-ciliated squamous cells (a mixed population of flattened secretory and basal cells) comprised 5.4% of the epithelium, 11% of the mitoses and 14% of the³HTdR labeling. When only the labeled cells were considered as discrete populations at 24 h post-injury labeled secretory cells had 88% of their number in mitosis and 12% in interphase, compared to labeled basal cells with 45% in mitosis and 55% in interphase. These data indicate that secretory cells passed through the DNA synthesis phase into mitosis at twice the rate of basal cells. Thus secretory cells played the dominant role in tracheal regeneration due to the number of cells involved and their proliferative rate.

Between 24 and 60 h, proliferative activity gradually declined and epidermoid metaplasia developed in the epithelium. By 72 h the LI and MR had declined to almost control values. Preciliated cells, first seen at 48 h, were most evident at 72 h when LI (0.7%) and MR (0.4%) were low. Pre-ciliated cells were not labeled by³HTdR, indicating that these post-mitotic cells did not synthesize DNA. Thereafter, LI and MR remained low and the majority of the epithelial cells in the wound assumed normal morphology by 120 h. Nevertheless, some epidermoid metaplastic cells did remain.

The wound sites also contained considerable submucosal mesenchymal proliferation during the period of epithelial regeneration. At 24 h capillary endothelial cells were labeled and dividing, and by 36 h large numbers of fibroblasts were labeled and in mitosis. This mesenchymal proliferation remained sustained for a longer period than the epithelial proliferation, similar to the situation observed in developing fetal and neonatal airways.