Fig. 4. Potential mechanisms involved in epithelial repair in the trachea and main bronchi. Clara-like cells are shown in blue, ciliated cells in green, neuroendocrine cells in pink, basal cells in orange and mucous cells in purple. Regions rich in blood vessels and nerves are indicated. Black line represents the basal lamina. Smooth muscle cells and fibroblasts are not shown. Putative regenerative cell populations are numbered 1-4. (1) K14-positive cells: these are possibly basal cells that self-renew and give rise to Clara cells and ciliated cells after destruction of Clara cells by naphthalene. (2) Label-retaining cells, which have been identified in submucosal gland ducts and in intercartilage regions. (3) Epithelial cells in submucosal glands, which can regenerate the entire luminal epithelium in grafts. (4) Clara cells, which can replace ciliated cells damaged by oxidants.