Expression of phenotypic markers during regeneration of rat tracheal epithelium following mechanical injury.

We examined epithelial regeneration in mechanically injured rat trachea using phenotypic markers that identify unique differentiated stages of epithelial cells. Following a focal denuding wound, the cells from the adjacent nonwounded epithelium flattened and migrated into the wounded site during the first 12 h. At 24 h, these cells dedifferentiated into poorly differentiated (PD) cells that did not precisely resemble any of the mature tracheal cells. Proliferation of PD cells produced a multilayered epithelium by 48 h. Mitotic activity, measured as mitotic rate (MR) following a 6-h colchicine metaphase blockade, was high at 24 h (MR 23.4%) and 48 h (MR 24.0%). These PD cells expressed keratin 14 and Griffonia simplicifolia I-isolectin B4 (GSI-B4) lectin binding sites, which are specific for basal cells in normal epithelium but did not react with secretory or ciliated cell markers. At 72 h, MR fell to 1.8% (control MR 0.38%). The wound was covered with a pseudostratified epithelium; secretory cell markers were present at the apex of differentiating columnar cells, and a few preciliated cells expressing ciliated cell markers appeared. Basal cells also became distinctly recognizable and expressed keratin 14 and GSI-B4 binding sites. Newly appearing secretory or ciliated cells also expressed these markers but lost them gradually as they acquired new sets of specific markers. During epithelial regeneration after mechanical injury, "dedifferentiation," "proliferation," and "redifferentiation" of epithelial cells occurred, and the PD cell was pivotal in this process.