Stem cells of the respiratory tract.

Childhood respiratory diseases are accompanied by various forms of epithelial repair and remodelling. Respiratory stem cells are likely to be of great importance in these processes but their identification remains uncertain. By contrast, an enormous effort has been made to identify specific markers of stem cells of other organs; this has given rise to the new concept of plasticity. This proposes that stem cell differentiation is influenced by the niches in which the cells reside rather than their site of origin. Many studies provide evidence in support of this concept, including one demonstrating that respiratory epithelium can arise from bone marrow stem cells. Recent findings from human and animal studies suggest that Clara cell-specific protein-expressing cells with pre-Clara cell phenotypes may be candidate stem cells for the entire respiratory tract.