Detection of a novel stem cell probably involved in normal turnover of the lung airway epithelium

Regeneration of the lung airway epithelium after injury has been extensively studied. In contrast, analysis of its turnover in healthy adulthood has received little attention. In the classical view, this epithelium is maintained in the steady-state by the infrequent proliferation of basal or Clara cells. The intermediate filament protein nestin was initially identified as a marker for neural stem cells, but its expression has also been detected in other stem cells. Lungs from CD1 mice at the age of 2, 6, 12, 18 or 24 months were fixed in neutral-buffered formalin and paraffin-embedded. Nestin expression was examined by an immunohistochemical peroxidase-based method. Nestin-positive cells were detected in perivascular areas and in connective tissue that were in close proximity of the airway epithelium. Also, nestin-positive cells were found among the cells lining the airway epithelium. These findings suggest that nestin-positive stem cells circulate in the bloodstream, transmigrate through blood vessels and localize in the lung airway epithelium to participate in its turnover. We previously reported the existence of similar cells able to differentiate into lung chondrocytes. Thus, the stem cell reported here might be a bone marrow-derived mesenchymal stem cell (BMDMSC) able to generate several types of lung tissues. In conclusion, our findings indicate that there exist a BMDMSC in healthy adulthood that participates in the turnover of the lung airway epithelium. These findings may improve our knowledge about the lung stem cell biology and also provide novel approaches to therapy for devastating pulmonary diseases.