SUMMARYInteractions between epithelial cells and the extracellular matrix through integrins play a key role in the development of the lung by modulating branching morphogenesis, epithelial cell polarization, and differentiation. To determine the role of integrins during the different stages of lung development, we investigated the distribution of eight integrin subunits in the trachea and lung from human fetuses. In distal airways, during the early pseudoglandular stage of development, the ␣2-, ␣5-, ␣6-, ␣v-, and ␤1-subunits were detected in all epithelial cell plasma membranes, and polarized but undifferentiated tracheal epithelial cells expressed ␣3-, ␣6-, and ␤1-subunits in the plasma membrane of the cells facing the basement membrane. The ␣6-and ␤4-chains were detected along the basal plasma membrane of the basal cells in differentiated tracheal epithelia. The ␣4-subunit was detected in all respiratory cells throughout fetal development. In the submucosal glands, myoepithelial cells expressed the integrin subunits found in the undifferentiated cells of the developing airways, whereas the secretory cells expressed only ␣2-, ␣3-, ␣4-, ␣6-, and ␤1-subunits. These results demonstrate differential expression of integrins during lung development and suggest that integrins may play multiple roles in organogenesis and maturation of respiratory surface epithelium and glands. (J Histochem Cytochem 46:803-810, 1998)

We have developed an in vivo assay for progenitor cells of the human tracheobronchial epithelium relying on the transplantation of human prenatal respiratory tissues into severe combined immunodeficiency mice. Engrafted embryonic or fetal open tracheobronchial rudiments are rapidly closed at each end by a neoformed membrane that we named the operculum. After 2–4 weeks, differentiated human respiratory epithelium covers both the native airway matrix and the new operculum. Human epithelial cells dissociated from either emerging embryonic lung primordia or mature xenografts were seeded in host human airway grafts, of which native epithelium had been eliminated by several cycles of freezing and thawing. All grafts seeded with donor epithelial cells and implanted back into SCID mice recovered a surface mucociliary epithelium expressing expected markers and secreting mucus. Spontaneous epithelium regrowth was never observed in control unseeded, denuded grafts. In some experiments, donor epithelial cells and host denuded airway were sex-mismatched and the donor origin of newly formed epithelial structures was confirmed by sex chromosome detection. After two rounds of seeding and reimplantation, a normal epithelium was observed to line the 3rd generation operculum. These observations substantiate a functional assay for human candidate airway epithelium stem cells.