The aggregation of lymphoid cells in the bronchial mucosa has been named the bronchus-associated lymphoid tissue (BALT) and investigated in comparison with the gut-associated lymphoid tissue (GALT). To elucidate precisely the structure and function of the BALT, the present study examined the age-related change in the mouse BALT by light microscopy. We also observed the characteristics of the overlying epithelium, especially the lectin-binding properties of the epithelial cells, by the combined use of light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
  By LM, Iymphoid aggregates were not recognizable in the bronchial mucosa of young (8-10 week-old) mice, while they were commonly found at the second to fourth branching portions of the bronchial tree in older (32-40 week-old) mice. The epithelium overlying the lymphoid aggregates of the mature mice often contained a large number of mononuclear cells. Lectin cytochemistry revealed that UEA1 ( Ulex europaeus agglutinin 1) positive cells were not only restricted to the overlying epithelium of the BALT in the older mice but also found in a cell group in the mucous epithelium at the branching portions in the young mice. Comparison between the LM and SEM images of the UEA1-stained whole mount specimens clarified the surface morphology of the lectin-stained epithelial cells, showing them to be non-ciliated cells with a large number of short nticrovillous projections on the apical surface. TEM studies further demonstrated that the UEA1 reaction products appeared on the plasma membrane of the non-ciliated cells which often enfolded lymphocytes in the old mice. Latex microbeads, which were administrated intratracheally, were selectively taken up by the UEA1-positive cells of the BALT.
  These results indicate that the mouse BALT has specialized epithelial cells similar to the UEAI positive M cells in the GALT and probably functions as a part of the mucosal immune system. This study also showed the possibility that the UEAI positive cells appear in the mucous epithelium before the formation of the BALT.