Lectin binding beneath the epithelium and in smooth muscle cells in the developing bronchial tree

Components of the subepithelial stratum in developing rat lung reacted transiently with Maclura pomifera agglutinin (MPA) and Aleuria aurantia agglutinin (OFA) conjugated to horseradish peroxidase. These lectins possess selective affinity for and serve to localize glycoconjugates (GCs) with terminal Gal/GalNAc and Fuc, respectively. Staining was strongest with both lectins in the proximal bronchial tree and decreased peripherally to growing buds where it was absent. MPA staining of subepithelial structures decreased from the pseudoglandular through the canalicular period and disappeared by the terminal sac stage. Disappearance of this subepithelial reactivity coincided with appearance of apical MPA-positive glycoconjugate in the canalicular period. OFA stained selectively a layer of flattened cells and a thin extracellular stratum under the epithelium of proximal bronchi in the canalicular period. This lectin affinity extended farther peripherally in the pseudoglandular interval and diminished thereafter. The layer of OFA-positive cells underlying the epithelium was identified immunohistochemically as immature smooth muscle. These muscle cells gained contractile protein while losing surface lectin reactivity during fetal development. The high iron diamine method localized sulfated GC in basement membrane of proximal respiratory passages in the fetal lung. The results attest to the involvement of specific GCs in mediating epithelial-mesenchymal cell interaction during critical stages of bronchial morphogenesis.