Intimate cell contacts at the epithelial/mesenchymal interface in embryonic mouse lung

The epithelial-mesenchymal interface in 11-, 12-, and 13-day lung was analyzed at the ultrastructural level with emphasis on extracellular matrix organization and the occurrence of cell contacts. The basal lamina is intact in 11-day lung (primary bronchus), but occasionally cell surface extensions establish intimate cell contacts between epithelium and mesenchyme in the distal region of the primordium. In 12- and 13-day lung the basal lamina is partly absent along distal outgrowths and sometimes the domains of epithelium and mesenchyme are difficult to define. Intimate cell contacts between epithelium and mesenchyme cells (50–90 A separation) are regularly seen. Collagen fibers (130 A thick) having a 30 A striation pattern are found mainly along quiescent regions. In general, extracellular matrix material is very scanty along distal outgrowths. Aggregates of electron-dense vacuoles are found almost exclusively in the basal cytoplasm of epithelial cells. A relationship with RER is shown. These organelles exhibit morphological characteristics reminiscent of collagen-secreting vacuoles. The findings are discussed in connection with current ideas regarding branching morphogenesis. It is suggested that transient intimate cell contacts between epithelium and mesenchyme may be instrumental in the initiation of bifurcation.