Three-dimensional organization of the lamina reticularis in the rat tracheal basement membrane zone

The airway basement membrane zone is a region specialized for the attachmen t of the epithelium with the matrix. The epithelium is attached to the lami na densa, which, in turn, is connected to types I and III collagen of the l amina reticularis with anchoring fibrils. The purpose of this study was to define the three-dimensional organization of the structural proteins of the lamina reticularis in the rat trachea. We approached this problem by using whole mounts to look down on the flat surface of the basement-membrane zon e rather than a cross section of its thin profile. Fluorescent microscopy w ith long working distance water immersion objectives and scanning electron microscopy revealed that the structural proteins are arranged as a mat of l arge fibers oriented along the longitudinal axis of the airway. Smaller fib ers are crosslinked with the larger fibers to complete this structure. Othe r small fibers are oriented around the large fibers and an amorphous materi al covers individual fibers. The large fibers oriented along the longitudin al axis of the airway are consistent with prior descriptions of fibers comp osed of collagen III with some collagen I and V; small fibers encircling th e large fibers may be collagen VI, The crosslinking fibers are made up of e lastin and probably elastin-associated microfibrils. The amorphous proteins covering the fibrous framework may contain proteoglycans and other nonstru ctural proteins reported to be in the lamina reticularis. The present studi es demonstrate that the structural proteins of the lamina reticularis in th e rat trachea are arranged as fibers in a highly organized manner.