Basement membrane pores in human bronchial epithelium: a conduit for infiltrating cells?

Abstract

This study reports the presence of oval-shaped pores in the basement membrane of the human bronchial airway that may be used as conduits for immune cells to traffic between the epithelial and mesenchymal compartments. Human bronchial mucosa collected after surgery was stripped of epithelial cells without damaging the basement membrane. Both scanning and transmission electron microscopy showed oval-shaped pores 0.75 to 3.85 microm in diameter in the bronchial basement membrane at a density of 863 pores/mm2. Transmission electron microscopy showed that the pores spanned the full depth of the basement membrane, with a concentration of collagen-like fibers at the lateral edges of the pore. Infiltrating cells apparently moved through the pores, both in the presence and absence of the epithelium. Taken together, these results suggest that immune cells use basement membrane pores as predefined routes to move between the epithelial and mesenchymal compartments without disruption of the basement membrane. As a persistent feature of the basement membrane, pores could facilitate inflammatory cell access to the epithelium and greatly increase the frequency of intercellular contact between trafficking cells.

Figure 1.

Transmission electron micrographs of normal human bronchial epithelium. A: The pseudostratified structure of the bronchial epithelium with upper ciliated cells (open star), resting on a layer of basal cells attached to the upper basement membrane layer of lamina densa (arrowhead). The underlying cellular lamina propria is separated from the lamina densa by the lamina reticularis (star). B: Higher magnification of the lamina densa and overlying basal cell showing the lamina densa as an afibrous layer (arrowheads), whereas fibrous collagen was found in the lamina reticularis cut in mostly transverse section (star). Scale bars, 5 μm (A) and 2 μm (B).

Figure 2.

Scanning electron micrograph of an untreated bronchial epithelial sample, illustrating a limited area of damage to the epithelium to the level of the basement membrane caused by nonchemical means. Some columnar and basal cells remain attached in surrounding areas. Pores are found as oval structures distinct from the nodular basement membrane (arrowheads). Inset: Higher magnification view of a basement membrane pore showing the concentric rings of fibers lining the pore. Scale bar, 20 μm (2 μm in inset).

Figure 3.

Transmission electron micrograph of bronchial epithelial basement membrane with a pore in longitudinal section. A: The pore (arrowheads) traverses the full thickness of the basement membrane with epithelium (E) intact and connects with what appears to be a cell-filled channel at the lamina reticularis/lamina propria boundary (star). LR, lamina reticularis. B: Higher magnification of the lateral edges of the pore. The lamina densa (arrowheads) ends at the lip of the pore and the lateral edges of the pore appear lined with collagen-like fibrils. Scale bars, 5 μm (A) and 1 μm (B).

Figure 4.

Scanning electron micrograph of bronchial basement membrane denuded of epithelium following a stripping procedure involving coverslip scraping. No columnar or basal cells remain attached to the basement membrane and the basement membrane retains its nodular appearance without apparent damage. Basement membrane pores are clearly evident (arrowheads). Scale bar, 20 μm.

Figure 5.

Scanning electron micrograph of bronchial epithelial basement membrane after 24 hours’ culture. A: Epithelial cells (e) display a flattened morphology consistent with their migration across a denuded basement membrane, a nonepithelial cell with contrasting morphology is located between two pores (small arrowheads). B: Cells of nonepithelial morphology located on the basement membrane near pores (small arrowheads), with one cell apparently exiting a pore (large arrowhead). C–F: Morphology of cells consistent with different stages of cell migration through basement membrane pores. C: A cytoplasmic process appears to extrude from the lumen of a pore. D: A pore is blocked by the main body of a cell. E and F: The majority of a cell rests on the basement membrane surface adjacent to a pore, whereas cell processes clearly extend into the pore (F, magnified image of B). Scale bars, 10 μm (A and B), 2 μm (C and D), and 4 μm (E and F).

Figure 6.

Transmission electron micrographs of cells traversing basement membrane through pores in a sample with an intact bronchial epithelium. Based on the positioning of their nuclei, the cells appear to be traversing into (A) and out of (B) the epithelium (E). The typical hourglass shape of one cell (B) is consistent with its migration through the limited space of the pore (arrowhead). Both pores appear to be continuous with channels (star) in the lamina propria and appear lined with collagen-like fibrils seen mostly in transverse section. LR, lamina reticularis. Scale bar, 3 μm (A and B).