All rabbit pleural adhesions examined in this study were mesothelial-covered fibrovascular bands containing well-developed blood and lymphatic vessels establishing a structural continuity between both pleural layers (Fig 1, top, A). Myelinated nerve fibers were also present in adhesions from 20% (2 of 10 rabbits). In addition, polarized light microscopy revealed that foreign body granulomas of variable size were common histopathologic findings in most of the adhesions studied. The anatomic location of the adhesion within the pleural cavity did not appear to influence its overall morphology.
Light microscopy showed the surfaces of the adhesions to be covered by a continuous monolayer of flattened mesothelial cells (Fig 1, center, B). Complete epithelialization of the adhesions was already observed at 1 week after instillation. At that time, electron microscopy revealed that this newly formed mesothelium was composed of poorly differentiated cells without microvilli resting on a thin basal lamina (Fig 1, bottom, C). Mesothelial cell-cell adhesions consisted of tight junctions located at the apical part of the cells (Fig 1, bottom, C inset).
One week after talc instillation, light microscopy showed the adhesions to be composed of loose irregular collagenous connective tissue in various stages of maturity (Fig 2, top, A). The collagen fibers were arranged in discrete bundles and were often aligned parallel to the longitudinal axis of the adhesion. The thicker collagen fibers were observed at the edges of the adhesion. By this stage, there were numerous fibroblasts among bundles of collagen fibers, and a variable number of macrophages, lymphocytes, and mast cells (Fig 2, top, A). The fibroblasts were enlarged with strongly stained cytoplasm, lax chromatin, and long cytoplasmic processes. At 1 month, the cellular content of the adhesions became sparser and included elongated spindle-shaped fibroblasts lying between thick dense bundles of collagen fibers (Fig 1, center, B). By transmission electron microscopy, collagen fibers showed numerous densely packed fibrils of uniform diameter (80 nm) that exhibited a characteristic cross-striated banding pattern with a period of 50 nm (Fig 2, bottom, B), typical of type I collagen. Long cytoplasmic processes of fibroblasts were often observed in close contact with these collagen fibrils (Fig 2, bottom, B).
Figure 1. Top, A: Cross section of a pleural adhesion (pa) connecting the visceral and parietal (not seen) pleura; lp = lung parenchyma (hematoxylin-eosin, bar = 200 |j,m). Center, B: Semithin section of a pleural adhesion showing complete epithe-lialization and irregular dense connective tissue (methylene blue; bar = 20 |j,m). Bottom, C: Ultrastructural observation of a poorly differentiated mesothelial cell lining a pleural adhesion (bar = 0.5 |j,m). The overview shows a transmission electron microscopy image of a mesothelial cell-cell adhesive junction (bar = 0.25 ^m).
Figure 2. Extracellular matrix. Top, A: Semithin section of a pleural adhesion showing irregular collagenous connective tissue in various stages of maturity. At 1 week, only the region beneath the mesothelium (m) has a loose appearance, containing numerous enlarged fibroblasts (methylene blue; bar = 200 |j,m). Bottom, B: Ultrastructural observation of a fibroblast process organizing the collagenous matrix (bar = 0.5 |j,m).