Macroscopic examination showed a typical whitish graft in early deaths (< 3 months). Later on, grafts consistently showed a thickened, pink, and flaccid aspect resembling an esophagus. In most cases, there was up to 50% longitudinal contraction of the graft. A cleavage plane could be found between the graft and the surrounding mediastinal tissue in the four animals examined > 9 months postoperatively. At that time, cartilage-like fragments were seen, some of which were oriented transversally, as well as folds of a “posterior membrane” oriented longitudinally.
Partial replacement of the normal histologic structure of the aorta by saffron-stained connective tissue (collagen) with a dense inflammatory infiltrate and persistence of normal elastic tissue foci was seen on microscopic examination on the 10th week postoperatively. At that time, the aortic graft was covered by fibrinous exudates; foci of metaplastic squamous epithelium were seen at the periphery in contact with native tracheal epithelium (Fig 2). At 3 months, elastic tissue had retracted at the periphery of the graft where small islands of fibers could be seen. The internal face of the graft was lined with metaplastic surface epithelium. At 21 weeks, foci of cartilage appeared on transverse sections of the graft in the vicinity of anastomoses on longitudinal sections (Fig 3). Epithelial buds, covered with squamous and mucous cells, were observed expanding within the graft from the metaplastic surface epithelium. At 50 weeks, numerous islets of cartilage could be seen on both transverse (Fig 4) and longitudinal sections, together with a remaining islet of elastic fibers. Some of the cartilaginous formations showed osteoblastic activity and osseous proliferation. Surface epithelialization of the graft remained incomplete, showing squamous and mucous cells without reproducing a ciliated respiratory epithelium at this stage. Alcian blue-eosin enabled us to detect extracellular matrix and therefore to visualize islets of cartilage in the 50-week graft (Fig 5). Since collagen2a1 gene is expressed very early during the cartilage formation, we performed collagen2a1 RNA in situ hybridization on several aortic graft slides. At 50 weeks, in situ hybridization revealed expression in the cartilage islets (Fig 5).
Figure 2. Aortic graft at 10 weeks. Shown is squamous metaplasia lining graft infiltrated by dense polymorphous inflammatory cells (HES, original X 100). °Tracheal lumen.
Figure 3. Aortic graft at 21 weeks, showing replacement of aortic graft by collagen tissue and retraction of elastic fibers at periphery (single arrow) and islets of cartilaginous dystrophic structures (double arrows) [orcein, original X 100; insert, X 200].
Figure 4. Aortic graft at 44 weeks, with low magnification showing transversally disposed cartilaginous structure (Alcian blue). * Tracheal lumen.
Figure 5. Left, A: Aortic graft at 50 weeks, with dense inflammatory collagen tissue and islets of cartilage (rectangle) admixed with bony (arrows) structures (HES, original X 100). °Tracheal lumen. Inserts correspond to magnification (X 150) of islets of cartilage within rectangle showing Alcian blue stainable extracellular matrix (top right, B) and expression of Collagen2a1 as shown by RNA in situ hybridization (center right, C).