Evolution of cell phenotype and extracellular matrix in tissue-engineered heart valves during in-vitro maturation and in-vivo remodeling

Background and aim of the study: Contemporary tissue valves are non-viable, and unable to grow, repair or remodel. It was postulated that tissue-engineered heart valves (TEHV) fabricated from autologous cells and a biodegradable scaffold could yield a dynamic progression of cell phenotype and extracellular matrix (ECM), in vitro and in vivo, and ultimately recapitulate native valve microscopic architecture. Methods: Trileaflet valve constructs were fabricated from poly-4-hydroxybutyrate-coated polyglycolic acid seeded with ovine endothelial and carotid artery medial cells, cultured in vitro for 4-14 days in a pulse duplicator, implanted as pulmonary valves in five lambs, and explanted at 4-20 weeks. ECM composition and collagen architecture were examined by histology (including Movat pentachrome stain and picrosirius red under polarized light), and cell phenotype by immunohistochemistry. Results: Cells from in-vitro constructs (14 days) were activated myofibroblasts, with strong expression of alpha-actin (microfilaments), vimentin (intermediate filaments) and SMemb (non-muscle myosin produced by activated mesenchymal cells). Cells from in-vivo explants at 16-20 weeks were fibroblast-like, with predominant vimentin expression and undetectable levels of alpha-actin (similar to native valve). Collagen elaboration and cellular expression of MMP-13 (collagenase 3) were evident in vitro at 14 days. In-vivo explants had increased collagen accumulation and strong MMP-13 expression at 4-8 weeks, but less activation (decreased expression of SMemb) and patchy endothelial cells at 16-20 weeks. Moreover, the ECM architecture of 16- to 20-week explanted TEHV resembled that of native valves. Conclusion: Cell phenotype and ECM in TEHV prepared in vitro and implanted in vivo are dynamic, and reflect the ability of a vital tissue to remodel and, potentially, to grow.