Axonal growth arrests after an increased accumulation of Schwann cells expressing senescence markers and stromal cells in acellular nerve allografts

Acellular nerve allografts (ANAs) and other nerve constructs do not reliably facilitate axonal regeneration across long defects (>3 cm). Causes for this deficiency are poorly understood. In this study, we determined what cells are present within ANAs before axonal growth arrest in nerve constructs and if these cells express markers of cellular stress and senescence. Using the Thy1-GFP rat and serial imaging, we identified the time and location of axonal growth arrest in long (6 cm) ANAs. Axonal growth halted within long ANAs by 4 weeks, while axons successfully regenerated across short (3 cm) ANAs. Cellular populations and markers of senescence were determined using immunohistochemistry, histology, and senescence-associated β-galactosidase staining. Both short and long ANAs were robustly repopulated with Schwann cells (SCs) and stromal cells by 2 weeks. Schwann cells (S100β(+)) represented the majority of cells repopulating both ANAs. Overall, both ANAs demonstrated similar cellular populations with the exception of increased stromal cells (fibronectin(+)/S100β(−)/CD68(−) cells) in long ANAs. Characterization of ANAs for markers of cellular senescence revealed that long ANAs accumulated much greater levels of senescence markers and a greater percentage of Schwann cells expressing the senescence marker p16 compared to short ANAs. To establish the impact of the long ANA environment on axonal regeneration, short ANAs (2 cm) that would normally support axonal regeneration were generated from long ANAs near the time of axonal growth arrest (“stressed” ANAs). These stressed ANAs contained mainly S100β(+)/p16(+) cells and markedly reduced axonal regeneration. In additional experiments, removal of the distal portion (4 cm) of long ANAs near the time of axonal growth arrest and replacement with long isografts (4 cm) rescued axonal regeneration across the defect. Neuronal culture derived from nerve following axonal growth arrest in long ANAs revealed no deficits in axonal extension. Overall, this evidence demonstrates that long ANAs are repopulated with increased p16(+) Schwann cells and stromal cells compared to short ANAs, suggesting a role for these cells in poor axonal regeneration across nerve constructs

Fibrochondrogenesis of hESCs: Growth Factor Combinations and Cocultures

The successful differentiation of human embryonic stem cells (hESCs) to fibrochondrocyte-like cells and characterization of these differentiated cells is a critical step toward tissue engineering of musculoskeletal fibrocartilages (e.g., knee meniscus, temporomandibular joint disc, and intervertebral disc). In this study, growth factors and primary cell cocultures were applied to hESC embryoid bodies (EBs) for 3 weeks and evaluated for their effect on the synthesis of critical fibrocartilage matrix components: glycosaminoglycans (GAG) and collagens (types I, II, and VI). Changes in surface markers (CD105, CD44, SSEA, PDGFRα) after the differentiation treatments were also analyzed. The study was conducted in three phases: (1) examination of growth factors (TGF-β3, BMP-2, BMP-4, BMP-6, PDGF-BB, sonic hedgehog protein); (2) comparison of two cocultures (primary chondrocytes or fibrochondrocytes); and (3) the combination of the most effective growth factor and coculture regimen. TGF-β3 with BMP-4 yielded EBs positive for collagens I, II, and VI, with up to 6.7- and 4.8-fold increases in GAG and collagen, respectively. Analysis of cell surface markers showed a significant increase in CD44 with the TGF-β3 + BMP-4 treatment compared to the controls. Coculture with fibrochondrocytes resulted in up to a 9.8-fold increase in collagen II production. The combination of the growth factors BMP-4 + TGF-β3 with the fibrochondrocyte coculture led to an increase in cell proliferation and GAG production compared to either treatment alone. This study determined two powerful treatments for inducing fibrocartilaginous differentiation of hESCs and provides a foundation for using flow cytometry to purify these differentiated cells