Barriers to mesenchymal stromal cells for low back pain

Figure 1 Annulus fibrosus structure. Annulus fibrosus (AF) is composed of 15-25 concentric lamellar layers with oblique collagen fibers in alternating directions lying parallel within each lamella. From the edge of the disc inward through the annulus, the angle-ply fiber orientation (θ) decreases from ± 62° from the vertical axis to ± 45° in a linear manner. From the outer to inner AF regions, glycosaminoglycan increases from 3% to 8% per wet weight, while the ratio of type I collagen to type II collagen decreases. The inner AF mainly contains rounded fibrocartilage cells, and the outer AF mainly contains elongated fibroblast-like cells, while other cell types located in or near the AF include peripheral cells, interlamellar cells, and stem/progenitor cells.