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Abstract
In the end stage of intervertebral disc degeneration, cartilage, bone, endothelial cells, and neurons appear in association with the worsening condition. The origin of the abnormal cells is not clear. This study investigated the properties of progenitor cells in the annulus fibrosus (AF) using one in vitro and two in vivo models. Cultivation of rabbit AF cells with chondrogenic media significantly increased expressions of collagen and aggrecan. Upon exposure to osteogenic conditions, the cultures showed increased mineralization and expression of osteopontin, runx2, and bmp2 genes. Two models were used in the in vivo subcutaneous implantation experiments: 1) rabbit AF tissue in a demineralized bone matrix (DBM) cylinder (DBM/AF), and, 2) rat intact and needle punctured lumbar discs. Bone formation in the AF tissue was detected and hypertrophic chondrocytes and osteoblasts were present 1 month after implantation of the DBM/AF to nude mice. In addition to collagen I and II, immunostaining shows collagen X and osteocalcin expression in DBM/AF specimens 4 months after implantation. Similar changes were detected in the injured discs. Almost the entire needle punctured disc had ossified at 6 months. The results suggest that AF cells have characteristics of progenitor cells and, under appropriate stimuli, are capable of differentiating into chondrocytes and osteoblasts in vitro as well as in vivo. Importantly, these cells may be a target for biological treatment of disc degeneration.
Highlights
Degenerative disc disease affects the majority of the population and treatment costs up to 100 billion dollars annually [1,2]
The expression of runx2, bmp-2, and osteopontin were markedly increased in annulus fibrosus (AF) cells treated with osteogenic medium compared to that treated with growth medium (Fig. 2B), which indicates the osteogenic potential of these cells
While the nucleus pulposus (NP) is considered to be a remnant of the notochord, the AF is thought to be derived from mesoderm
Summary
Degenerative disc disease affects the majority of the population and treatment costs up to 100 billion dollars annually [1,2]. Current methods of treatment include non-surgical therapies such as physical therapy, spinal fusion, and disc replacement surgeries [3,4]. None of these methods restore full spine structure or function. The IVD comprises three distinct regions: nucleus pulposus (NP), annulus fibrosus (AF), and endplates Each of these regions is structurally and biochemically different, but function in concert to maintain the structural integrity of the spine. This is achieved through the disc’s network of various structural components including types of collagen, aggrecan, and elastin fibers that work to maintain hydrostatic pressure to relieve the effects of compression on the spine [5,6]. The etiology of disc degeneration is multifactorial, with no clear biological causes [8,9]
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