The regulation of nerve and blood vessel ingrowth in aneural and avascular intervertebral disc and articular cartilage

Introduction This review will discuss the regulatory mechanisms of both innervation and vascularisation within normally aneural and avascular tissues, and how they may become altered in degeneration enabling new nerve and blood vessel formation which is hypothesised to be a source of pain. Conclusion Normal intervertebral discs and articular cartilage are the largest aneural and avascular tissues in the human body yet during intervertebral disc degeneration and osteoarthritis these tissues become increasingly vascularised by small blood vessels and innervated by peptide containing sensory nerve fibres. The mechanism by which this process occurs remains largely unknown. Published data suggests that various factors present within the healthy tissues such as aggrecan, chondromodulin and semaphorins may act as repulsive barriers to neurite and endothelial cell invasion. During degeneration however, the synthesis of these molecules becomes disrupted, potentially leading to vascularisation and innervation of the tissue.