About Intervertebral Discs

Picture from M.A. Adams et al, 2010

Having learnt in anatomy class (when I was a physiotherapy student) that our intervertebral discs (IVD) are avascular (has no blood supply), I was instantly surprised when recent research showed that it may not be totally true.

picture from springer link

A little anatomy lesson before I tell you more. Our IVD's are fibrocartilaginous joints that are thought to be the largest avascular structures in the human body. They are made up of three distinct and interdependent tissues. The outer most cartilage endplates are thin layers of hyaline cartilage that anchor the IVD to the adjacent vertebral bones. The vertebral end plates have plenty of blood supply and this allows for diffusion of nutrients into the IVD through the cartilage end plate.

The annulus fibrosus (AF) is a series of super strong well organized concentric lamellae of fibrocartilage that surround and protect the nucleus pulposus (NP) of the IVD. 

The NP is the innermost jelly like substance made up mainly of water and proteoglycans. The NP helps distribute pressure evenly across the IVD and prevent excessive forces loading the spine. This is what can herniate through the AF, causing what is commonly know as a 'slipped disc' or prolapsed intervertebral disc (PID).

A group of researchers performed a comprehensive *scoping review on peer-review publications on the blood supply of human IVD's excluding disc herniations. 22 out of 3122 articles found met the inclusion criteria of fetal to > 90 years old, various health status and both sexes using gross dissection, histology or medical imaging to assess if there is blood supply.

Consistent observations from this review were that there is no blood supply in the NP of the IVD throughout life. 

Both the cartilage endplates and AF have considerable blood supply during fetal development and in infants, but decreases over our lifespan. A common feature of the cartilage endplate was the presence of channels throughout the tissue, likely from the well vascularized vertebral endplate from the adjacent vertebrae. Between birth and ten years of age, there is a drastic decrease in blood vessels within these channels; which are not seen at all in adults.

However, there are blood vessels growing into the endplates and inner layers of the AF especially when there is damaged or disrupted tissue regardless of age. This is more common in older adults. Location of blood vessels are variable. 

It is thought that annular fissures or tears associated with degenerated discs are perhaps more conducive to the ingrowth of blood vessels since there is a loss of proteoglycans (a protein compound found in connective tissue) due to the healing process. Interestingly, there are also nerves found together with the blood vessels suggesting some patients may get more pain than others with such conditions.

Through this scoping review, we now know that the IVD is not entirely avascular as often thought and cited. This is great news for patients. We always knew that you can heal from a "slipped disc", but the discs having a blood supply means a better chance that it can heal from an injury.

Reference

Fournier DE, Kiser PK, Shoemaker JK et al (2020). Vascularization Of The Human Intervertebral Disc: A Scoping Review. JOR Spine. 15: 3(4): e1123. DOI: 10.1002/jsp2.1123.

*A scoping review has a broader scope compared to traditional systematic reviews with correspondingly more expansive inclusion criteria.

* you can read more about slipped discs and how slipped discs can heal here.