The Inner Annulus Fibrosus Encroaches on the Nucleus Pulposus in the Injured Mouse Tail Intervertebral Disc.
Yulong Wei, Zuozhen Tian, Robert J Tower and 6 others
PMID 32858534WHAT IT FOUND
In injured mouse discs, inner annulus fibrosus cells migrate inward to fill the space left by herniated nucleus pulposus.
These inner cells express N-cadherin and collagen, replacing the gelatinous core with fibrocartilage. This mechanism explains how degeneration progresses in this animal model.
Key findings
01Inner annulus fibrosus cells labeled red by lineage tracing were found invading the space previously occupied by the nucleus pulposus after needle injury, while the nucleus pulposus cells themselves were not labeled.
02N-cadherin protein, which aligns with cell membranes in healthy nucleus pulposus cells, was lost in injured discs as early as 2 days post-injury and did not reappear by 4 weeks.
03Injured motion segments showed a statistically significant increase in neutral zone modulus (stiffness) compared to intact controls, though other biomechanical measures like compressive modulus and creep did not differ significantly.
STILL TO COME
How it was doneWhat they found
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What it does not show
The study was conducted entirely in mice; the authors explicitly state that the premise that this model translates to human work remains to be confirmed. The needle puncture model causes acute herniation and partial loss of nucleus pulposus tissue, which may only reflect a subset of human patients with herniation rather than general degeneration. The observation period was short (up to 4 weeks), whereas human disc degeneration develops over years. The study did not determine if the inner annulus cells proliferate in response to injury or simply migrate. Biomechanical comparisons used different mice for injured and intact groups because tail level affects stiffness, introducing potential inter-individual variability.
Declared interests
The authors declare no competing interests. The study was supported by the NIH.
The easy way to misread this
Do not assume these findings apply directly to human disc degeneration or clinical practice. The authors state that the translation of this mouse model to human work is unconfirmed, and the mechanism described is specific to the acute injury and herniation model used in the study.