The Human Intervertebral Disc Download

The Human
Intervertebral Disc
Developmental, Anatomic and
Physiologic Considerations for Potential
Regenerative Therapies
Benjamin D. Levy, MD, FAAPMR
Interventional Pain Management
Ambulatory Care Service
U.S. Department of Veterans Affairs
VA New Jersey Health Care System
Topics of Discussion
Cellular and Molecular Biology
Implications for Regenerative
Financial Disclosures
Happily employed by the United
States Federal Government
Disc Anatomy
30° angle-ply architecture1
Disease Models & Mechanisms 4, 31-41 (2011)
Disc Embryology
• Mesoderm-derived involved in cell signaling and
• Becomes nucleus pulposus
• Blocks of mesoderm flanking the notochord paraxially
• Cells of somites become sclerotome
• Sclerotomes become alternating more & less condensed2:
More condensed: Around notorchord to become annulus
Less condensed: Become vertebral bodies
From Orthop Clin N Am 42 (2011) 447–464
Disc Genetic Factors
Key Developmental Genes:
• Sox (5,6,9):
Critical to collagen II, inner annulus and matrix
Reduced Sox9 expression correlated with degenerative
• TGFβ:
Regulates cell proliferation and matrix production.
In murine model, remains active at maturity4
Cellular Biology
Nucleus Pulposus
• Cells very similar to notochord cells at birth.1
Large with vacuoles containing glycosaminoglycans.
By 10 years of age, notochordal cells disappear.
In other species, connote disc repair6
• NP cells:
Appear similar to chondrocytes.
Humans are termed “chondrodystrophoid”6
Aggrecan and some Collagen Type II
Express FasL, which induces apoptosis of any
cell with Fas receptor7:
• T-cells7
• Nucleus pulposus cells8
Cellular Biology
Annulus Fibrosus
• Outer annulus fibroblastic cells1,2
Collagen type I (like tendon)
• Inner annulus chondrocyte-like cells1,2
Collagen type II (like hyaline cartilage, eye vitreus)
Molecular Biology
Main molecules in nucleus:
• Aggrecan: Large proteoglycan for water
retention (220 kDa). Anionic chondroitin
sulfate GAG chains
• Biglycan: Small proteoglycan with chondroitin
/ dermatan sulfate GAG chans. (38 kDa).
• Collagen type II, elastin
Disc homeostasis1:
• Balance of proteoglycan synthesis and degradation
• Ratio of small to large proteoglycans
Vascular Supply
• Fetal/infant (up to 2 years old)5:
Inner and outer annulus
Anterior, central, posterior endplates
• Juvenile/adolescent:
Avascular except small capillaries in outermost annulus
• Adult ( > 21 years old):
Avascular except small capillaries in outermost annulus
May have vascular ingrowth with annular tears or complete
disc destruction/scar
Vascular Supply
segmental radicular artery
interosseous artery
capillary tuft
disc annulus
Disc Nutrition
Diffusion from limited blood vessels9:
• Glucose and oxygen most important.
• Endplates (vertebral) vessels only. Terminate in loops.
• Any vascular portion of annulus only supplies the
• Endplate vessels have muscarinic receptors: will
constrict in response to cigarette smoke.9
• Movement of solutes from periphery to center of disc
from changes in mechanical load.
• MINIMAL contribution compared to diffusion down
concentration gradient.
• Zero-gravity state can cause hyperhydration10
Disc Nutrition
Endplate selective permeability9:
• Small solutes (oxygen, glucose) easy.
• Growth factors and matrix macromolecules cannot pass.
• Prevention of lactic acid build-up; pH > 6.7
Proteoglycan role:
• Impedes movement of larger proteins.
• Higher proteoglycan concentration = smaller diffusion
pore size.
Effect of diurnal cycle:
• Fluid loss decreases disc height by 20% = higher
proteoglycan concentration.
• Smaller disc height decreases distance for diffusion.
Classically begun with tear of
Endplate microfractures now felt to
be sentinel event (~65% of time).
Subclinical avulsion + time = disc
Acute annular tear with disc
herniation also common
Neovascularization / innervation
Endplate compromise = loss of nutrition
From Spine (Phila Pa 1976). 2005 Jan 15;30(2):167-73.
Calcification of endplate = decreased pore
Change in pore size = disruption of diffusion
From Spine (Phila Pa 1976). 2005 Jan 15;30(2):167-73.
Normal animal model
Human disc herniation
Decreased oxygen tension, glucose and
pH = cell death
Reduced proteoglycan concentration
Loss of selective permeability
Inflammatory cytokines (TNF, IL-1, IL-6,
etc) can enter nucleus
Cytokines upregulate MMP expression;
TIMP cannot keep up.
Additional proteoglycan destruction
Loss of water content and disc
Goals for New Therapies
Efficacy/survival in hostile
Maintain immune privilege
Restore matrix milieu
Reduce clinical symptoms!
Potential Targets
Chemodenervation of annular nerve
ingrowth: methylene blue
Recruitment of remaining NP cells:
platelet rich plasma (via TGFβ, IGF1)
Replacement of NP cells
Careful Considerations
Stem cell implantation:
• Embryonic stem cells controversial and may retain
tumorigenic potential.6
• Cell type needs to be similar to NP cells.
Mesenchymal is derived from mesoderm embryologically.
• Need cells to survive in low oxygen tension / low pH.
Bone marrow derived mesenchymal stem cells may survive better
than adipose (in rat model).12
• Should NOT provoke immune response6
• Need to keep cells within nucleus.13
• Identify ideal cell amount: prevent oxygen deprivation
and over-pressurization6,14
Careful Considerations
Platelet rich plasma:
• Inject to coax remaining cells to produce
proteoglycans / collagen type II
• Possible transient efficacy
• Incomplete knowledge of effects…
Ex. PRP contains VEGF,15 but disc milieu is avascular
Some preparations contain white blood cells16, but NP cells
express FasL. May induce IL-1 and TNF-α17
Thrombin can be used to activate PRP, but may induce
antibodies against it18
• May interfere with clotting cascade (post-op bleeding)
• Animal studies implicate anti-thrombin antibodies in lupus-type
Combination therapy:
Pig model of PRP and MSC showed osteogenic differentiation
instead of Collagen II / Aggrecan production19
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