Type II Collagen is Primarily Involved in the Annulus Fibrosus of the Intervertebral Disc
Type II collagen is the primary collagen type involved in the annulus fibrosus of the intervertebral disc, particularly in the inner regions, while the outer annulus contains predominantly type I collagen. 1, 2
Collagen Distribution in the Annulus Fibrosus
The annulus fibrosus has a complex, radial-ply connective tissue structure consisting of concentric lamellae of oriented collagen. Its composition varies from the outer to inner regions:
Outer annulus fibrosus:
- Contains predominantly type I collagen
- More fibrous in nature
- Lateral spacing of collagen molecules is approximately 14 Å in wet native state 2
Inner annulus fibrosus:
- Contains predominantly type II collagen
- More cartilaginous in nature
- Lateral spacing of collagen molecules is approximately 16-17 Å in wet native state 2
Nucleus pulposus:
- Contains exclusively type II collagen 2
Functional Significance of Collagen Types
The distribution of different collagen types within the annulus fibrosus serves important biomechanical functions:
Type II collagen:
- Contains more water than type I collagen (approximately 50-100% more) 2
- This higher hydration level helps dissipate compressive forces
- Critical for the cartilaginous properties of the inner annulus
Type I collagen:
- Provides tensile strength in the outer annulus
- Contributes to the fibrous nature of the outer annulus
Development and Remodeling
During development, the annulus fibrosus undergoes significant changes in its collagen composition:
In fetal development, the outer annulus is fibrous (containing type I collagen) while the inner part is cartilaginous (containing type II collagen) 3
With post-natal growth and aging, there is a gradual switch from type I to type II collagen and an increase in proteoglycan synthesis in the outer annulus - a process known as fibrocartilaginous metaplasia 3
This remodeling appears to occur in response to compressive loading and is driven by growth factors such as TGF-β1 and IGF-1 3
Biomechanical Implications
The specific arrangement and interaction between collagen types contribute significantly to the mechanical properties of the annulus fibrosus:
- Fiber stretch and shear interactions dominate contributions to circumferential direction stress
- Normal and shear interactions between fibers and matrix dominate axial stress 4
- These interactions are likely facilitated by crosslinks, elastin, or minor collagens 4
Understanding the collagen composition and structure of the annulus fibrosus is critical for developing effective treatments for intervertebral disc pathologies and for informing future repair and regenerative strategies.