Hierarchical structure of the intervertebral disc

Area of Science:

• Biomaterials Science
• Biomechanics
• Anatomy

Background:

• The human intervertebral disc (IVD) is a complex structure crucial for spinal function.
• Understanding the hierarchical organization of collagenous components is key to deciphering IVD mechanics.
• Previous studies have provided limited insight into the detailed morphological gradients within the annulus fibrosus.

Purpose of the Study:

• To characterize the hierarchical structure and morphological gradients of collagenous components in the human intervertebral disc.
• To quantify variations in lamellar thickness, fiber inclination, and collagen fiber crimp within the annulus fibrosus.
• To develop a hierarchical model of the intervertebral disc incorporating observed morphological gradients.

Main Methods:

• Optical microscopy techniques were employed to examine the collagenous structures of the human intervertebral disc.
• Measurements of lamellar thickness, interlamellar fiber angles, and collagen fiber crimp morphology were systematically performed.
• Analysis focused on different regions of the annulus fibrosus (anterior, lateral, posterior) and radial gradients from the disc edge to the nucleus.

Main Results:

• Lamellar thickness increases abruptly in the anterior annulus fibrosus, defining peripheral and transitional regions.
• A broad distribution of lamellar thicknesses was observed in the lateral and posterior annulus fibrosus.
• Interlamellar fiber angles decrease radially inward (from +62° to +45°), and collagen fiber crimp angle increases (from 20° to 45°) with a decreasing crimp period (from 26 to 20 µm).

Conclusions:

• The human intervertebral disc exhibits significant radial morphological gradients in its collagenous components.
• These gradients, including variations in lamellar structure and collagen fiber organization, are critical for the disc's biomechanical function.
• A novel hierarchical model integrating these morphological findings provides a more comprehensive understanding of intervertebral disc structure.