Three dimensional mesoscale analysis of translamellar cross‐bridge morphologies in the annulus fibrosus using optical coherence tomography

Abstract

The defining characteristic of the annulus fibrosus (AF) of the intervertebral disc (IVD) has long been the lamellar structures that consist of highly ordered collagen fibers arranged in alternating oblique angles from one layer to the next. However, a series of recent histologic studies have demonstrated that AF lamellae contain elastin- and type VI collagen-rich secondary “cross-bridge” structures across lamellae. In this study, we use optical coherence tomography (OCT) to elucidate the three-dimensional (3D) morphologies of these translamellar cross-bridge in AF tissues. Mesoscale volumetric images by OCT reveal a highly heterogeneous spatial network and distribution of 3-D translamellar cross-bridges. The results of this study confirm the translamellar cross-bridge is identified as a distinguishable structure, which is laid in the interbundle space of adjacent lamellae and crisscrosses multiple lamellae in the radial direction. In contrast to previously proposed models extrapolated from 2-D sections, results from this current study show that translamellar cross-bridges exist as a complex, interconnected network. We also found much greater variation in lengths of cross-bridges within the interbundle space of lamellae (0.8-1.4 mm from the current study versus 0.3-0.6 mm from 2-D sections). OCT-based 3-D morphology of translamellar cross-bridge provides novel insight into the AF structure.

Optical Coherence Tomographic Elastography Reveals Mesoscale Shear Strain Inhomogeneities in the Annulus Fibrosus

“…Together with our previous results in ovine IVDs, 19 these data indicate that the ability for OCT to visualize the ITL matrix in AF tissues across species.…”

“…42 Within each lamella at the microscale level, it has been shown that motion segment bending induces fibrillar sliding of collagen on the convex side that is in tension, 34 resulting in greater relative displacements across collagen fibers than within collagen fibers, similar to that which has been observed in tendons. 43,44 At the intermediate mesoscale level, the ITL matrix traverses through and between lamellae, 19 but its mechanical function in the AF during tissue loading is still subject to speculation. Previous studies have suggested that the ITL compartment contributes little to AF mechanics in radial tension.…”

“…We have previously demonstrated the use of OCT in characterizing 3-D morphologies of the ITL network at relatively high resolution. 19 In this study, we show that elastographic analysis can be performed using OCT images obtained before and after loading. Specifically, we observed that under high AF tissue deformations the ITL compartment undergoes much higher shear strain than lamellae, suggestive of matrix damage.…”

“…Considering the structural inhomogeneity of the AF, 19–23 it is possible that certain regions within the annulus tissue may be more acutely susceptible to these types of stress gradients during IVD loading. One plausible candidate is the proteoglycan- and elastin-rich interlamellar and translamellar cross-bridge (ITL) compartment.…”

Annulus fibrosus cell phenotypes in homeostasis and injury: implications for regenerative strategies

“…This angle‐ply structure contributes to the nonlinear and anisotropic mechanical properties of the AF . Adding to this structural complexity, individual lamellae are separated by a disorganized interlamellar tissue containing proteoglycan‐rich matrix, elastic fibers, and cells, and are connected by networks of interlamellar cross‐bridges consisting of elastin and type VI collagen, which are thought to be important for maintaining healthy AF function . Elastic fibers are also known to bridge layers, and these have been speculated to be functional tie fibers and/or remnants of vascular channels that regress following IVD development .…”