Ultrastructural organization of elastic fibres in the partition boundaries of the annulus fibrosus within the intervertebral disc

The relationship between elastic fibre disorders and disc degeneration, aging and progression of spine deformity have been discussed in a small number of studies. However, the clinical relevance of elastic fibres in the annulus fibrosus (AF) of the disc is poorly understood. Ultrastructural visualization of elastic fibres is an important step towards understanding their structure-function relationship. In our previous studies, a novel technique for visualization of elastic fibres across the AF was presented and their ultrastructural organization in intra- and inter-lamellar regions was compared. Using the same novel technique in the present study, the ultrastructural organization of elastic fibres in the partition boundaries (PBs), which are located between adjacent collagen bundles, is presented for the first time. Visualization of elastic fibres in the PBs in control and partially digested (digested) samples was compared, and their orientation in two different cutting planes (transverse and oblique) were discussed. The ultrastructural analysis revealed that elastic fibres in PBs were a well-organized dense and complex network having different size and shape. Adjacent collagen bundles in a cross section (CS) lamella appear to be connected to each other, where elastic fibres in the PBs were merged in parallel or penetrated into the collagen bundles. There was no significant difference in directional coherency coefficient of elastic fibres between the two different cutting planes (p =.35). The present study revealed that a continuous network of elastic fibres may provide disc integrity by connecting adjacent bundles of CS lamellae together. Compared to our previous studies, the density of the elastic fibre network in PBs was lower, and fibre orientation was similar to the intra-lamellar space and inter-lamellar matrix. Statement of Significance: A detailed ultrastructural study in the partition boundaries of the annulus fibrosus within the disc revealed a well-organized elastic fibre network with a complex ultrastructure. The continuous network of elastic fibres may provide disc integrity by connecting adjacent bundles of cross section lamellae together. The density of the elastic fibre network in PBs was lower, and fibre orientation was similar to the intra-lamellar space and the inter-lamellar matrix.