Early efforts have confirmed the presence of a sparse and irregular network of elastic fibers in the intervertebral disc (IVD), however, later light microscopic studies reported that IVD elastic network is highly organized and their distribution and orientation vary from region to region. Even though, due to the masking effect of the extracellular matrix, the visualization of the ultrastructural organization of the elastic fibers in the IVD has not been possible until recently [1].
We developed a new method to partially digest all non-elastin components and evaluate the ultrastructural organization of elastic fibers in the IVD using scanning electron microscopy technique [2]. Our results revealed that elastic fibers form a network across the nucleus polposus (NP) consisting of straight and thick (890 nm) parallel fibers that were interconnected by wavy fine fibers (215 nm). Our further analysis identified a dense elastic fiber network at the interface of the NP and annulus fibrosus (AF) consisting of major thick elastic fibers (> 1000 nm) that were interconnected with delicate (< 200 nm) elastic fibers. Within the AF intra-lamellar space, a loose network of elastic fibers was observed which was comprised of almost parallel thick fibers (300-500 nm) and fine interconnecting fibers of less than 300 nm thickness. Visualization of the interlamellar matrix (ILM) under high magnification revealed a dense and complex network of elastic fibers, including thick (2000 nm) and thin (100 nm) elastic fibers. Similar to the ILM, elastic fibers within the partition boundaries (PB) formed a complex and dense network comprised of thick (1000-2000 nm) and thin (100 nm) fibers [3, 4].
A detailed examination of the elastic network in the IVD led to a new understanding of IVD structure and its interconnectivity that contribute to fundamental structural information relevant to IVD tissue engineering and mechanical modeling. The IVD can be viewed as a modular structure organized into compartments of collagen bundles enclosed by an elastic sheath.