Vertebral Endplate Architecture and Vascularization: Application of Micro-Computerized Tomography, a Vascular Tracer, and Immunocytochemistry in Analyses of Disc Degeneration in the Aging Sand Rat

Abstract

Lower lumbar vertebral endplates from young and old sand rats were assessed in an Institutional Animal Care and Use Committee approved study for architectural endplate features using micro-computerized tomography (CT) 3-dimensional (3D) models and vascularization studies by an in vivo vascular tracer or immunocytochemical identification of blood vessels. To assess endplate porosity and vascularization using μCT architectural analysis, an in vivo vascular tracer, and immunocytochemical identification of blood vessels in the endplate. The vertebral endplates, also called cartilage endplates, form the superior and inferior, or cranial and caudal, boundaries of the disc. In the human being and sand rat, the cartilaginous endplate undergoes calcification with aging and is replaced by bone. Endplate sclerosis has long been thought to play a role in disc degeneration by decreasing nutrient availability to the disc, but this is still poorly understood. Previous work has identified increasing bone mineral density with aging and disc degeneration in the sand rat model. μCT models of the lower lumbar endplates of vertebrae at L5–6 and L6–7 were constructed from 6 younger (mean age 11 months) and 21 older (mean age 25.6 months) sand rats. Architectural features were scored on a semiquantitative scale for smoothness of the endplate face, irregularities on the endplate margin, and endplate thickness. There were 2 smaller sets of animals (n = 18) evaluated for endplate vascularity following in vivo injection of a fluorescent vascular tracer or by the use of immunocytochemistry to identify blood vessels. μCT revealed a solid bony surface to the endplate, which was not penetrated by vasculature; with aging/disc degeneration, there was roughening and pitting of the plate surface, and the development of irregularmargins. In L5–6 and L6–7, sites of prominent disc degeneration evident on radiographs, the proportion of abnormalities in surface smoothness, margin irregularity, and endplate thickening were all statistically significant in both younger and older animals (P ≤ 0.0027). More severe changes were evident in the caudal versus cranial endplate surfaces. Histologic study of vascular tracer showed that there was no penetration of the disc by vascular supply from the endplate; this was verified by immunocytochemical identification of blood vessels. The canal system within the endplate was a complex 3D interconnected network. Findings show that disc degeneration in the sand rat occurs concomitantly with marked architectural bony changes on the endplate face, including loss of smoothness and development of irregular bony margins. Vascular connections were not present between the endplate and disc; this was verified with μCT studies, in vivo vascular tracers, and traditional immunocytochemistry. The canal system within the imaged endplate was revealed to consist of a complex 3D interconnected network.