THE NATURAL HISTORY OF AGE-RELATED DISC DEGENERATION: THE INFLUENCE OF AGE AND PATHOLOGY ON CELL POPULATIONS IN THE L4???5 DISC

Abstract

Introduction: The age-related reduction of water-trapping proteoglycans needed to maintain optimal disc hydration may be caused by reduced synthetic ability or fewer chondrocytes but there is a paucity of objective quantitative studies of disc cellularity. Methods: Sagittal sections of L4–5 discs were subdivided into a nucleus zone and 18 annulus zones prior to determining cell density (cells/mm2) in the mid-sagittal plane of 10 male and 10 female discs aged 13–79 years; 7 parasagittal slices through a single disc; discs showing various types of tears. Results: Most chondrocytes were unicellular but bicellular and multicellular chondrons were common in the margins of large tears and the nucleus in degenerate discs. Cellularity was highest in the right posterior quadrant, lowest in the left anterior quadrant, about equal in the left posterior and right anterior quadrants, and substantially higher in the right half of the nucleus. The correlation of increasing age with declining cell density was much stronger for the nucleus than for the annulus. Nucleus cellularity continued to decline throughout life whereas the annulus ceased its decline after the age of 50. Cell density was low in the vicinity of tears but elsewhere the disc was unaffected. Extensive inferior and superior end-plate separations reduced cellularity throughout the disc. Increased thickness of the cartilage end-plate was associated with higher cellularity in the nucleus. Discussion: The findings that cell density is higher in the posterior annulus and in the right half of the disc, tends to be increased if the cartilage end-plate is thicker, and is not uniformly diminished by large tears, indicates that disc cellularity is influenced by a complex interplay of factors which needs to be understood before attempts are made to restore the structural and functional integrity of degenerate discs.