ISSLS prize winner:: Collagen fibril sliding governs cell mechanics in the anulus fibrosus -: An in situ confocal microscopy study of bovine discs

Study Design. In situ investigation of collagen and cell mechanics in bovine caudal discs using novel techniques of confocal microscopy. Objective. To measure simultaneously the in situ intercellular and collagen matrix mechanics in the inner and outer anulus fibrosus of the intervertebral disc subjected to flexion. Summary of Background Data. Mechanobiology studies, both in vivo and in vitro, clearly demonstrate that mechanical factors can influence the metabolic activity of disc cells, altering the expression of key extracellular matrix molecules. Essential to elucidating the mechanotransduction mechanisms is a detailed understanding of the in situ mechanical environment of disc cells in response to wholebody mechanical loads. Methods. Confocal microscopy was used to simultaneously track and capture in situ images of fluorescently labeled cells and matrix during an applied flexion. The position of the nuclear centroids was calculated before and after applied flexion to quantify the in situ intercellular mechanics of both lamellar and interlamellar cells. The deflection patterns of lines photobleached into the extracellular matrix were used to quantify collagen fibril sliding and collagen fibril strains in situ. Results. The extracellular matrix was observed to deflect nonuniformly due to the relative sliding of the collagen fibrils. Intercellular displacements within the lamellar layers were also nonuniform, both along a cell row and-between adjacent rows. Within a cell row, the intercellular displacements were small ( < 1%). Conclusions. The in situ cell mechanics of anular cells was found to be strongly influenced by collagen fibril sliding in the extracellular matrix and could not be inferred directly from applied tissue loads.