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M. Johnson, E. H. Zhou, R. Krishnan, K. Perkumas, W. D. Stamer, C. R. Ethier, J. Fredberg; Stiffness of Schlemm’s Canal Cells in vitro and in vivo . Invest. Ophthalmol. Vis. Sci. 2009;50(13):4879.
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The Schlemm's canal (SC) cell is exposed to a basal-to-apical pressure gradient that is mechanically supported by its stiffness. Determination of this stiffness will allow an evaluation of the maximum pressure loading the SC cell can support and, perhaps, the extent to which this layer can generate outflow resistance.
Optical magnetic twisting cytometry (OMTC) was used to measure the shear modulus of 3 lines of human SC cells in culture from donors of ages 29, 64 and 78. Magnetic beads 4.5 µm in diameter were attached to SC cells using poly-L-Lysine and twisted by a magnetic field. The resulting bead displacement was measured and the moduli of these cells determined. Estimates for cell modulus in vivo were determined by applying elasticity theory to data of Grierson and Lee (AJO,84: 234) for giant vacuole volume in SC cells as a function of IOP in monkey eyes.
The moduli in the three cultured cell lines were 352±18 Pa (mean±SE, n=411 cells), 490±56 Pa (n=1010) and 432±20 Pa (n=610), in order of increasing age. These values are similar to upper bound estimates of modulus in another SC cell line (64 years) determined using magnetic pulling cytometery (204-742 Pa) (Zeng et al, in review).Analysis of monkey SC cells in vivo indicated that the modulus increased with increasing IOP. Assuming that 10% of outflow resistance is generated by inner wall pores, we found that at low IOP (8-15 mmHg), the modulus was 583-1328 Pa, in agreement with values determined in culture, while at high IOP (22-30 mm Hg), the modulus increased to 2711-5404 Pa. Assuming all of the resistance was in the inner wall led to estimates of SC modulus >20 times higher than measured in culture and higher than any known endothelial cell type.
The stiffnesses of isolated SC cells measured using OMTC were comparable to previous measurements from magnetic pulling cytometry. In vivo histologic studies suggest that this stiffness increases under physiological load. Our results suggest that SC cells may not be sufficiently stiff to support more than a small fraction (<10%) of the pressure drop across the conventional outflow tissues.
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