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R. G. Aranibar, H. G. Rylander, III, N. J. Kemp, J. C. Dwelle, S. E. Byers, M. K. Markey, C. S. Specht, T. E. Milner; Relationship Between Birefringence and Neurotubule Density in the Primate Retinal Nerve Fiber Layer. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3344.
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© ARVO (1962-2015); The Authors (2016-present)
To experimentally determine the level of correlation between retinal nerve fiber layer (RNFL) birefringence and retinal ganglion cell (RGC) neurotubule density (NTD, number of neurotubules per unit RNFL area) in the primate RNFL.
Phase retardation per unit depth (PR/UD, proportional to birefringence) and NTD were measured in the peripapillary RNFL of a healthy 11-year old female cynomolgus monkey eye. PR/UD was measured with an enhanced polarization-sensitive optical coherence tomography (EPS-OCT) instrument. A novel algorithm was used to estimate NTD in transverse sections of 3 RGC nerve fiber bundles per RNFL octant (45° sector) imaged with a transmission electron microscope (TEM). The algorithm calculates NTD using a twice-weighted sum of linear-estimated RGC axoplasmic neurotubule densities (NTDx, number of neurotubules per unit RGC axoplasmic area). NTDx estimates are individually weighted by probabilities of their associated RGC axoplasmic areas (Ax, axon area less non-cytoskeletal organelle area) in the nerve fiber bundle and summed together. This sum is weighted by the area fraction of RGC axoplasm in the nerve fiber bundle, yielding NTD. PR/UD and NTD were then correlated using conventional (i.e., single-level) and multilevel techniques.
Statistically insignificant (P > 0.10) and marginally significant (0.05 < P < 0.10) correlation coefficients (0.45 < r < 0.61) were found between PR/UD and NTD. In order for the correlation coefficient calculated between sector-averaged PR/UD and NTD to be statistically significant (P < 0.05) and powerful (Power > 80%), measurements must be made in at least 16 sectors. A much stronger and statistically significant (P = 0.01) correlation coefficient (r = 0.81) was observed between octant-averaged PR/UD and Ax mode.
Axoplasmic regions of RGC axons are the only areas in which neurotubules can be positioned. A strong relationship should exist between Ax and neurotubule number (k) if neurotubules are the primary support system of RGC axons and structural requirements are the same for all RGC axons. Error due to linear-based estimations of this relationship (i.e., NTDx) may explain lower and higher correlations of PR/UD with NTD and Ax mode, respectively, and non-estimate based methods for measuring NTD may be required to determine an accurate mathematical relationship between RNFL birefringence and NTD.
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