The Relationship between Retinal Ganglion Cell Axon Constituents and Retinal Nerve Fiber Layer Birefringence in the Primate

Abstract

To determine the degree of correlation between spatial characteristics of the retinal nerve fiber layer (RNFL) birefringence (Delta n(RNFL)) surrounding the optic nerve head (ONH) with the corresponding anatomy of retinal ganglion cell (RGC) axons and their respective organelles. RNFL phase retardation per unit depth (PR/UD, proportional to Delta n(RNFL)) was measured in two cynomolgus monkeys by enhanced polarization-sensitive optical coherence tomography (EPS-OCT). The monkeys were perfused with glutaraldehyde and the eyes were enucleated and prepared for transmission electron microscopy (TEM) histologic analysis. Morphologic measurements from TEM images were used to estimate neurotubule density (rho(RNFL)), axoplasmic area (A(x)) mode, axon area (A(a)) mode, slope (u) of the number of neurotubules versus axoplasmic area (neurotubule packing density), fractional area of axoplasm in the nerve fiber bundle (f), mitochondrial fractional area in the nerve fiber bundle (x(m)), mitochondria-containing axon profile fraction (m(p)), and length of axonal membrane profiles per unit of nerve fiber bundle area (L(am)/A(b)). Registered PR/UD and morphologic parameters from corresponding angular sections were then correlated by using Pearson's correlation and multilevel models. In one eye there was a statistically significant correlation between PR/UD and rho(RNFL) (r = 0.67, P = 0.005) and between PR/UD and neurotubule packing density (r = 0.70, P = 0.002). Correlation coefficients of r = 0.81 (P = 0.01) and r = 0.50 (P = 0.05) were observed between the PR/UD and A(x) modes for each respective subject. Neurotubules are the primary source of birefringence in the RNFL of the primate retina.