Measurement of the absolute value of the optical birefringence of myelin in primate brain tissue.

SIGNIFICANCE: Researchers require quantitative biomarkers to accurately identify neurodegenerative diseases and quantitatively monitor disease progression. The structural anisotropy of myelin leads to strong optical birefringence, enabling quantitative imaging with polarized-light microscopy imaging for detailed myelin assessment in neurodegenerative disease states and aging studies.

AIM: Our aim is to measure the absolute refractive index difference (birefringence) of the myelin sheath of primate brain tissue using birefringence microscopy (BRM).

APPROACH: Three-micron cryo-sectioned samples from the paraformaldehyde-fixed corpus callosum of a 22-year-old male rhesus macaque were analyzed using a BRM system. To quantify the absolute birefringence, transversely oriented axons were imaged with a 40× objective under red light-emitting diode illumination ( λ = 625    nm ). Measurements focused on large myelinated axons (diameters ∼ 2 to 8    μ m ) mounted in 85% glycerol, ensuring high-resolution characterization of thick myelin sheaths.

RESULTS

The myelin birefringence was determined to be Δ n = 0.012 ± 0.001 , representing the first absolute measurement of this optical property for myelin in primate brain tissue.

CONCLUSIONS

This absolute birefringence value enables quantification of myelin volume fraction and assessment of myelin loss in ex vivo measurements, providing an accurate optical biomarker for neurodegenerative diseases and aging studies through polarized light microscopic imaging.