Optical properties of mutant versus wild-type mouse skin measured by reflectance-mode confocal scanning laser microscopy (rCSLM)

Separation of the two optical scattering properties, the scattering coefficient (μ _s) and the anisotropy of scattering (g), has been experimentally difficult in tissues. A new method for measuring these properties in tissues uses reflectance-mode confocal scanning laser microscopy (rCSLM). Experimentally, the focus at depth z is scanned down into the tissue. The measured data is the exponential decay of the confocal reflectance signal as a function of the depth of the focal volume, R(z)=ρexp(-μz), summarized as a local reflectivity (ρ) and an exponential decay constant (μ). The ρ and μ map uniquely into the μ _s and g of the tissue. The method was applied to three mouse skin tissues: one wild-type (wt/wt), one heterozygous mutant (oim/wt), and one homozygous mutant (oim/oim), where oim indicates the mutation for osteogenesis imperfecta, a bone disease that affects type I collagen structure. The mutation affects the collagen fibrils of the skin and the assembly of collagen fiber bundles. The anisotropy of scattering (g) at 488 nm wavelength decreased from 0.81 to 0.46 with the added mutant allele. There was a slight increase in the scattering coefficient (μ _s) with the mutation from 74to94cm ^-1. The decrease in g (toward more isotropic scattering) is likely due to the failure of the mutant fibrils to assemble into the larger collagen fiber bundles that yield forward scattering.