Correlation of histology and linear and nonlinear microscopy of the living human cornea

The morphology and the function of cellular and non-cellular structures in the living human cornea can be determined with modern correlative linear and nonlinear optical microscopic techniques and histology. Correlative microscopy is based on the use of different optical techniques to study the same specimen, ideally at the same location within the specimen, in order to increase the functional and/or morphological understanding of the specimen. A case study to assess the effect of overnight lid-closure on in vivo human corneal morphology is presented to illustrate correlative linear microscopy and optical low-coherence reflectometry. Nonlinear multiphoton excitation microscopy provides functional information on cellular metabolism based on the intrinsic fluorescence from the reduced pyridine nucleotides and the oxidized flavoproteins. Second-harmonic generation microscopy, a scattering process that does not deposit net energy into the tissue, provides structural information on corneal collagen organization. Molecular third-harmonic generation microscopy generates a signal in all materials and it an emerging technique. Coherent anti-Stokes Raman scattering microscopy provides chemical imaging for biology and medicine. The comparison and limitations of these microscopic modalities, linear and nonlinear microscopy applied to the cornea, and a review of some key findings is analyzed. A correlative integration and correlation of linear and nonlinear microscopies to study corneal function and structure is proposed to validate the clinical interpretation of microscopic images of the cornea.