Refractive indices of the collagen fibrils and extrafibrillar material of the corneal stroma

Abstract

Ultrastructural data from x-ray diffraction studies of the cornea were used to estimate the refractive indices of the collagen fibrils and extrafibrillar material of human, ox, trout, and rabbit corneas. X-ray diffraction measurements of the size and spacing of the collagen fibrils and the separation between the constituent molecules of the fibrils were taken from a previous species study. The tissue volume fractions occupied by the stromal components were estimated and their refractive indices were calculated using the Gladstone-Dale law of mixtures. For the fibrils and extrafibrillar material, the refractive indices in the human cornea were 1.411 and 1.365; for the ox 1.413 and 1.357; for the rabbit 1.416 and 1.357; and for the trout 1.418 and 1.364, respectively. An alternative estimate based on the physical properties and chemical composition of bovine cornea, accounting for interfibrillar type VI collagen and cellular water, produced similar estimates of 1.416 and 1.356 for the fibrils and extrafibrillar material, respectively.

Transparency, swelling and scarring in the corneal stroma

“…The refractive index of the hydrated collagen fibrils, the refractive index of the interfibrillar matrix, and the ratio of these have previously been obtained using X-ray diffraction measurements in a number of different species. 2 The relative positions of the individual fibrils, and the diameter of each fibril, are obtained from electron micrographs. There is a problem in using measurements from electron microscopy in that it has been shown that a number of microscope preparation protocols result in considerable shrinkage of fibril diameters and, particularly, interfibril spacings.…”

“…Also, recent X-ray diffraction data have unambiguously confirmed earlier evidence for a difference in the refractive indices of the collagen fibrils and of the extrafibrillar material. 2 Most modern models are based on the lattice theory put forward by Maurice. 3 By approximating the collagen fibril to perfect, infinitely long cylinders, an estimate of the scattering from an individual fibril can be calculated.…”

“…The amount by which it falls can be estimated by measuring the change in the refractive index of the stroma as a function of tissue thickness or hydration ( Figure 8) and then applying Gladstone and Dales's law of refractive indices to the system. 2 Since the imbibed fluid does not enter the fibrils themselves, 21 this fall in the refractive index as the stroma swells leads to an increase in the ratio of the refractive index of the fibrils to that of the interfibrillar matrix and to a corresponding increase in light scattering. We found that between physiological hydration and H ¼ 3.8, there was a 0.15% reduction in the refractive index of the matrix and a corresponding 0.1% increase in the ratio of the refractive indices of the fibrils and the matrix (S Khan, S Dennis, and K Meek, unpublished results).…”

Optical clearing of biological tissues: prospects of application in medical diagnostics and phototherapy

“…The scattering particles (organelles, protein fibrils, membranes, and globules) have greater density of proteins and lipids and, hence, higher refractive index (n s = 1.39-1.47) in comparison with the base substance of the cytoplasm [6]. The refractive index values of the connective tissue fibrils lie in the range 1.41-1.53 and depend upon the degree of hydration of their major component, the collagen [9]. The refractive index of the interstitial fluid and the blood plasma amounts to nearly 1.33-1.35 depending on the wavelength [3,10].…”

Average refractive index of tendon as a function of water content

“…The significant deviation of the actual n g (C w ) curve from this approximating line in the region C w ≤ 0.52 may be attributed to a change of the mode of hydration. In the literature, two modes of collagenous tissue hydration are distinguished [33][34][35][36]. For relatively high values of H (H > H c , where H c is a critical value, the so-called fibrillar saturation point [35]), changes in the total water content in the tissue lead to changes of the water content in extrafibrillar space while the water content in collagen fibrils remains unchanged [7,[33][34][35][36].…”