The depth-dependent anisotropy of articular cartilage by Fourier-transform infrared imaging (FTIRI)

To study the anisotropic characteristics of individual histological zones in articular cartilage using Fourier-transform infrared imaging (FTIRI) at 6.25microm pixel resolution.

A canine humeral cartilage-bone block was paraffin-embedded and microtomed into 6microm sections. Each of the five sections was infrared (IR)-imaged 26 times with identical acquisition parameters, for a 5-10 degrees increment of a wire grid polarizer introduced before the detector in 0-180 degrees angular space. Following the IR imaging experiments, the same tissue sections were also imaged by polarized light microscopy (PLM).

The IR absorption components of cartilage (amide I, amide II, amide III, and sugar) exhibit distinctly different anisotropies, which vary differently as a function of the tissue depth. A new type of image, "the absorbance anisotropy map", was constructed for each major component, which shows that (1) the absorbance of the amide components in most parts of the tissue is anisotropic, (2) the anisotropic behavior in the radial and the superficial zones of the tissue is opposite, (3) the absorption profile of amide I is inverse to those of amide II and amide III, and (4) the IR absorption of the sugar component is almost isotropic. The anisotropic variations of the amide components were fitted to an empirical equation.

The IR anisotropy map is a powerful tool to monitor the individual chemical components in articular cartilage. The ability to examine the same tissue section using both FTIRI and PLM offers the possibility of correlating the tissue's morphology with chemical distribution.